DOI: 10.1007/s11224-021-01755-5

A ¹³C chemical shifts study of iodopyrazoles: experimental results and relativistic and non-relativistic calculations

This work reports the ¹³C chemical shifts of 49 iodopyrazoles and the ¹⁵N chemical shifts of 6 iodopyrazoles, most of them from the literature but a number of significant cases from the present work. Most experimental data were from solution studies but some of them correspond to the solid state (CPMAS). The calculations include non-relativistic calculations (nr-GIAO) and relativistic ones (ZORA, mDKS/B3LYP/VDZ and mDKS/B3LYP/VTZ). In the case of NH-pyrazoles, problems of tautomerism and desmotropy arise that have been also studied. The manuscript is dedicated to develop some equations containing corrections for heavy atoms to predict ¹³C and ¹⁵N chemical shifts.

DOI: 10.1007/s11224-020-01661-2

Multinuclear magnetic resonance studies of five silver(I) trinuclear pyrazolate complexes

Experimental multinuclear determination of the chemical shifts, especially 15N and 109Ag, of five silver(I) trinuclear pyrazolate complexes, (PzAg)3, coupled with ZORA (zeroth-order regular approximation) and GIAO (gauge-independent atomic orbital) calculations, has shed new light on these compounds. An almost complete collection of spin-spin coupling constants has been measured for the first time for several of these compounds. The aromaticity of (PzAg)3 was examined using NICS (nucleus-independent chemical shift) and ACID (anisotropy of the induced current density) approaches reaching the conclusion of its absence.

DOI: 10.1002/mrc.5026

Calculated coupling constants 1J(X–Y) and 1K(X–Y), and fundamental relationships among the reduced coupling constants for molecules HmX–YHn, with X, Y ═ ¹H, ⁷Li, ⁹Be, ¹¹B, ¹³C, ¹⁵N, ¹⁷O, ¹⁹F, ³¹P, ³³S, and ³⁵Cl

Equation‐of‐motion coupled‐cluster singles and doubles (EOM‐CCSD) calculations have been performed to determine coupling constants 1J(X–Y) for 65 molecules H_mX–YH_n, with X,Y ═ ¹H, ⁷Li, ⁹Be, ¹¹B, ¹³C, ¹⁵N, ¹⁷O, ¹⁹F, ³¹P, ³³S, and ³⁵Cl. The computed ¹J(X–Y) values are in good agreement with available experimental data. The reduced coupling constants ¹K(X–Y) have been derived from ¹J(X–Y) by removing the dependence on the magnetogyric ratios of X and Y. Patterns are found for the reduced coupling constants on a ¹K(X–Y) surface that are related to the positions of X and Y in the periodic table.

DOI: 10.1016/j.ssnmr.2020.101676

A GIPAW versus GIAO-ZORA-SO study of 13C and 15N CPMAS NMR chemical shifts of aromatic and heterocyclic bromo derivatives

Theoretical simulation of NMR parameters in compounds bearing heavy atoms generally requires the application of relativistic corrections. We report herein the theoretical characterization of ¹³C and ¹⁵N CPMAS NMR of known bromo-derivative crystals by using both the GIPAW and the combined GIAO-ZORA-SO approximation methods. Several statistical analyses were performed to compare both approaches, with non-relativistic GIPAW method being more useful to predict the ¹³C and ¹⁵N chemical shifts. The problem of applying GIPAW to crystal structures showing static or dynamic crystalline disorder of the special class resulting in half-protons will be discussed in detail.

DOI: 10.1002/mrc.5002 

An experimental and computational NMR study of organometallic nine‐membered rings: Trinuclear silver(I) complexes of pyrazolate ligands

This work reports the calculation of the nuclear magnetic resonance (NMR) chemical shifts of eight trinuclear Ag(I) complexes of pyrazolate ligands using the relativistic program ZORA. The data from the literature concern exclusively ¹H, ¹³C, and ¹⁹F nuclei. For this reason, one of the complexes that is derived from 3,5‐bis‐trifluoromethyl‐1H‐pyrazole has been studied anew, and the ¹⁵N and ¹⁰⁹Ag chemical shifts determined for the first time in solution. Solid‐state NMR data of this compound have been obtained for some nuclei (¹H, ¹³C, and ¹⁹F) but not for others (¹⁴N, ¹⁵N, and ¹⁰⁹Ag).

DOI: 10.1002/poc.4043 

A theoretical and spectroscopic (NMR and IR) study of indirubin in solution and in the solid state

The infrared spectrum of indirubin in the solid state and the ¹H, ¹³C and ¹⁵N NMR spectra in DMSO‐d6 solution as well as the ¹³C and ¹⁵N CPMAS spectra have been recorded. Different types of calculations (B3LYP, GIAO, and GIPAW) have been carried out. Experimental and calculated values were compared yielding commensurate results. E/Z isomerism was explored, as was oxo/hydroxy tautomerism.

DOI: 10.1002/mrc.4926

Theoretical study of some λ5‐phosphinines and their NMR spectra

DOI: 10.1007/s11224-019-01310-3

Multinuclear NMR spectra and GIAO/DFT calculations of N-benzylazoles and N-benzylbenzazoles

The ¹H, ¹³C, and ¹⁵N chemical shifts of almost the whole series of N-benzyl azoles and benzazoles, with the exception of the unknown 1-benzyl-1H-pentazole (10) and the very unstable 2-benzyl-2H-isoindole (12), have been measured. In addition, the X-ray crystal structure of 1-benzyl-1H-indazole (14) was solved (monoclinic, space group P21/n), its geometry being very close to that used for the calculations. The absolute chemical shieldings were calculated at the gauge-independent atomic orbital (GIAO)/Becke, 3-parameter, Lee-Yang-Parr (B3LYP)/6-311++G(d,p) level and then transformed with very robust empirical equations into chemical shifts of the three nuclei showing an excellent agreement with the 313 experimental values.

DOI: 10.1002/mrc.4892

Assignment of ¹H and ¹³C NMR data for three pairs of diastereomers of 4′‐X benzo[1,3]cyclopropa[1,2‐b]chromene‐4,5‐diones (X = H, OCH₃, and Cl)

DOI: 10.1002/mrc.4754

Libration of phenyl groups detected by VT‐SSNMR:Comparison with X‐ray crystallography

The X‐ray crystal structure of 2‐benzyl‐1H‐benzimidazole, 2BnBzIm, was determined at 293 K showing no dynamic phenomena (disorder) of any class. On the other hand, some 13C NMR signals were absent in the CPMAS spectrum (100 MHz, 300 K). We decided to carry out variable‐temperature SSNMR and discovered that the missing signals are orthoand meta carbons of the phenyl ring of the benzyl group. Line‐shape analysis and the Eyring equation were used to determine the barrier, which was compared with the calculated DFT for the gas phase that it is much lower.

DOI:  10.1002/mrc.4422

The ¹H NMR spectrum of pyrazole in a nematic phase

The experimental ¹H nuclear magnetic resonance (NMR) spectrum of 1H-pyrazole was recorded in thermotropic nematic liquid crystal N-(p-ethoxybenzylidene)-p-butylaniline (EBBA) within the temperature range of 299–308 K. Two of three observable dipolar D_HH-couplings appeared to be equal at each temperature because of fast prototropic tautomerism. Analysis of the Saupe orientational order parameters using fixed geometry determined by computations and experimental dipolar couplings results in a situation in which the molecular orientation relative to the magnetic field (and the liquid crystal director) can be described exceptionally by a single parameter. 

DOI:  10.1002/mrc.4438

A theoretical and ex perimental NMR study ofBODIPY 493/503: difluoro{2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron

DOI: 10.1002/mrc.4260

A multinuclear magnetic resonance study of fluoro derivatives of hydroxybenzaldehydes

NMR study of the prototropic tautomerism of tris(tetrabutylammonium)hydrogen pyrophosphate salt in solution and in the solid-state

The structure of tris(tetrabutylammonium)hydrogen pyrophosphate (HPP) has been studied in solution and in the solid-state by ³¹P NMR. GIAO/DFT calculations of the isolated molecule in its open and closed conformations, of their water complexes and the solvent effects (continuum model) have been useful to discuss the experimental NMR data. Contrary to literature reports, the ³¹P NMR spectra of HPP in solution have a single band, that broadened considerably when the temperature is lowered till 193 K without reaching the coalescence. In the solid-state, two very close signals are observed that the calculations do not reproduce adequately.

DOI: 10.1016/j.molstruc.2014.07.020

The origin of the splitting of ¹³C and ¹⁵N NMR signals of 3(5)-phenyl-5(3)-methylpyrazolium chloride and bromide in the solid state: Quantum Espresso calculations

A combination of ¹³C and ¹⁵N CPMAS NMR spectroscopy and theoretical methods (DFT and DFT-D) was used to discuss the observation of large splittings affecting some atoms in 3(5)-phenyl-5(3)-methylpyrazolium chloride and bromide. Conventional calculations using fully optimized structures with C₂symmetry reproduce solution spectra, but the large splitting observed for the signals of several pyrazolium carbon and nitrogen atoms in the solid-state can only be explained by calculations employing the experimental P2₁/n geometry and periodic boundary calculations.

DOI: 10.1080/00268976.2013.802820

Ramsey terms for two-, three-, and four-bond coupling involving ¹⁵N and ¹⁷O in hydrogen-bonded and nonhydrogen-bonded systems: are coupling constants sensitive to RAHBs?

Ab initio EOM-CCSD/(qzp,qz2p) calculations have been performed on complexes with intermolecular hydrogen bonds involving ¹⁵N and ¹⁷O, and molecules with and without intramolecular hydrogen bonds involving these nuclei. Coupling constants across intermolecular hydrogen bonds are well approximated by the Fermi-contact (FC) term. In general, ^2hJ(X–Y) for intramolecular coupling across X–H^…Y hydrogen bonds are not sensitive to the presence of resonance-assisted hydrogen bonds (RAHBs). However, ^2hJ(O–O) for coupling across the intramolecular hydrogen bond in malonaldehyde is greater than ^2hJ(O–O) for its saturated counterpart, so that ^2hJ(O–O) is sensitive to the presence of the RAHB. This is also the case for the sulphur analogues of malonaldehyde. For these unsaturated hydrogen-bonded molecules, molecules with carboxyl groups, and trans-glyoxal, J is dominated by the paramagnetic spin orbit (PSO) term. For these systems, the primary mode of coupling transmission is through the conjugated chain. For complexes with intermolecular hydrogen bonds, saturated molecules with intramolecular hydrogen bonds, unsaturated and saturated molecules in which the hydrogen bond has been broken, and unsaturated molecules with intramolecular N–H^…N or O–H^…N hydrogen bonds, J is dominated by the FC term. FC domination in hydrogen-bonded systems indicates that the primary transmission mode is across the hydrogen bond.

DOI: 10.1002/mrc.3983

The structures of two aldazines: [1,1'-(1E,1'E)-hydrazine-1,2-diylidenebis(methan-1-yl-1-ylidene)dinaphthalen-2-ol] (Lumogen) and 2,2'-(1E,1'E)-hydrazine-1,2-diylidenebis(methan-1-yl-1-ylidene)diphenol (salicylaldazine) in the solid state and in solution

A combination of NMR spectroscopy and theoretical methods Density functional theory including dispersion corrections (DFT-D) was used to study the structures of Lumogen and salicylaldazine. In the solid state, Lumogen exists as the dihydroxy tautomer 1a (an azine, C=N–N=C) as was already known from an X-ray determination. In a deuterated dimethyl sulfoxide solution, another tautomer is observed besides 1a; its structure corresponds to the hydroxy-oxo tautomer 1b (a hydrazone, C=N–NH–Csp²). In what concerns salicylaldazine, we have observed only the dihydroxy tautomer 2a

DOI:/10.1016/j.tet.2013.06.072,

A theoretical study on the aromaticity of benzene and related derivatives incorporating a C–C≡C–C fragment

Continuing with our interest in aromaticity, we have studied the influence that replacement of formal C–C single bonds by C–C≡C–C fragments, in a series of mono- (cyclobutadiene, benzene, and cyclooctatetraene) and fused-carbocycles (naphthalene and azulene), has in their properties, focusing mostly on NMR and aromaticity. We have analyzed the effect of such substitution not only in the aromaticity of the different structures, but also in the influence of low and high spin states by means of NICS values over the rings and 3D NICS isosurfaces. We have found that, in most of the cases, the substitution induces a loss of aromaticity in singlet states (both restricted and unrestricted) that can be recovered when triplet states are taken into account.

DOI: 10.1007/s10593-013-1237-x

Structure of NH-benzazoles (1H-benzimidazoles, 1H- and 2H-indazoles, 1H- and 2H-benzotriazoles)

The structure and properties (crystallography, NMR, theoretical calculations) of the three N-unsubstituted benzazoles (1H-benzimidazoles, 1H- and 2H-indazoles, 1H- and 2H-benzotriazoles) have been reviewed for the period 2000–2012 with some results from previous years. The study of these compounds will greatly increase in the coming years and it is expected that the present review will contribute to it.

Arkivoc, ii, 19-35 (2014)

A theoretical study of the mechanism of oxidation of 1H-pyrazolines to 1H-pyrazoles by molecular bromine

The bromine oxidation of NH-pyrazolines to pyrazoles involves a bromo substituted 2- or 1-pyrazoline. The present paper explores theoretically the different possibilities comparing the published NMR results to the experimental ones. Besides, geometries and energies are reported and discussed.

DOI: 10.1080/00387010.2012.691597

Theoretical and Experimental NMR Study of a Series of Five Nitrobenzene-1,2-Diamines

¹H-, ¹³C-, and ¹⁵N-NMR studies of five nitrobenzene-1,2-diamines in solution and solid state have been achieved and the experimental chemical shifts and coupling constants agree with the theoretical values obtained at the B3LYP/6-311 + +G(dp) computational level using the geometries fully optimized with the hybrid HF/DFT B3LYP method and the 6-31G(d) basis set. The GIAO approximation has been used to calculate the absolute shieldings. The contribution of the substituents to the ¹⁵N chemical shifts of the amino groups could be quantified using a presence/absence matrix and a multiple regression.

DOI: 10.1002/jhet.1076

Multinuclear NMR Characterization of Cyanuric Fluoride (2,4,6-Trifluoro-1,3,5-triazine)

Although 2,4,6-trifluoro-1,3,5-triazine, C₃F₃N₃, is a highly symmetrical molecule, its NMR parameters can be obtained by reducing its symmetry through the introduction of ¹⁴N/¹⁵N and ¹²C/¹³C isotopomers. Experimental and computed chemical shifts of cyanuric fluoride have been obtained for ¹³C, ¹⁵N, and ¹⁹F. Spin-spin coupling constants have been measured and compared with previous experimental data and with the complete set of computed EOM-CCSD coupling constants.

DOI:10.1016/j.comptc.2012.07.002

Analysis of the interactions between difluoroacetylene and one or two hydrogen fluoride molecules based on calculated spin–spin coupling constants

A theoretical study of FCCF:(HF)_n complexes, with n = 1 and 2, has been carried out by means of ab initio computational methods. Two types of complexes are formed: those with FH⋯π interactions and those with FH⋯FC hydrogen bonds. The indirect spin–spin coupling constants have been calculated at the CCSD/aug-cc-pVTZ-J computational level. Special attention has been paid to the dependence of the different intramolecular coupling constants in FCCF on the distance between the coupled nuclei and the presence or absence of the hydrogen fluoride molecule. The sensitivity shown by these coupling constants to the presence of hydrogen fluoride is quite notorious.

DOI: 10.1002/mrc.2868

The structure of glibenclamide in the solid state

The structure of glibenclamide, 5-chloro-N-(2-{4-[(cyclohexylamino)carbonyl] aminosulfonyl}phenyl) ethyl)-2-methoxybenzamide, an important antidiabetic drug, has been studied both in solution and in the solid state by a combination of NMR spectroscopy and theoretical calculations. The possibility that glibenclamide suffers a tautomerization under melting to afford a desmotrope was rejected

DOI: 10.1021/jp300763d

Structures, Binding Energies, and Spin-Spin Coupling Constants of Geometric Isomers of Pnicogen Homodimers (PHFX)2, X = F, Cl, CN, CH3, NC

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to determine the structures and binding energies of homodimers (PHFX)₂ for X = F, Cl, CN, CH₃, and NC. Geometric isomers of these complexes with C_i symmetry exist, which are differentiated in terms of the nature of the atoms (F–P···P–F, H–P···P–H, or A–P···P–A, with A being the atom of X directly bonded to P), which approach a nearly linear alignment. Of these, isomers having F–P···P–F linear are the most stable. Binding energies, intermolecular distances, and EOM-CCSD spin–spin coupling constants are sensitive to both the nature of X and the atoms that assume the linear alignment.

DOI: 10.1021/jp211451y

FCl:PCX Complexes: Old and New Types of Halogen Bonds

MP2/aug′-cc-pVTZ calculations have been performed to investigate the halogen-bonded complexes FCl:PCX, for X = NC, CN, F, H, CCH, CCF, CH₃, Li, and Na. Although stable complexes with a F–Cl···P halogen bond exist that form through the lone pair at P (configuration I), except for FCl:PCCN, the more stable complexes are those in which FCl interacts with the C≡P triple bond through a perturbed π system (configuration II). In complexes I, the nature of the halogen bond changes from traditional to chlorine-shared and the interaction energies increase, as the electron-donating ability of X increases. The anionic complex FCl:PC^– has a chlorine-transferred halogen bond. SAPT analyses indicate that configuration I complexes with traditional halogen bonds are stabilized primarily by the dispersion interaction. The electrostatic interaction is the most important for configuration I complexes with chlorine-shared halogen bonds and for configuration II complexes except for FCl:PCNa for which the induction term is most important. The F–Cl stretching frequency is red-shifted upon complexation. EOM-CCSD/(qzp,qz2p) spin–spin coupling constants have been obtained for all FCl:PCX complexes with configuration I. ¹J(F–Cl) decreases upon complexation. ^2XJ(F–P) values are quadratically dependent upon the F–P distance and are very sensitive to halogen-bond type. ^1XJ(Cl–P) tends to increase as the Cl–P distance decreases but then decreases dramatically in the chlorine-transferred complex FCl:PC^– as the Cl–P interaction approaches that of a covalent Cl–P bond. Values of ¹J(F–Cl) for configuration II are reduced relative to configuration I, reflecting the longer F–Cl distances in II compared to those of the neutral complexes of I. Although the F–P and Cl–P distances in configuration II complexes are shorter than these distances in the corresponding configuration I complexes,^2XJ(F–P) and ^1XJ(Cl–P) values are significantly reduced, indicating that coupling through the perturbed C–P π bond is less efficient. The nature of F–P coupling for configuration II is also significantly different, as evidenced by the relative importance of PSO, FC, and SD components.

DOI: 10.1002/mrc.2866

Prototropic tautomerism of 5-aryloxy-1(2)H-tetrazoles

The structure of 5-(2,6-dimethylphenoxy)-1H- and 2H-tetrazoles together with those of 5-(2,6-diisopropyl-phenoxy)-1H and 2H-tetrazoles have been theoretically studied including absolute shieldings and energies. The conclusion of these studies is that a slow tautomerism between 1H- and 2H-tetrazoles cannot explain the experimental observations reported recently in the literature.

DOI: 10.1016/j.cplett.2012.04.034

Variations in the structures and binding energies of binary complexes with HBO

Ab initio MP2/aug’-cc-pVTZ calculations have been carried out to determine the structures and binding energies of binary complexes formed by HBO with a series of small molecules A. Three different types of structures have been identified, which depend on the nature of A. In one structure A:HBO, HBO acts as a weak proton donor. In the second HBO:A, HBO is a relatively strong base. The third type of complex A||HBO has HBO and A in an approximately parallel arrangement. The dipole moment of A influences both the type of complex formed and its binding energy.

DOI: 10.1016/j.cplett.2012.04.039

Homo- and heterochiral dimers (PHFX)2, X = Cl, CN, CH3, NC: To what extent do they differ?

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine if intermolecular P–P distances, Z–P–P angles, binding energies, ³¹P chemical shieldings, or EOM–CCSD spin–spin coupling constants can differentiate between corresponding C₂ (homochiral) and C_i (heterochiral) dimers (PHFX)₂, X = Cl, CN, CH₃, NC. With one exception, C_i isomers have shorter P–P distances than corresponding C₂ isomers. Neither binding energies, Z–P–P angles, chemical shieldings, nor spin–spin coupling constants ^1pJ(P–P) exhibit patterns which distinguish between corresponding C₂ and C_i isomers. ^1pJ(P–P) values correlate linearly with P–P distances, so that experimental values of ^1pJ(P–P) could be used to extract intermolecular P–P distances.

DOI: 10.1016/j.tet.2012.05.056

A theoretical NMR study of the structure of benzynes and some of their carbocyclic and heterocyclic analogs

This work reports the theoretical study of the aromaticity of a series of carbocycles (benzene, cyclohexane, bent and planar cyclooctatetraene) and heterocycles (pyridine, furan, thiophene, pyrrole) and their didehydro forms (arynes and hetarynes). As aromaticity probe Schleyer's NICS were used and represented in two 3D isosurfaces of the electron density. The spatial 3D representation of the NICS is shown to be a powerful tool to visualize the aromaticity (or its absence) of different molecules

DOI: 10.1021/jp2094164

DOI: 10.1021/jp203576j

Ab Initio Study of Ternary Complexes X:(HCNH)+:Z with X, Z = NCH, CNH, FH, ClH, and FCl: Diminutive Cooperative Effects on Structures, Binding Energies, and Spin-Spin Coupling Constants Across Hydrogen Bonds

Ab initio calculations have been performed on a series of complexes in which (HCNH)⁺ is the proton donor and CNH, NCH, FH, ClH, and FCl (molecules X and Z) are the proton acceptors in binary complexes X:HCNH⁺ and HCNH⁺:Z, and ternary complexes X:HCNH⁺:Z. These complexes are stabilized by C–H⁺···A and N–H⁺···A hydrogen bonds, where A is the electron-pair donor atom of molecules X and Z. Binding energies of the ternary complexes are less than the sum of the binding energies of the corresponding binary complexes. In general, as the binding energy of the binary complex increases, the diminutive cooperative effect increases. The structures of these complexes, data from the AIM analyses, and coupling constants ¹J(N–H), ^1hJ(H–A), and ^2hJ(N–A) for the N–H⁺···A hydrogen bonds, and ¹J(C–H), ^1hJ(H–A), and^2hJ(C–A) for the C–H⁺···A hydrogen bonds provide convincing evidence of diminutive cooperative effects in these ternary complexes. In particular, the symmetric N···H⁺···N hydrogen bond in HCNH⁺:NCH looses proton-shared character in the ternary complexes X:HCNH⁺:NCH, while the proton-shared character of the C···H⁺···C hydrogen bond in HNC:HCNH⁺ decreases in the ternary complexes HNC:HCNH⁺:Z and eventually becomes a traditional hydrogen bond as the strength of the HCNH⁺···Z interaction increases.

DOI: 10.1016/j.cplett.2011.07.043

³¹P-³¹P spin-spin coupling constants for pnicogen homodimers

Ab initio calculations have been carried out in a systematic investigation of pnicogen homodimers (PH₂X)₂, for X = F, OH, NC, NH₂, CCH, CN, CH₃, H, and BH₂. Complex binding energies range from 7 to 34 kJ mol⁻¹, which is within the range observed for neutral hydrogen-bonded complexes. One-bond spin–spin coupling constants across the pnicogen interaction ^1pJ(P–P) exhibit a quadratic dependence on the P–P distance, similar to the dependence of ^2hJ(X–Y) on the X–Y distance for complexes with X–H⋯Y hydrogen bonds. Thus, computed values of ^1pJ(P–P) could be used to extract P–P distances from experimentally measured coupling constants.

DOI: 10.1039/C1CP20480G

DOI: 10.1021/jp206801x

Structures, Energies, and Spin-Spin Coupling Constants of Methyl-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes: Four-member B-P-B-P Rings B₂P₂(CH₃)_nH_8-n, with n = 0, 1, 2, 4

An ab initio study has been carried out to determine the structures, relative stabilities, and spin–spin coupling constants of a set of 17 methyl-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B₂P₂(CH₃)_nH_8–n, for n = 0, 1, 2, 4, with four-member B–P–B–P rings. The B–P–B–P rings are puckered in a butterfly conformation, in agreement with experimental data for related molecules. Isomers with the CH₃ group bonded to P are more stable than those with CH₃ bonded to B. If there is only one methyl group or if two methyl groups are bonded to two different P or B atoms, isomers with equatorial bonds are more stable than those with axial bonds. However, when two methyl groups are present, the gem isomers are the most stable for molecules B₂P₂(CH₃)₂H₆ with P–C and B–C bonds, respectively. Transition structures present barriers to the interconversion of two equilibrium structures or to the interchange of axial and equatorial positions in the same isomer. These barriers are very low for the isomer with two methyl groups bonded to B in axial positions for the isomer with four axial bonds and for the isomer with geminal B–C bonds at both B atoms. Coupling constants ¹J(B–P), ¹J(P–C), ¹J(B–C), ²J(P–P), and ³J(P–C) are capable of providing structural information. They are sensitive to the number of methyl groups present and can discriminate between axial, equatorial, and geminal bonds, although not all do this to the same extent. The one-bond coupling constants ¹J(B–P), ¹J(P–C), and ¹J(B–C) are similar in equilibrium and transition structures, but ³J(P–C) and ²J(P–P) are not. These coupling constants and those of the corresponding fluoro-derivatives of the 1,3-diborata-2,4-diphosphoniocyclobutanes demonstrate the great sensitivity of phosphorus coupling to structural and electronic effects.

DOI: 10.1007/s00706-011-0473-y

An experimental NMR and computational study of 4-quinolones and related compounds

We report the synthesis and structural study of eight compounds, either quinolin-4(1H)-ones or quinolines. Tautomerism as well as (E) → (Z) and rotational isomerism were studied both experimentally (¹H and ¹³C NMR) and theoretically [B3LYP/6-311++G(d,p)].

DOI: 10.1016/j.tet.2011.04.067

A ¹³C and ¹⁵N experimental NMR and theoretical study of the structure of linear primary aliphatic amines and ammonium salts: from C₁ to C₁₈

Eighteen aliphatic linear amines, from methylamine to stearylamine, have been experimentally studied by NMR and theoretically calculated at the GIAO/B3LYP/6-311++G(d,p) level. A partial exploration of their conformation has been carried out, mainly to determine the effect on the chemical shifts. In solution and for neutral amines, ¹⁵N chemical shifts indicate a mixture of two conformations. In the solid state (CPMAS NMR) only the subset of solid amines has been studied (from C14 to C18). The ¹⁵N signals of the corresponding ammonium salts in the solid state depend on the counteranions, Cl⁻ and CF₃CO₂⁻, a result that is theoretically proven.

DOI: 10.1016/j.cplett.2011.03.085

Do nitrogen bases form chlorine-shared and ion-pair halogen bonds?

Ab initio calculations have been carried out to investigate the structures, binding energies, and spin–spin coupling constants of complexes with FCl as the Lewis acid and a series of sp, sp², and sp³ hybridized nitrogen bases. These properties indicate that neutral complexes are stabilized by traditional F–Cl…N halogen bonds, although the chlorine-shared character of the bond is greater with sp² and sp³ bases. Thus, these complexes are dramatically different from neutral complexes FCl:CNX with sp hybridized carbon bases, which exhibit traditional, chlorine-shared, and ion-pair halogen bonds.

DOI: 10.1021/jp200535g

Structures, Energies, and Spin-Spin Coupling Constants of Fluoro-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes: Four-Member B-P-B-P Rings B2P2FnH8_n with n = 0, 1, 2, 4

An ab initio study has been carried out to determine the structures, relative stabilities, and spin−spin coupling constants of a set of 15 fluoro-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B₂P₂F_nH_8−n, for n = 0, 1, 2, 4, with four-member B−P−B−P rings. Except for B₂P₂F₄H₄ with four fluorines bonded to two borons, these rings are puckered in a butterfly conformation. For a fixed number of fluorines, the isomers with B−F bonds are significantly more stable than those with P−F bonds. As the number of fluorines increases, the energy difference between the most stable isomer and the other isomers increases. Transition structures which interconvert axial and equatorial positions present relatively small inversion barriers. Coupling constants involving ³¹P, namely, ¹J(B−P), ¹J(P−F), ²J(P−P), ²J(P−F), and³J(P−F) are large and are capable of providing structural information. They are sensitive to the number of fluorines present and can discriminate between axial, equatorial, and geminal B−F and P−F bonds, although not all do this to the same extent. ¹J(B−P) and ²J(P−P) are similar in equilibrium and transition structures. Although transition structures no longer discriminate between axial and equatorial bonds, ¹J(P−F) and ³J(P−F) remain sensitive to the number of fluorine atoms present.

DOI: 10.1515/HC.2010.008

A theoretical multinuclear NMR study of pyrazolylborates

The experimental chemical shifts and coupling constants of five borates of general formula BH_nPz_4–n [from the borohydride to tetrakis(pyrazol-1-yl)borate] anions were compared with calculations carried out at the B3LYP/6-311++G(d,p) level (GIAO for absolute shieldings), in general with satisfying results. The most stable conformations of pyrazolylborate anions are similar to those of neutral pyrazolylmethanes.

DOI: 10.1002/mrc.2586

A theoretical NMR study of ortho and para-substituted benzenes compared with silabenzenes, pyridines and phosphabenzenes

Calculations of absolute shieldings and nuclear-independent chemical shift [NICS(1)] values of 84 compounds (benzenes, silabenzenes, pyridines and phosphabenzenes) allowed to discuss the absolute shieldings in the function of Brown σ_p⁺ substituent constant and to predict 71 new chemical shifts. The NICS(1) analysis of the aromaticity leads to the unexpected result for the silabenzenes that it is almost insensitive to substituent effects. In these set of compounds, it can be concluded that the aromaticity and its sensitivity to substituent effects are orthogonal. 

DOI: 10.1021/jp110295n

Do Traditional, Chlorine-shared, and Ion-pair Halogen Bonds Exist? An ab Initio Investigation of FCl:CNX Complexes

Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to determine the structures, binding energies, and bonding of complexes FCl:CNX, with X = CN, NC, NO₂, F, CF₃, Cl, Br, H, CCF, CCH, CH₃, SiH₃, Li, and Na. Equation-of-motion coupled cluster calculations have also been carried out to determine the coupling constants ¹J(F−Cl), ^1XJ(Cl−C), and ^2XJ(F−C) across these halogen bonds. As the strength of the base is systematically increased, the nature of the halogen bond changes from traditional, to chlorine-shared, to ion-pair. The type of halogen bond present in a complex can be readily determined from its structure, binding energy, AIM bonding analyses, and spin−spin coupling constants. Coupling constants across halogen bonds are compared with corresponding coupling constants across traditional, proton-shared, and ion-pair hydrogen bonds.

DOI: 10.1021/jp1085747

Structural and Electronic Effects on One-Bond Spin?Spin Coupling Constants 1J(B-N), 1J(B-H), and 1J(B-F) for Complexes of Nitrogen Bases with BH3 and Its Fluoro-Substituted Derivatives.

Ab initio equation-of-motion coupled cluster (EOM-CCSD) one-bond spin−spin coupling constants ¹J(B−N), ¹J(B−H), and ¹J(B−F) have been evaluated for complexes X:BH_nF_3−n with X = N₂, NCH, NCLi, H₂CNH, NF₃, and NH₃, for n = 0−3. These complexes can be classified as either covalent or van der Waals complexes, on the basis of their binding energies and B−N distances. ¹J(B−N) for covalent complexes varies significantly from −19 to +9 Hz, whereas ¹J(B−N) is less than 2 Hz for van der Waals complexes. An absolute value of ¹J(B−N) of 3 Hz or greater indicates that the complex is covalently bonded, but a small value of this coupling constant does not necessarily mean that it is a van der Waals complex, in view of the variation among these complexes found for ¹J(B−N) as a function of the B−N distance. Deformation of the boron acid upon complex formation and electron donation by the nitrogen base has opposing effects on both ¹J(B−H) and ¹J(B−F). These effects are relatively small in van der Waals complexes. In covalent complexes, electron donation has the dominant effect on ¹J(B−H), and on ¹J(B−F) in complexes with BH₂F and BHF₂, but acid deformation has the dominant effect on ¹J(B−F) in complexes with BF₃. Values of both ¹J(B−H) and ¹J(B−F) reflect the van der Waals or covalent nature of the B−N bond.

DOI: 10.1016/j.jmr.2010.07.015

¹⁵N-¹⁵N spin-spin coupling constants through intermolecular hydrogen bonds in the solid state

A ^2hJ_NN intermolecular spin–spin coupling constant (SSCC) of 10.2 ± 0.4 Hz has been measured for the powdered tetrachlorogallate salt of pyridinium solvated by pyridine (pyridine–H⁺⋯pyridine cation 3). Density Functional Theory (DFT) calculations at the B3LYP/6-311++G(d, p) level reproduced this value and two others reported in the literature for ^2hJ intermolecular SSCCs, which were measured for complexes in solution.

DOI: 10.1016/j.molstruc.2010.06.022

An experimental and theoretical study of ¹J(¹³C-¹⁴N) coupling constants in nitro-aromatic and nitro-heteroaromatic compounds

Data for 12 nitro derivatives (benzenes, pyrroles, furazans and pyrazoles) are reported, of which some furazans and the three pyrazoles are new. These couplings, in the 9–18 Hz range, were compared with B3LYP/6-311++G(d,p) calculations. Although the agreement is not very good, several interesting consequences can be drawn: the value of the coupling constant is not related to the position in the ring nor to the torsion angle but the dimensionless parameter η. For large η (slow quadrupole relaxation) the triplets are observed while for small η (rapid quadrupole relaxation) a broad triplet or even a broad singlet is observed.

DOI: 10.1007/s11224-010-9608-8

Influence of the basis set on the calculation of the absolute ¹³C shieldings of methyl derivatives (CH₃X with X = CH₃, CN, NH₂, NO₂, OH, F) with special emphasis in the cases of X = Cl, Br, SH, SeH, and PH₂
Within the set of 13 methane derivatives, the best results concerning ¹³C absolute shieldings were obtained using the GIAO/MP2/6-311++G(d,p) or the GIAO/MP2/aug-cc-pVTZ approximations. An important finding is that the deviations found with the GIAO/B3LYP/6-311++G(d,p) approach appears to be related to the atomic weight of the X atom in CH₃X: the higher the atomic weight the larger the deviation.

DOI:10.1080/00268971003692018

Two-, three-, and four-bond N-F spin-spin coupling constants in fluoroazines

Ab initio EOM-CCSD calculations have been performed to investigate 2-, 3- and 4-bond ¹⁵N–¹⁹F coupling constants in mono-, di-, and trifluoroazines. ²J(N–F) values are negative and are dominated by the Fermi-contact (FC) term. Absolute values of ²J(N–F) tend to decrease as the number of N atoms in the ring increases, and may also be influenced by the number and positions of C–F bonds. ³J(N–F) values are positive with three exceptions, are usually dominated by the FC term, and also tend to decrease as the number of N atoms increases. The three molecules which have negative values of ³J(N–F) have dominant negative paramagnetic-spin orbit (PSO) terms, and are structurally similar insofar as they have an intervening C–F bond between the N and the coupled F. ⁴J(N–F) values are negative because the PSO, FC, and spin-dipole (SD) terms are negative, with only one exception. Four molecules have significantly greater values of ⁴J(N–F). These are structurally similar with the coupled N bonded to two other N atoms. The computed EOM-CCSD ⁿJ(N–F) coupling constants are in good agreement with the few experimental values that are available.

DOI:10.1021/jp100649k

Ab Initio EOM-CCSD Investigation of One-Bond C-C, N-C, and N-N Spin-Spin Coupling Constants in Fluoroazines

Ab initio EOM-CCSD calculations were carried out to examine one-bond ¹J (C−C), ¹J(N−C), and ¹J(N−N) spin−spin coupling constants in benzene, pyridine, the diazines, and selected triazines, tetrazines, and pentazine and their fluoro-substituted derivatives. Relative to benzene, ¹J(C−C) decreases in the azines as N atoms are introduced into the ring, but this decrease does not exceed 5 Hz. In the fluoro-substituted derivatives, ¹J(C−C) may increase only slightly if the coupled carbon atoms form C−H bonds, or increase dramatically if either or both of the coupled atoms participate in C−F bonds. The value of ¹J(C−C) also depends on the nature of the bonding of the coupled atoms in the ring. The largest increase is found when both carbons participate in C−F bonds, and both are ortho to N atoms. Relative to pyridine,¹J(N−C) increases as N atoms are introduced into the ring, with the magnitude of the increase depending on the bonding of the coupled atoms. It is negligible if neither atom is bonded to another N, increases if one of the coupled atoms is bonded to another N atom, and increases further if both are bonded to other N atoms. Fluoro-substitution has an opposing effect on¹J(N−C), making this coupling constant less positive or negative when the coupled C participates in a C−F bond. The decrease in ¹J(N−C) relative to the parent molecule is enhanced if either of the coupled atoms is bonded to another N atom or to another C−F group. A further enhancement occurs if both coupled atoms are so bonded, with the largest increases associated with the bonding scheme in which the coupled C is bonded to another N and the coupled N to another C−F. Fluoro-substitution has a small effect on ¹J(N−C) if the coupled C forms a C−H bond, and on ¹J(N−N). Thus, the effects of fluoro-substitution on one-bond couplings tend to be localized.

DOI:10.1016/j.cplett.2010.02.079

An ab initio investigation of the properties of H₂:HX hydrogen-bonded complexes

Eight complexes H₂:HX formed with the σ-bond of the H₂ molecule as the proton acceptor and proton donors HCCH, HCCLi, HCCF, HCN, HNC, H₂O, HF, and HCl have been optimized at MP2/aug-cc-pVTZ. Analyses of the electron densities indicate that these are weakly-bound hydrogen-bonded complexes, in contrast to H₂:HH which is a van der Waals complex. H–H bond stretching frequencies of the H₂ molecule, ¹H chemical shieldings, and indirect spin–spin coupling constants have been computed in order to identify the most promising spectroscopic tool for characterizing these complexes. The H₂ stretching vibration is the property which is most sensitive to complex formation.

DOI:10.1016/j.cplett.2010.02.051

A theoretical study of the stationary structures of the methane surface with special emphasis on NMR properties

The seven stationary points of the methane hypersurface were first explored concerning geometries and energies to check previous data. On these geometries, absolute ¹H and ¹³C NMR shieldings as well as¹J(CH) and ²J(HH) coupling constants were calculated. The results show important variations in the NMR parameters depending on the stationary point considered. Relationships have been found between the ¹H and¹³C shieldings and between these NMR parameters and the relative energy of the different species.

DOI:10.1021/jp1003159

Ab Initio Study of Nonadditivity Effects: Spin-Spin Coupling Constants for Tetrafluoroethene in Ternary pi Complexes

C₂F₄ coupling constants have been evaluated at EOM-CCSD/(qzp,qz2p) in binary complexes with electron donors X (X = HLi, Cl⁻, CN⁻) and with the electron acceptor FH, and in ternary complexes FH:C₂F₄:X in which X and FH are located on opposite faces of the C₂F₄ π cloud. The electron donors X and the electron acceptor FH have opposite effects on ¹J(C−C), ¹J(C−F), ²J(C−F), and ³J(F−F) in binary complexes. Effects of X and FH on a particular coupling constant in a ternary complex are additive if the change in the coupling constant in this complex relative to C₂F₄ is within 1 Hz of the sum of the changes in the corresponding binary complexes. This is the case for ¹J(C−F). Both positive and negative nonadditivities are computed for the remaining coupling constants. Although the values of most coupling constants lie between the values for FH:C₂F₄ and C₂F₄:X, that is not the case for ²J(C−F), and the effect of FH is enhanced by the presence of X. Moreover, values of ³J(F−F) trans and cis for FH:C₂F₄:X when X is Cl⁻ or CN⁻ bonded through C are within 1 Hz of the values for the corresponding binary complex C₂F₄:X. Significant differences can be found between the relative contributions of the PSO, FC, and SD terms to total J and to the nonadditivities of J in ternary complexes FH:C₂F₄:X.

DOI: 10.1021/jp910040w

Probing ¹J(C-F) and ⁿJ(F-F) Spin-Spin Coupling Constants for Fluoroazines: An Ab Initio Theoretical Investigation

Ab initio equation-of-motion coupled cluster singles and doubles calculations have been carried out to evaluate one-bond C−F coupling constants ¹J(C−F) and three-, four-, and five-bond F−F coupling constants ⁿJ(F−F) for a series of mono-, di-, and trifluoroazines. The computed ¹J(C−F) and ⁿJ(F−F) values for these are in good agreement with available experimental coupling constants. The values of ¹J(C−F) vary as the number and positions of N atoms and the number and relative positions of C−F bonds change, but it is difficult to discern general patterns for these changes due to opposing effects of the Fermi contact and paramagnetic spin−orbit terms. The majority of ¹J(C−F) values lie in a range that includes the three monosubstituted pyridines. For trifluoroazines, ¹J(C−F) for a C−F bond that is ortho to two other C−F bonds is greater than ¹J(C−F) for the other two bonds. F−F coupling constants arise in these molecules when the two C−F bonds are ortho, meta, or para. Values of ³J(F−F) are relatively large and negative, whereas values of ⁵J(F−F) are relatively large and positive.⁴J(F−F) may be positive or negative and large or small. The value of this coupling constant depends on the nature of the atom that links the two C−F bonds and the number and positions of N atoms in the ring. The calculations carried out in this study at a reliable level of theory give values for one-bond C−F and n-bond F−F spin−spin coupling constants for the fluoroazines that are not available experimentally. In addition, the patterns that describe the changes that occur in these molecules provide a basis for predicting their values in larger, related systems in the absence of experimental data and direct calculations.

DOI: 10.1016/j.molstruc.2009.11.019

A SOPPA theoretical study of the spin-spin coupling constants of all fluorobenzenes C₆H_nF_6-n (n = 0-5)

The 295 experimental spin–spin coupling constants (SSCC) determined for fluorobenzenes were compared with the calculated ones using the second-order polarization propagator approximation (SOPPA). The agreement is, on the whole, good and the discrepancies were analyzed comparing couplings of the same class of coupling families.

DOI: 10.1002/mrc.2543

Difluorobenzenes revisited: an experimental and theoretical study of spin-spin coupling constants for 1,2-, 1,3-, and 1,4-difluorobenzene

The experimental spin–spin coupling constants (SSCCs) for 1,3- and 1,4-difluorobenzene have been determined anew, and found to be consistent with previously determined values. SSCCs for 1,2-, 1,3-, and 1,4-difluorobenzene have been analyzed by comparing them with the coupling constants computed using the second-order polarization propagator approximation (SOPPA) and the equation-of-motion coupled cluster singles and doubles method (EOM-CCSD). Eighty experimental values have been analyzed using SOPPA calculations, and a subset of 40 values using both SOPPA and EOM-CCSD approaches. One-bond coupling constants ¹J(CC) and ¹J(CF) are better described by EOM-CCSD, whereas one-bond ¹J(CH) values are better described by SOPPA. An empirical equation is presented which allows for the prediction of unknown coupling constants from computed SOPPA values. A similar approach may prove useful for predicting coupling constants in larger systems.

DOI:10.1007/7081_2008_1

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How Aromaticity Affects the Chemical and Physicochemical Properties of Heterocycles: A Computational Approach

Our publications dealing with problems related to aromatic heterocycles are discussed with the appropriate references from the literature. The three main topics are theoretical calculations, tautomerism, and NMR spectroscopy but other aspects are also discussed, such as crystal structures, proton transfer, hydrogen bonds, IR, etc.

DOI:10.1002/mrc.2382

A theoretical structural analysis of the factors that affect ¹JNH, ^1hJNH and ^2hJNN in N-H...N hydrogen-bonded complexes

Calculations of ¹J_NH, ^1hJ_NH and ^2hJ_NN spin–spin coupling constants of 27 complexes presenting N–H·N hydrogen bonds have allowed to analyze these through hydrogen-bond coupling as a function of the hybridization of both nitrogen atoms and the charge (+1, 0, − 1) of the complex. The main conclusions are that the hybridization of N atom of the hydrogen bond donor is much more important than that of the hydrogen bond acceptor. Positive and negative charges (cationic and anionic complexes) exert opposite effects while the effect of the transition states ‘proton-in-the-middle’ is considerable.

DOI: 10.1002/mrc.2483

Molecular complexes between pi-excedent heterocycles (indoles and carbazole) and pi-deficient polynitrobenzenes

Five charge-transfer complexes 1–5 derived from indoles (including a carbazole) and halogenopolynitrobenzenes (ClDNB, FDNB, ClTNB) as well as their individual components have been studied in the solid state by ¹³C CPMAS NMR. The stacking effects on the ¹³C chemical shifts have been rationalized by means of M05-2X functional and GIAO/B3LYP/6-311 ++G(d,p) calculations. The results, although only semiquantitative, are very promising for studying such structures.

DOI: 10.1021/jp902089g

A Systematic Comparison of Second-Order Polarization Propagator Approximation and Equation-of-Motion Coupled Cluster Singles and Doubles C-C, C-N, N-N, C-H, and N-H Spin-Spin Coupling Constants

Ab initio one-, two-, and three-bond C−C, C−N, and N−N spin−spin coupling constants, and one-bond C−H and N−H coupling constants have been computed using two different theoretical methods, SOPPA/(qzp,qz2p) and EOM-CCSD/(qzp,qz2p). Both EOM-CCSD (equation-of-motion coupled cluster singles and doubles) and SOPPA (second-order polarization propagator approximation) coupling constants correlate linearly with experimental data. In the great majority of cases, the computed EOM-CCSD C−C, C−N, N−N, and N−H coupling constants are in better agreement with experimental data than SOPPA values, although both levels of theory provide reasonable estimates of these couplings. EOM-CCSD consistently underestimates one-bond C−H coupling constants by about 10 Hz, and SOPPA values of ¹J(C−H) are in better agreement with experimental data. The performance of SOPPA supports its use in future studies of coupling constants involving C, N, and H in larger chemical and biological systems.

DOI:10.1007/s00214-009-0599-8

A theoretical study of diborenes HLB=BLH for L=CO, NH3, OH2, PH3, SH2, ClH: structures, energies, and spin-spin coupling constants

Ab initio calculations were carried out to investigate the structures, binding energies, bonding, and NMR spin–spin coupling constants of complexes HLB=BLH, for L=CO, NH₃, OH₂, PH₃, SH₂, and ClH. Both B–B and B–H bonds lengthen on complex formation relative to singlet HBBH, and except for L=CO, the B–B bonds are double bonds. The order of stability of the trans isomers correlates with the ordering of ligands in the spectrochemical series of ligand field theory. The trans isomer is always more stable than the corresponding cis. Inverse correlations are found between ¹ J(B–B) and ¹ J(B–H) and the corresponding B–B and B–H distances. For the trans isomers, ¹ J(B–B) appears to be related to the ordering of ligands in the spectrochemical series, while ¹ J(B–H) is related to the protonation energy of the ligand L.

DOI:10.1021/jp9035919

Characterizing Complexes with F-Li...N, H-Li...N, and CH3Li.. Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants

Ab initio calculations have been carried out to determine the structures, binding energies, and spin−spin coupling constants of complexes stabilized by X−Li···N bonds with F−Li, H−Li, and CH₃Li as the Lewis acids. Complexes of these acids with the nitrogen bases N₂, HCN, 1,3,5-triazine, pyrazine, 1,2,3-triazine, pyridine, and NH₃ have linear X−Li···N bonds. Methylamine forms a nonlinear lithium bond only when F−Li is the lithium donor. Two bases, HN═CH₂ and aziridine, form nonlinear X−Li···N bonds with each acid. Except for complexes with N₂, which have small binding energies of about 5 kcal/mol, the binding energies of lithium-bonded complexes are appreciable, varying between 15 and 23 kcal/mol. The one-bond coupling constant ¹J(F−Li) may increase or decrease upon complexation, but ¹J(H−Li) and ¹J(C−Li) decrease significantly. These coupling constants have their smallest values in complexes with nonlinear X−Li···N bonds. No correlations appear to exist between ¹J(X−Li) and the X−Li distance and ^1liJ(Li−N) and the Li−N distance. Values of the two-bond coupling constants ^2liJ(X−N) are extremely small. Comparisons of ^2liJ(F−N) with ^2hJ(F−N) for coupling across a hydrogen bond and ^2xJ(F−N) for coupling across a halogen bond suggest that the extremely small values of ^2liJ(X−N) are not due to long X−N distances but to the low valence electron density on Li in lithium-bonded complexes.

DOI:10.1021/ct900128v

An Ab Initio Study of the Structures and Selected Properties of 1,2-Dihydro-1,2-azaborine and Related Molecules

An ab initio study has been carried out to investigate the effect of replacing [HC−CH]_n linkages in benzene by the isoelectronic [HN−BH]_n linkages for n = 1, 2, and 3. Such replacements give rise to azaborine, a set of diazaborines, borazine, and pseudoborazine. These replacements lead to significant rearrangements of electron densities in these molecules due primarily to the introduction of the polar B−N bond. As a result, azaborine and diazaborines exhibit much more localized structures than that of benzene. They are also less aromatic than benzene but have a higher degree of aromaticity than borazine. The bonding patterns can be related to the relative stabilities of the diazaborines. Among these molecules, the most stable isomer contains an N−B−N−B linkage, while the two least stable isomers have either a B−B or a N−N bond. Changes in bonding patterns are also reflected in changes in the N1−B2 coupling constant. When N1 and B2 are bonded to the less electronegative atoms C and B, ¹J(N1−B2) increases relative to borazine, but when either N1 or B2 is bonded to N, ¹J(N1−B2) decreases. Computed NMR chemical shifts and coupling constants are in good agreement with available experimental data.

DOI:10.1021/jp9020917

Characterizing Complexes with F-Li+-F Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants

Ab initio MP2/aug-cc-pVTZ calculations have been performed to determine the structures and binding energies of complexes with F−Li⁺−F bonds formed from the fluorine bases LiF, CH₃F, HF, ClF, and FF. There is only a single minimum across the Li⁺ transfer coordinate, and in each series, the lithiated homodimer is stabilized by a symmetric F···Li⁺···F bond. Complexes having LiF, CH₃F, and HF as the base have similar structures, with linear F−Li⁺−F bonds and a head-to-tail alignment of the F−Li⁺ bond dipole with the dipole moment vector of the base. In each series with a given acid, the binding energy decreases as the difference between the lithium ion affinities increases. EOM-CCSD coupling constants ¹J(F−Li), ^1liJ(Li−F), and ^2liJ(F−F) have also been evaluated. In complexes with essentially linear bonds, ^2liJ(F−F) values are small and positive and increase quadratically as the F−F distance decreases. ^1liJ(Li−F) and¹J(F−Li) also vary systematically with distance. Comparisons are made between structural, energetic, and coupling constant properties of these complexes and corresponding complexes stabilized by F−H⁺−F hydrogen bonds.

DOI:10.1021/jp902983r

The Effect of Perfluorination on the Aromaticity of Benzene and Heterocyclic Six-Membered Rings

Despite having six highly electronegative F’s, perfluorobenzene C₆F₆ is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C₆F₆and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the F’s induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C₆F_nH_(6−n), n = 1−6) have similar ring-LMO-NICS_πzz values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C₅H₅Y heterocycles (Y = BH⁻, N, NH⁺) are as aromatic as benzene, based on their ring-LMO-NICS_πzzand ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O⁺) are involved.

DOI:10.1002/mrc.2433

A DFT and AIM analysis of the spin-spin couplings across the hydrogen bond in the 2-fluorobenzamide and related compounds

In 1975 a large number of coupling constants were measured in 2-fluorobenzamide labeled with ¹⁵N. Some of them were assigned to couplings through intramolecular NH···F hydrogen bonds (HBs). These couplings change dramatically when CDCl₃ is replaced by DMSO-d₆. In this theoretical paper we provide density functional theory (DFT) calculations that justify the existence of a weak HB in the absence of solvent, while solvents that act as HB acceptors break down the intramolecular hydrogen bond (IMHB) of 2-fluorobenzamide. Atoms in molecules (AIM) analyses and Steiner-Limbach plots were used to analyze the structure of the compounds.

DOI:10.1016/j.molstruc.2009.03.028

A theoretical and NMR experimental study of N1,N3-di(3-aminoacridin-6-yl)-isophthalamide and N2,N6-di(3-aminoacridin-6-yl)-2,6-dicarboxamide

Two bis-flavine derivatives linked by a 1,3-diamidobenzene and a 1,3-diamidopyridine have been prepared and their conformation determined by a combination of NMR experiments (¹H and ¹³C) and DFT calculations including GIAO absolute shieldings. In the case of the pyridyl derivative, the conformation of minimum energy is stabilized by intramolecular hydrogen bonds.

DOI:10.1002/mrc.2414

The behavior of Gliclazide in solution and in the solid state: a case of organic compound presenting a solid-solution structure

The structure of the hypoglycemic agent Gliclazide has been studied by ¹H, ¹³C, and ¹⁵N NMR in solution (CDCl₃ and DMSO-d₆) and in the solid state. In the solid state, the compound crystallizes as an EZ isomer without dynamic properties. In CDCl₃ solution, the structure is still EZ but with a slow nitrogen inversion about the pyrrolidine nitrogen: two invertomers have been observed and characterized. In DMSO-d₆, the rate is faster and only averaged signals were observed. GIAO calculated absolute shieldings were used to confirm the nature of the observed species. In the solid state, Gliclazide presents the phenomenon of solid-solution with two disordered conformations present in the crystal at a 90:10 ratio. 

DOI: 10.1021/ct800321b

Systematic Comparison of Second-Order Polarization Propagator Approximation (SOPPA) and Equation-of-Motion Coupled Cluster Singles and Doubles (EOM-CCSD) Spin-Spin Coupling Constants for Molecules with C, N, and O Double and Triple Bonds and Selected F-Substituted Derivatives 

Ab initio EOM-CCSD and SOPPA calculations with the Ahlrichs (qzp,qz2p) basis set have been carried out to evaluate one-, two-, and three-bond spin−spin coupling constants for molecules H_mX=YH_n and H_mX­YH_n for X, Y = ¹³C, ¹⁵N, and ¹⁷O, and selected ¹⁹F-substituted derivatives. In the great majority of cases, EOM-CCSD one-bond C−C, C−N, C−O, C−F, N−N, N−O, and N−F coupling constants and three-bond F−F coupling constants are smaller in absolute value than the corresponding SOPPA coupling constants, with the EOM-CCSD values in better agreement with experimental data. SOPPA tends to significantly overestimate the absolute values of large one- and three-bond couplings involving fluorine. The majority of two-bond SOPPA coupling constants are in better agreement with experiment than EOM-CCSD, although differences between EOM-CCSD and experimental values are not dramatic. A statistical analysis of thirty EOM-CCSD and SOPPA coupling constants versus experimental coupling constants demonstrates that better agreement with experiment is found when EOM-CCSD is the computational method.

DOI: 10.1021/ct8002699

Structures, Bonding, and One-Bond B-N and B-H Spin-Spin Coupling Constants for a Series of Neutral and Anionic Five-Membered Rings Containing BN Bonds

The structures and bonding of a series of five-membered rings with BN bonds C_xN_yB_zH₅ (x + y + z = 5) and their most stable deprotonated anions C_xN_yB_zH₄⁻ as well as anionic rings C_xN_yB_zH₅⁻ have been investigated at the MP2/6−311++G(d,p) level of theory. The great majority of these rings present BN bond orders close to that found in borazine, suggesting that there is substantial electron delocalization in these rings. This observation is also supported by both NBO and ELF analyses. Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have also been performed to obtain the ¹⁵N−¹¹B and ¹H−¹¹B spin−spin coupling constants. For neutral systems, the former range from −10 to −35 Hz, thereby bracketing the value of ¹J(B−N) for borazine, which is −29 Hz. ¹J(B−N) spans an even greater range in the anions, from −3 to −36 Hz. The absolute value of ¹J(B−N) decreases upon deprotonation if coupling involves the deprotonated nitrogen or a boron atom bonded to the deprotonated N. ¹J(B−H) always decreases upon nitrogen deprotonation.

DOI: 10.1021/jo801362q

Structure of N,N'-Bis(amino acids) in the Solid State and in Solution. A ¹³C and ¹⁵N CPMAS NMR Study

Three bis(amino acids) linked by the amino groups have been prepared and structurally characterized. We have named them Gly-Gly, Ala-Ala and Gly-Ala (or Ala-Gly). These compounds have been characterized by NMR both in solution and in the solid state. They exist as zwitterions with the ammonium group proximal to the carboxylate anion. In the case of Gly-Ala, a dynamic situation is observed by CPMAS NMR (¹³C and ¹⁵N) corresponding to a double proton migration between two proximal tautomers.

DOI: 10.1007/s11224-008-9365-0

A computational study of the effect of C-lithiation on the NMR properties (chemical shifts and coupling constants) of aziridines

A DFT (B3LYP/6-311++G(d,p) study of a series of N-H, N-methyl and N-propyl aziridines and their C-lithium derivatives has been carried out in order to explore their configurational as well as their NMR properties (¹H and ¹³C). The results agree fairly well with experimental observations [Org Lett 9:1263, 2007 and J Org Chem 73, 2008 (73:3197)] and reveal the existence of lithium-N(lone pair) and lithium C(aromatic) interactions.

DOI: 10.1002/mrc.2304

Resolving an apparent discrepancy between theory and experiment: spin-spin coupling constants for FCCF

Ab initio equation of motion coupled cluster singles and doubles (EOM–CCSD) and second-order polarization propagator approximation (SOPPA) calculations have been performed to evaluate spin–spin coupling constants for FCCF (difluoroethyne). The computed EOM-CCSD value of ³J(FF) obtained at the experimental geometry of this molecule supports the previously reported experimental value of 2.1 Hz, thereby resolving an apparent discrepancy between theory and experiment. This coupling constant exhibits a strong dependence on the CC and CF distances, and its small positive value results from a sensitive balance of paramagnetic spin-orbit (PSO) and spin-dipole (SD) terms. The three other unique FCCF coupling constants ¹J(CC), ¹J(CF), and ²J(CF) have also been reported and compared with experimental data. While ¹J(CF) is in agreement with experiment, the computed value of ²J(CF) is larger than our estimate of the experimental coupling constant. 

DOI: 10.1016/j.ssnmr.2007.12.002

A study in desmotropy

A solid-state NMR/theoretical study of the tautomerism of 4-substituted pyrazolinones was carried out in search of new desmotropes. Thus, the systematic exploration by ¹³C and ¹⁵N CPMAS NMR of a series of 4-acylpyrazolin-5-ones has allowed to determine their structure in the solid state and to find a new example of desmotropy that of 1-phenyl-4-acetyl-pyrazolinone.

DOI: 10.1016/j.ssnmr.2008.03.003

Cation dinitrogen complexes [N2...X...N2]+, X= H+, Li+, Na+, Be2+, Mg2+

The complexes of dinitrogen with five cations (H⁺, Li⁺, Na⁺, Be²⁺ and Mg²⁺) up to four N₂ molecules have been calculated at the MP2/6-311++G(d,p) level. Energetic and geometric aspects have been determined together with absolute shieldings (GIAO). The atoms in molecules methodology has been used to analyze energy, charge and volume of these complexes.

DOI: 10.1021/jp804119r

Spin-Spin Coupling across Intermolecular F-Cl...N Halogen Bonds

Ab initio EOM-CCSD calculations have been performed to determine one- and two-bond spin−spin coupling constants ¹J(F−Cl), ^1XJ(Cl−N), and ^2XJ(F−N) across F−Cl···N halogen bonds in complexes with F−Cl as the Lewis acid and N₂, FCN, HCN, (CH₃)CN, LiCN, Z-HNNH, H₂CNH, NH₂F, NH₃, cyclic-NH(CH₂)₂, and NH₂(CH₃) as Lewis bases. The structures of these complexes were optimized at MP2 with the aug′-cc-pVTZ basis set. The absolute value of ^2XJ(F−N) increases in these complexes as the F−N distance decreases, a behavior similar to that of^2hJ(F−N) for complexes stabilized by F−H···N hydrogen bonds. ^1XJ(Cl−N) also tends to increase in absolute value with decreasing F−N distance. ¹J(F−Cl) is always positive, decreases upon complex formation as the F−Cl distance increases, and appears to be sensitive to the hybridization of the nitrogen base. The relatively large differences in the values of these coupling constants in the various complexes and their variation along the chlorine-transfer coordinate for F−Cl···NH₃ suggest that they should be amenable to experimental investigation.

DOI: 10.1002/mrc.2272

Experimental measurements and theoretical calculations of the chemical shifts and coupling constants of three azines (benzalazine, acetophenoneazine and cinnamaldazine)

Three azines, two of them doubly labeled with ¹⁵N, have been studied by multinuclear magnetic resonance in solution and in the solid state. The spectral parameters obtained by iterative analyses have been compared with DFT/B3LYP calculated values (absolute shieldings and coupling constants). The agreement is generally good. Some anomalies have been discussed in relation to the structure of these compounds. 

Doi: 10.1021/jp801519v

Ab Initio EOM-CCSD Spin-Spin Coupling Constants for Hydrogen-Bonded Formamide Complexes: Bridging Complexes with NH3, (NH3)2, H2O, (H2O)2, FH, and (FH)2

EOM-CCSD spin−spin coupling constants across hydrogen bonds have been computed for complexes in which NH₃, H₂O, and FH molecules and their hydrogen-bonded dimers form bridging complexes in the amide region of formamide. The formamide one-bond N−H coupling constant [¹J(N−H)] across N−H···X hydrogen bonds increases in absolute value upon complexation. The signs of the one-bond coupling constants ^1hJ(H−X) indicate that these complexes are stabilized by traditional hydrogen bonds. The two-bond coupling constants for hydrogen bonds with N−H as the donor [^2hJ(N−X)] and the carbonyl oxygen as the acceptor [^2hJ(X−O)] increase in absolute value in the formamide/dimer relative to the corresponding formamide/monomer complex as the hydrogen bonds acquire increased proton-shared character. The largest changes in coupling constants are found for complexes of formamide with FH and (FH)₂, suggesting that bridging FH monomers and dimers in particular could be useful NMR spectroscopic probes of amide hydrogen bonding.

Doi: 10.1002/mrc.2209

A review with comprehensive data on experimental indirect scalar NMR spin-spin coupling constants across hydrogen bonds

Scalar NMR spin–spin coupling constants across hydrogen bonds are fundamental in structural studies and as test grounds for theoretical calculations. Since they are scattered among many articles of different kinds, it seems useful to collect them in the most comprehensive way. 

Doi: 10.1002/mrc.2199

Spin-spin coupling across intramolecular N...H+...N hydrogen bonds in models for proton sponges: an ab initio investigation

Ab initio calculations have been performed to obtain structures and coupling constants ¹J(NH), ^1hJ(HN), and ^2hJ(NN) for models of proton sponges with symmetric and asymmetric NH⁺N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy, a longer NN distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of ^2hJ(NN) for the models are significantly less than predicted values based on the distance dependence of ^2hJ(NN) for complexes with intermolecular NH⁺N hydrogen bonds. However, the reduced values of ^2hJ(NN) cannot be attributed solely to the distortion of the hydrogen bond in the models, but also reflect differences in s electron populations at the nitrogens in both the ground state and the excited states which couple to it through the Fermi-contact (FC) operator. Values of ^2hJ(NN) for IMHBs can be related quadratically to the NN distances in the models, and demonstrate that there is no discrepancy between computed values of^2hJ(NN) at the short NN distances found in these systems and experimental data for proton sponges.

Doi: 10.1021/ct800111j

A systematic comparison of second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants for selected singly bonded molecules, and the hydrides NH3, H2O, and HF and their protonated and deprotonated ions and hydrogen-bonded complexes

Second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singles and doubles (EOM−CCSD) methods have been employed for the calculation of one-bond spin−spin coupling constants in series of small molecules and ions, and of one- and two-bond coupling constants across X−H···Y hydrogen bonds. For isolated molecules, one-bond SOPPA coupling constants ¹J(X-Y) involving ¹³C, ¹⁵N, ¹⁷O, and ¹⁹F have larger absolute values than corresponding EOM−CCSD coupling constants, with the EOM−CCSD values being in significantly better agreement with available experimental data. The difference between SOPPA and EOM−CCSD tends to increase as the number of nonbonding electrons on the coupled atoms increases, and the SOPPA values for O−F coupling are significantly in error. Similarly, the absolute values of SOPPA one-bond coupling constants¹J(X−H) for the hydrides NH₃, H₂O, and FH and their protonated and deprotonated ions are greater than EOM−CCSD values, with the largest differences occurring for F−H coupling. One- and two-bond coupling constants ¹J(X-H), ^1hJ(H−Y), and ^2hJ(X−Y) across X−H···Y hydrogen bonds in neutral, protonated, and deprotonated complexes formed from the hydrides are similar at SOPPA and EOM−CCSD, with the largest differences again found for ¹J(F−H) in complexes with F−H as the proton donor, and ^2hJ(F−F) for (FHF)⁻. The signs of ¹J(X−H), ^1hJ(H−Y), and ^2hJ(X−Y) are the same at both levels of theory, as is their variation across the proton-transfer coordinate in F−H···NH₃. SOPPA would appear to provide a reliable and more cost-effective alternative approach for computing coupling constants across hydrogen bonds, although couplings involving F may be problematic.
.

Doi: 10.1002/mrc.2181

A computational study of 2JHH(gem) indirect spin-spin coupling constants in simple hydrides of the second and third periods

Several theoretical methods have been used to compute ²J_HH in neutral, anionic and cationic HXH hydrides, X being the 14 nuclei from Li to Cl (28 molecules). Since the calculations also provide ¹J_XH spin–spin coupling constants (SSCC), these have also been analyzed. The best results were obtained using Second-order polarization propagator approximation (SOPPA)/sadJ. The geminal coupling constants appear to be dependent on the electronegativity of the X-atom.

DOI: 10.1021/jp073519r

HCP and H3C-CP as Proton Acceptors in Protonated Complexes Containing Two Phosphorus Bases: Structures, Binding Energies, and Spin-Spin Coupling Constants

Ab initio calculations at the MP2/aug'-cc-pVTZ level have been carried out to investigate the structures and binding energies of cationic complexes involving protonated sp, sp², and sp³phosphorus bases as proton donor ions and the sp-hybridized phosphorus bases H−C≡P and H₃C−C≡P as proton acceptors. These proton-bound complexes exhibit a variety of structural motifs, but all are stabilized by interactions that occur through the π cloud of the acceptor base. The binding energies of these complexes range from 6 to 15 kcal/mol. Corresponding complexes with H₃C−C≡P as the proton acceptor are more stable than those with H−C≡P as the acceptor, a reflection of the greater basicity of H₃C−C≡P. In most complexes with sp²- or sp³-hybridized P−H donor ions, the P−H bond lengthens and the P−H stretching frequency is red-shifted relative to the corresponding monomers. Complex formation also leads to a lengthening of the C≡P bond and a red shift of the C≡P stretching vibration. The two-bond coupling constants ^2πhJ(P−P) and ^2πhJ(P−C) are significantly smaller than ^2hJ(P−P) and ^2hJ(P−C) for complexes in which hydrogen bonding occurs through lone pairs of electrons on P or C. This reflects the absence of significant s electron density in the hydrogen-bonding regions of these π complexes.

DOI: 10.1007/s11224-007-9208-4

The use of chemical shifts vs. coupling constants for studying tautomerism: a combined experimental and theoretical approach

When observing average NMR signals originated from a rapid equilibrium, the procedure to estimate the composition of the mixture is to use interpolation. To illustrate the difficulties of this approach, the much-studied case of the NH and OH tautomers of pyrazolinones will be reexamined. Calculated absolute shieldings and coupling constants were compared with experimental data. Although the large predominance of the OH tautomer in DMSO was confirmed, the result is a little disappointing because no consistency in the percentages was achieved using chemical shifts and coupling constants.

DOI: 10.1021/jp068695z

Probing P-H+-P Hydrogen Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants

Ab initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of 22 open and 3 cyclic complexes formed from the sp² [H₂C=PH and HP=PH (cis and trans)] and sp³ [PH₂(CH₃) and PH₃] hybridized phosphorus bases and their corresponding protonated ions. EOM-CCSD calculations have been carried out to obtain ³¹P−³¹P and ³¹P−¹H coupling constants across P−H⁺−P hydrogen bonds. Two equilibrium structures with essentially linear hydrogen bonds have been found along the proton-transfer coordinate, except for complexes with P(CH₃)H₃⁺ as the proton donor to the sp² bases. Although the isomer having the conjugate acid of the stronger base as the proton donor lies lower on the potential energy surface, it has a smaller binding energy relative to the corresponding isolated monomers than the isomer with the conjugate acid of the weaker base as the donor. The hydrogen bond of the latter has increased proton-shared character. All of the complexes are stabilized by traditional hydrogen bonds, as indicated by positive values of the reduced coupling constants ^2hK_P-P and ¹K_P-H, and negative values of ^1hK_H-P. ^2hJ_P-Pcorrelates with the P−P distance, a correlation determined primarily by the nature of the proton donor. For open complexes, ¹J_P-H always increases relative to the isolated monomer, while^1hJ_H-P is relatively small and negative. ^2hJ_P-P values are quite large in open complexes, but are much smaller in cyclic complexes in which the P−H⁺−P hydrogen bonds are nonlinear. Thus, experimental measurements of ^2hJ_P-P should be able to differentiate between open and cyclic complexes.

DOI: 10.1021/jp073112m

Theoretical study of complexes and fluoride cation transfer between N2F+ and electron donors

A theoretical study of the complexes formed by the N₂F cation (fluorodiazonium ion) and a series of small molecules containing nitrogen atoms have been carried out at the MP2 computational level. In addition, fluorine transfer has been studied. The electron density, NMR shielding and indirect coupling constants of the complexes have been evaluated. The covalent or halogen bonding characteristics of the N···F interactions observed in the complexes are defined by the interatomic distance. It has been determined that the limiting value is 1.6 Å.

DOI: 10.1002/mrc.2053

Statistical analysis of 13C and 15N NMR chemical shifts from GIAO/B3LYP/6-311++G** calculated absolute shieldings

The ¹³C and ¹⁵N absolute shieldings of 28 compounds have been calculated at the GIAO/B3LYP/6-311 + + G** level to complete a collection of data already published. This has allowed us to devise new equations relating δ and σ for these nuclei based on 461 points (¹³C) and 70(72) points (¹⁵N). 

DOI: 10.1002/mrc.1992

Pyrazolo[1,5-a]pyrimidines. A combined multinuclear magnetic resonance (¹H, ¹³C, ¹⁵N, ¹⁹F) and DFT approach to their structural assignment.

Multinuclear magnetic resonance spectroscopy together with GIAO-DFT calculations allowed establishment of the structure of the products obtained by condensation of 3(5)-amino-4-phenyl-1H-pyrazole and β-dicarbonyl compounds bearing a trifluoromethyl group. They are 3-phenyl-5-(R)-7-trifluoromethylpyrazolo[1,5-a]pyrimidines. 

DOI: 10.1016/j.molstruc.2006.10.015

An experimental and DFT analysis of coupling constants in [³¹P(CH₃)_nH_(4−n)]⁺ systems where n = 0–4 with a note on [¹⁴N(C₂H₅)₄]⁺ and [³¹P(C₂H₅)₄]⁺

The experimentally determined coupling constants of phosphonium cations of general formula [P(CH₃)_nH_(4−n)]⁺ where n = 0–4 have been gathered and those corresponding to  measured again. They have been compared with the coupling constants computed at the B3LYP/6-311++G(d,p)//B3LYP//6-311++G(d,p) level. The agreement is highly satisfactory save for ¹J_PC and for ¹J_PH. The last problem is probably related to specific solvation through hydrogen bonds. The cases of and  were also examined to provide a basis for the fact that β protons show a large coupling constant with ¹⁴N than α protons.

DOI: 10.1021/ct600351p

Spin-Spin Coupling Constants for Iminoboranes RBNH, HBNR, and RBNR and Comparisons with Corresponding Isoelectronic Acetylenes RCCH and RCCR, for R = H, CH3, NH2, OH, and F

Ab initio equation-of-motion coupled-cluster singles and doubles method calculations have been performed on iminoboranes RBNH, HBNR, and RBNR, for R = H, CH₃, NH₂, OH, and F, to evaluate substituent effects on one- and two-bond ¹⁵N−¹¹B, ¹¹B−¹H, and ¹⁵N−¹H spin−spin coupling constants. For comparison purposes, ¹³C−¹³C coupling constants were evaluated for corresponding isoelectronic molecules RCCH and RCCR. The absolute values of ¹J(N−B) and¹J(C−C) increase in the series HBNR and RCCH as the σ-electron-withdrawing ability of R increases. In contrast, NH₂ substitution at B leads to a decrease in the absolute value of ¹J(N−B), but OH and F substitution increase ¹J(N−B). Disubstitution has dramatically different effects on coupling constants for RCCR and RBNR. In the former, ¹J(C−C) more than doubles relative to the corresponding RCCH and HCCH; in the latter, disubstitution of OH and F decreases ¹J(N−B) relative to the corresponding monosubstituted derivatives, while NH₂substitution increases ¹J(N−B). Changes in one- and two-bond B−H and N−H coupling constants upon substitution are similar to changes observed for ¹J(N−B) in the corresponding monosubstituted derivatives RBNH and HBNR.

DOI: 10.1021/jp067823g

Attacking Boron Nucleophiles: NMR Properties of Five-Membered Diazaborole Rings

This paper reports computed NMR spectral data for the diazaborole anion (C₂H₄B₁N₂^-) and the corresponding neutral five-membered rings with B−H (C₂H₅B₁N₂, diazaborole) and B−Li (C₂H₄B₁Li₁N₂, Li−diazaborole) bonds, which are the central moieties of newly synthesized nucleophilic organoboryl five-membered rings, recently reported by Segawa et al. (Science2006, 314, 113). Both spin−spin coupling constants and chemical shifts were obtained using high-level ab initio calculations. These data are a necessary complement to the very scarce experimental information available.

DOI: 10.1021/jp0628737

An ab Initio Study of 15N-11B Spin-Spin Coupling Constants for Borazine and Selected Derivatives

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to investigate substituent effects on coupling constants for borazine and selected substituted borazines. For molecules in which F atoms are not bonded to adjacent atoms in the ring, F substitution increases the one-bond ¹¹B−¹⁵N coupling constants involving the atom at which substitution occurs but leaves the remaining one-bond B−N coupling constants essentially unchanged. For these molecules, the magnitudes of one-bond B−N coupling constants are only slightly dependent on the number of F atoms present. Fluorine substitution at adjacent B and N atoms in the borazine ring further increases the one-bond B−N coupling constant involving the substituted atoms and has the same effect on the other one-bond coupling constants as observed for corresponding molecules in which substitution occurs at alternate sites. In contrast to the effect of F substitution, substitution of Li at either N or B decreases one-bond B−N coupling constants relative to borazine. The effects of F and Li substitution on one-bond B−N coupling constants for borazine are similar to F and Li substitution effects on ¹³C−¹³C coupling constants for benzene.

Doi: 10.1002/mrc.1840

Computed coupling constants in X(CH3)nH(4-n) moieties where X = 13C and 15N+, and n = 0-4: comparisons with experimental data

Seventy-three unique spin–spin coupling constants have been analyzed for the ten species in the two series X(CH₃)_nH_4–n, where the central atom X is ¹³C or ¹⁵N⁺. Thirty-seven experimental values have been obtained from the literature, and several new coupling constants have been measured for the methyl-substituted ammonium ions. Both DFT with the B3LYP functional and ab initio EOM-CCSD calculations have been carried out on these same systems. Coupling constants computed by these two methods are in agreement with experimental values. Some problems related to coupling constants for the cationic ammonium systems have been resolved when these were recomputed at EOM-CCSD for complexes in which NH₄⁺ is hydrogen-bonded to H₂O molecules.

Doi: 10.1002/mrc.1775

A 1H, 13C and 15N NMR study in solution and in the solid state of six N-substituted pyrazoles and indazoles.

Three N-substituted pyrazoles and three N-substituted indazoles [1-(4-nitrophenyl)-3,5-dimethylpyrazole (1), 1-(2,4-dinitrophenyl)-3,5-dimethylpyrazole (2), 1-tosyl-pyrazole (3), 1-p-chlorobenzoylindazole (4), 1-tosylinda-zole (5) and 2-(2-hydroxy-2-phenylethyl)-indazole (6)] have been studied by NMR spectroscopy in solution (¹H, ¹³C, ¹⁵N) and in the solid state (¹³C, ¹⁵N). The chemical shifts have been compared with GIAO/DFT calculated absolute shieldings. Some discrepancies have been analyzed.

DOI: 10.1002/hlca.200590148

Solid-State NMR Study of the Tautomerism of Acetylacetone Included in a Host Matrix.

The tautomerism of the enol form of acetylacetone (=pentane-2,4-dione; 1) inside a host cavity has been studied by means of solid-state¹³C-NMR spectroscopy (SSNMR) using the variable-temperature CPMAS technique. It appears that the enol form, 4-hydroxypent-3-en-2-one (1a), exists in an equilibrium with an identical tautomer (1c) trough OH ⋅⋅⋅O proton transfer. The experimental results (energy barrier and chemical shifts) were rationalized by means of MP2 and GIAO calculations.

Doi: 10.1002/mrc.1671

A theoretical study of multinuclear coupling constants in pyrazoles

The 243 coupling constants of eight N-R-pyrazoles [R = H, CH₃, C₆H₅, COCH₃, NH₂, NO₂, SO₂CF₃, Si(CH₃)₃] have been calculated and compared with 131 experimental values. The agreement is good and can be used to estimate new couplings. The whole collection has been statistically analyzed.

DOI: 10.1016/j.cplett.2005.06.104

A theoretical investigation of N–H⋯O═P hydrogen bonds through¹⁵N–³¹P and ¹H–³¹P coupling constants

The Fermi contact terms corresponding to ^3hJ_NP and ^2hJ_HP for complexes involving N–H⋯O–P hydrogen bonds have been calculated at the EOM-CCSD level of theory. The N–H donors in these complexes are urea or two hydrogen isocyanide molecules, and the proton acceptor is . Experimentally measurable^3hJ_NP values are predicted only for linear or nearly linear N–H⋯O═P hydrogen bonds. Computed ^2hJ_HPvalues appear to be too small to detect the presence of these hydrogen bonds.

DOI: 10.1016/j.cplett.2005.06.061

Are RAHBs "resonance assisted"? A theoretical NMR study

The concept of resonance-assisted hydrogen bonds (RAHBs) is one of the most frequently used concepts in structural chemistry. Computed equation-of-motion coupled cluster singles and doubles (EOM–CCSD) O–O and N–N coupling constants through intramolecular X–H–X hydrogen bonds (^2hJ_X–X) and MP2 ¹H chemical shifts of the X–H–X protons have been used to investigate RAHBs in model saturated and unsaturated systems. The computed results suggest that the NMR properties of these molecules do not receive significant contributions from resonance, but are a consequence of the σ-skeleton framework.

DOI: 10.1021/jp051600t

Interaction Energies and NMR Indirect Nuclear Spin-Spin Coupling Constants in Linear HCN and HNC Complexes

The cooperativity effects on both the electronic energy and NMR indirect nuclear spin−spin coupling constants J of the linear complexes (HCN)_n and (HNC)_n (n = 1−6) are discussed. The geometries of the complexes were optimized at the MP2 level by using the cc-pVTZ basis sets. The spin−spin coupling constants were calculated at the level of the second-order polarization propagator approximation with use of the local dense basis set scheme based on the cc-pVTZ-J basis sets. We find strong correlations in the patterns of different properties such as interaction energy, hydrogen bond distances, and spin−spin coupling constants for both series of compounds. The intramolecular spin−spin couplings are with two exceptions dominated by the Fermi contact (FC) mechanism, while the FC term is the only nonvanishing contribution for the intermolecular couplings. The latter do not follow the Dirac vector model and are important only between nearest neighbors.

DOI: 10.1021/jp0406666

Ab Initio Study of the Influence of Trimer Formation on One- and Two-Bond Spin-Spin Coupling Constants Across an X-H-Y Hydrogen Bond: AH:XH:YH₃ Complexes for A, X = ¹⁹F, ³⁵Cl and Y = ¹⁵N,³¹P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond (¹J_X-H and ^1hJ_H-Y) and two-bond (^2hJ_X-Y) spin−spin coupling constants in AH:XH:YH₃complexes, where A and X are ¹⁹F and ³⁵Cl and Y is either ¹⁵N or ³¹P. The changes in both one- and two-bond spin−spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X−H−Y hydrogen bond. The proton-shared character of the X−H−Y hydrogen bond increases in the order XH:YH₃ < ClH:XH:YH₃ < FH:XH:YH₃. This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X−Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X−H and X−Y spin−spin coupling constants (¹K_X-H and ^2hK_X-Y) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. ^1hK_H-Y is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH₃ and FH:FH:NH₃. In these two complexes, the F−H−N hydrogen bond has sufficient proton-shared character to induce a change of sign in ^1hK_H-Y. The effects of trimer formation on spin−spin coupling constants are markedly greater in complexes in which NH₃ rather than PH₃ is the proton acceptor.

DOI: 10.1002/mrc.1569

Theoretical study of 31P, 31P coupling constants in Cyclotriphosphazenes

Several scalar coupling constants (mainly ³¹P, ³¹P) were calculated for 10 cyclotriphosphazenes and compared with experimental results when available. Although the experimental values cannot be reproduced, the calculated values are proportional to the experimental values. Some difficult cases, such as ¹⁹F, ¹⁹F couplings, are discussed. 

DOI: 10.1002/mrc.1409

The tautomerism of Omeprazole in solution: a ¹H and ¹³C NMR study

The tautomerism of 5(6)-methoxy-2-{[(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl] sulfinyl}-1H-benzimidazole (omeprazole) was determined in solution, K_T = 0.59 in THF at 195 K, in favor of the 6-methoxy tautomer. The assignment of the signals was made by comparison with its two N-methyl derivatives in acetone-d₆ and through theoretical calculations of the absolute shieldings (GIAO/DFT/6-311++G**).

DOI: 10.1016/j.theochem.2004.04.030

Theoretical study of peptide model dimers. Homo versus heterochiral complexes

The study of possible chiral recognition of a series of peptide models (For-Gly-NH₂, For-Ala-NH₂ and four of their fluoro substituted derivatives) has been carried out by means of DFT calculations. Homo (L,L) and heterochiral (L,D) dimers formed by hydrogen bond (HB) complexation have been considered. Initially, the conformational preferences of the monomers have been calculated and used to generate all the possible homo and heterochiral dimers. The energetic results show that in most cases, the β monomers are the most stable while in the dimers, the γ–γ complexes show the strongest interaction energies. In three of the four chiral cases studied, a heterochiral dimer is the most stable one. In addition, the electron density and nuclear shielding of the complexes have been studied.

DOI:10.1016/j.molstruc.2003.10.041

The structure of 1-formyl-3-phenyl-D2-pyrazoline in the gas phase (DFT calculations), in solution (NMR) and in the solid state (X-ray crystallography)

The molecular structure of 1-formyl-3-phenyl-Δ²-pyrazoline was determined by X-ray crystallography (triclinic,P-1). The geometry thus obtained was compared with that obtained by DFT calculations. The GIAO method was used to calculate absolute shieldings, which agree conveniently with those measured by ¹³C and ¹⁵N NMR. The title compound appears to be an essentially planar molecule.

DOI:10.1023/B:STUC.0000011246.24486.63

Fluorine-fluorine interactions: NMR and AIM analysis

The structure of a number of compounds that show experimental F⋅⋅⋅sF coupling constants across the space has been studied using HF-DFT methods (B3LYP) and Atoms in Molecules (AIM) methodologies. For all the cases with strong coupling constants a bond critical point and the corresponding bond path between the fluorine atoms involved has been found in the electron density map. In an attempt to predict NMR properties, new compounds for which no experimental F-F coupling constants are available, but with the same characteristics in the electron density maps, have been calculated.

DOI:10.1016/j.tet.2004.06.039

On the tautomerism of pyrazolones: the geminal 2J[pyrazole C-4,H-3(5)] spin coupling constant as a diagnostic tool

The tautomerism of pyrazolones unsubstituted at position 3(5) has been investigated by ¹³C- and ¹H NMR spectroscopic methods. Apart from chemical shift considerations and NOE effects the magnitude of the geminal ²J[pyrazole C-4,H3(5)] spin coupling constant permits the unambiguous differentiation between 1H-pyrazol-5-ol (OH) and 1,2-dihydro-3H-pyrazol-3-one (NH) forms. Whereas 1H-pyrazol-5-ols and 2,4-dihydro-3H-pyrazol-3-ones (CH-form) exhibit ²J values of approximately 9–11 Hz, in 1,2-dihydro-3H-pyrazol-3-ones this coupling constant is considerably reduced to 4–5 Hz. This can be mainly attributed to the removal of the lone-pair at pyrazole N−1 in the latter due to protonation or alkylation. According to the data obtained, 2-substituted 4-acyl-1,2-dihydro-3H-pyrazol-3-ones exist predominantly as pyrazol-5-ols in CDCl₃ or benzene-d₆ solution, whereas in DMSO-d₆ also minor amounts of NH tautomer may contribute to the tautomeric composition. 2,4-Dihydro-2-phenyl-3H-pyrazol-3-one (1-phenyl-2-pyrazolin-5-one) exists in benzene-d₆ solely in the CH-form, in CDCl₃ as a mixture of CH and OH-form, whereas in DMSO-d₆ a fast equilibrium between OH and NH isomer (with the former far predominating) is probable. For 11 compounds, including neutral and protonated molecules, we have calculated at the B3LYP/6-311++G** level, the²J(¹H,¹³C) coupling constants which are in good agreement with those measured experimentally.

DOI: 10.1002/mrc.1460

A GIAO/DFT study of 1H, 13C and 15N shieldings in amines and its relevance in conformational analysis

The ¹H, ¹³C and ¹⁵N absolute shieldings of 13 amines were calculated at the GIAO/B3LYP/6–311++G** level. For some compounds (ethylamine, piperidine and 1-methylpiperidine) two conformations were calculated. The ¹³C and ¹⁵N data could be correctly correlated with experimental chemical shifts, allowing the conformation of 1-methylpiperidine to be established. The ¹H NMR absolute shieldings, although less well correlated with δ values, were used to account for the anisotropy effects of the N lone pair. 

DOI:10.1016/j.tet.2004.06.005

Synthesis, experimental and theoretical NMR study of 2'-hydroxychalcones bearing a nitro substituent on their B ring

The synthesis of several 2′-hydroxynitrochalcones has been accomplished by an aldol reaction of equimolar amounts of the appropriate 2′-hydroxyacetophenones with nitrobenzaldehydes in alkaline medium. The reaction of 2′-hydroxyacetophenones bearing a 6′-methoxy with 2- or 4-nitrobenzaldehydes gave the expected 2′-hydroxynitrochalcones and also 4-methoxynitroaurones, being the latter ones the unique reaction products when using 2 molar equiv of nitrobenzaldehydes. The reaction mechanisms for the formation of both products are discussed. The ¹³C NMR chemical shifts have been discussed first by means of an empirical additive model and then by comparison with GIAO/B3LYP calculated absolute shieldings.

DOI:10.1016/j.tet.2004.01.031

Influence of the H/F replacement on the homoaromaticity of homotropylium ion: a GIAO/DFT theoretical study

The problem of homoaromaticity in mono-, di- and polyfluorinated- homotropylium cations is addressed by the B3LYP/6-311++G** DFT method. The energetic, structural and magnetic criteria are used for this purpose. They convincingly show that the ground state equilibrium species are aromatic, or in other words that the homoaromaticity is preserved by the (poly)fluorination. In contrast, a considerable decrease in the aromatic stabilization is observed in the transition structures (TS). According to the NICS(0) index, they vary form strongly antiaromatic, via weakly and non-aromatic to slightly aromatic transition states. However, the hierarchy of the aromaticity in fluorinated homotropylium ions predicted by NICS(0) is completely unrelated to that obtained by using the energy criterion assuming a kinetic definition of aromaticity. On the other hand the latter is closely related to geometric parameters of the equilibrium and transition structures.

DOI: 10.1007/s00214-003-0486-7

Karplus-type relationships betweeen scalar coupling constants: 3JHH molecular vs. 4hJHH supramolecular coupling constants

The ³ J _HH coupling constants in six H–X–Y–H systems (ethane, methylamine, methanol, hydrazine, hydroxylamine and hydrogen peroxide) and ^4h J _HH coupling constants in four H–...XH...Y–H, namely [H₃NHNH₃]⁺ (two arrangements), HOHNH₃ and HOHOH₂ have been calculated theoretically as a function of the torsion angle φ. For covalent situations, the corresponding Karplus equations have been fitted to calculated ³ J _HH=acos2 φ+bcos φ+c. The a, b and c terms have been analyzed as a function of the electronegativities of X and Y. In the case of ammonium/ammonia complexes (proton shared and not), water/ammonia, and water dimer the values are low (maximum 0.5 Hz) but follow closely a Karplus relationship.

DOI: 10.1063/1.1640342

¹⁹F–¹⁹F spin–spin coupling constant surfaces for (HF)₂ clusters: The orientation and distance dependence of the sign and magnitude of J_F–F 

Ab initio calculations using the equation-of-motion coupled cluster method have been carried out to investigate ¹⁹F–¹⁹F spin–spin coupling constants for a pair of HF molecules. The overall features of the J_F–F coupling surface with respect to the F–F distance and the orientation of the pair of HF molecules reflect those of the Fermi-contact (FC) surface, although the FC term may not be a good quantitative estimate of J_F–F. The hydrogen-bonded HF dimer exhibits unusual behavior compared to other hydrogen-bonded complexes, since both the FC term and ^2hJ_F–F exhibit variations in sign and magnitude as the F–F distance changes and the linearity of the hydrogen bond is destroyed. The FC term for F–F coupling is relative small and negative for the equilibrium dimer. At the dimer F–F distance, the maximum negative value for the FC term is found for the linear arrangement F–H⋯H–F, while the maximum positive value is found for the linear H–F⋯F–H arrangement, despite the fact that neither of these structures is bound. Changes in the sign and magnitude of the FC term are analyzed using the nuclear magnetic resonance triplet wave function model, which relates the orientation of magnetic nuclei to the phases of the wave functions for excited triplet states that couple to the ground state. The FC term for a particular orientation is a result of competing positive and negative contributions from different triplet states, the sign of each contribution being determined by the alignment of the nuclear magnetic moments in that state. Factors are identified which must play a role in determining which types of wave functions dominate. 

DOI: 10.1021/jp0400871

Computed Spin-Spin Coupling Constants (¹J_X-Y) in Molecules H_mX-YH_n for X and Y = ¹³C, ¹⁵N, and ³¹P: Comparisons with Experiment and Insights into the Signs of ¹J_X-Y

One-bond X−Y spin−spin coupling constants (¹J_X-Y) for 18 H_mX−YH_n molecules, with X and Y =¹³C, ¹⁵N, and ³¹P, have been computed using the equation-of-motion coupled-cluster singles and doubles method. The molecules investigated include all possible combinations of these three elements bonded with single, double, and triple bonds. The computed coupling constants are in good agreement with experiment over a range that extends from −250 to +200 Hz. With only two exceptions, the sign of the Fermi-contact (FC) term is the same as the sign of ¹J_X-Y, but the FC term may or may not be a good quantitative estimate of ¹J_X-Y. When reduced spin−spin coupling constants (¹K_X-Y) are used for comparing coupling constants involving different atoms, a linear relationship is observed between ¹K_X-N and ¹K_X-P. The signs of ¹J_X-Y for approximately half of the molecules included in this study are exceptions to the Dirac vector model. The recently proposed NMR triplet wave function model has been used to provide insight into the variation of the signs of these one-bond spin−spin coupling constants.

DOI: 10.1021/jp021619l

Two-Bond ¹³C-¹⁵N Spin-Spin Coupling Constants (^2hJ_C-N) Across C-H-N Hydrogen Bonds

Ab initio EOM-CCSD calculations have been performed to determine ¹³C−¹⁵N spin−spin coupling constants (^2hJ_C-N) across C−H−N hydrogen bonds in 17 neutral, 3 cationic, and 3 anionic complexes. The contributions of the paramagnetic spin−orbit, diamagnetic spin−orbit, and spin−dipole terms to the total ¹³C−¹⁵N spin−spin coupling constants (^2hJ_C-N) are negligible, so ^2hJ_C-N is determined solely by the Fermi contact term, which is distance-dependent. ^2hJ_C-N for complexes stabilized by C−H···N hydrogen bonds exhibits some dependence on the nature of the hybridization and the nature of the bonding at the C atom of the proton-donor C−H group. Nevertheless, a single curve can be constructed from ^2hJ_C-N and C−N distances for the equilibrium structures of the entire set of complexes that should be useful for estimating C−N distances from experimental measurements of coupling constants across C−H−N hydrogen bonds. Small deviations from linearity of the C−H−N hydrogen bond lead to only small changes in ^2hJ_C-N.

DOI: 10.1021/jp022621f

Two-Bond ¹⁵N-¹⁹F Spin-Spin Coupling Constants (^2hJ_N-F) across N-H⁺...F Hydrogen Bonds 

Two-bond ¹⁵N−¹⁹F NMR spin−spin coupling constants (^2hJ_N-F) have been computed using equation-of-motion coupled cluster singles and doubles theory (EOM-CCSD) for a variety of cationic complexes stabilized by traditional N−H⁺···F hydrogen bonds. The proton donors include protonated sp bases derived from HCN, protonated sp² aromatic rings and imines, and protonated sp³ bases derived from NH₃, with FH as the proton acceptor. ^2hJ_N-F is determined solely by the Fermi-contact term, which is distance dependent. The absolute values of N−F coupling constants for cationic complexes are significantly greater than the F−N coupling constants for neutral complexes stabilized by traditional F−H···N hydrogen bonds over a range of N−F distances. This may be attributed to the greater proton-shared character of hydrogen bonds in cationic complexes. Moreover, at a given distance, values of ^2hJ_N-F for complexes with sp and sp² nitrogens as proton donors are considerably greater than ^2hJ_N-F values for complexes with sp³ nitrogens as donors. When the cationic complexes are grouped according to the hybridization of the nitrogen, good correlations are found between ^2hJ_N-F and the N−F distance. Small perturbations of the N−H⁺···F hydrogen bond from linearity are associated with only small decreases in ^2hJ_N-F.

DOI: 10.3390/i4030064

Review on DFT and ab initio Calculations of Scalar Coupling Constants

The present review summarizes the information available on the ab initio calculations of spin-spin nuclear coupling constants through hydrogen bonds or in van der Waals complexes. It also reports the sources of experimental data on n^hJ_XY scalar couplings.

DOI:10.1524/zpch.217.12.1565.20476

Computed EOM-CCSD ¹⁹F-¹⁹F Spin-Spin Coupling Constants in Small Organic Molecules

Two-, three-, and four-bond 19F-19F spin-spin coupling constants (nJFF) for a set of small fluoro-substituted organic molecules have been computed using ab initio equation-ofmotion coupled cluster singles and doubles (EOM-CCSD) theory. The computed values reproduce the experimental signs and magnitudes of nJFF. The straight line that relates the experimental and computed coupling constants has a slope of approximately 1 and passes through the point (0, 0Hz) within the uncertainties of the fit. Hence, EOM-CCSD values of nJFF should be excellent predictors of experimental values when these are not available. All of the components of nJFF except for the diamagnetic spin-orbit term may be large and must be evaluated if agreement between theory and experiment is to be obtained.

DOI:10.1023/A:1024454011830

The Molecular Structure and NMR Properties of P-Phosphinoylmethyl Aminophosphonium Salts

The molecular structures of two aminophosphonium salts (bromide and tetrafluoroborate) have been determined by X-ray analysis. They have similar conformations and hydrogen bond (HB) networks: the N–H acid proton is bonded to the anion and, in the case of the fluoroborate, to the oxygen atom of the phosphine oxide, forming a pseudo six-membered ring closed by a weak N–H⋅⋅⋅O intramolecular hydrogen bond (IMHB). These compounds have been studied by multinuclear NMR in solution, including the ¹⁵N-labeled derivatives, to determine a complete set of coupling constants. A coupling of ∣1.5∣ Hz between the ¹⁵N and the ³¹P nuclei, separated by three bonds, was observed experimentally for the bromide in CDCl₃ solution, which appears to be a classical ³ J _N-P across the covalent bonds and not a ^3h J _N-P across the IMHB.

DOI:10.1023/A:1024402027760

Review: GIAO Calculations of Chemical Shifts in Heterocyclic Compounds

In this review, the GIAO calculations of absolute shieldings and their relationship with experimental chemical shifts for aromatic heterocycles will be summarized. Automatic assignment, conformational analysis, E/Z isomerism, and, in particular, tautomerism, will be discussed in detail. Solid-state and solvent effects will be examined, as well as the problem of heteroaromaticity. The review ends with the discussion of some methodological problems with special emphasis on the calculation of references, such as TMS and nitromethane.

DOI: 10.1039/B208803G 

The influence of chain-elongation on Karplus-type relationships: a DFT study of scalar coupling constants in polyacetylene derivatives

The coupling constants of a series of acetylenic derivatives have been calculated using the finite perturbation method. In the case of dimethylated derivatives a Karplus-type relationship has been obtained for coupling constants of hydrogen atoms separated up to 15 bonds. Additional relationships have been obtained between the interatomic distances and the coupling constants.

DOI: 10.1021/ja011755o

^3hJ(¹⁵N-³¹P) Spin-Spin Coupling Constants across N-H···O-P Hydrogen Bonds 

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to evaluate three-bond ¹⁵N−³¹P coupling constants (^3hJ_N-P) across N−H···O−P hydrogen bonds in model cationic and anionic complexes including NH₄⁺:OPH, NH₄⁺:OPH₃, NH₃:^-O₂PH₂, NFH₂:^-O₂PH₂, and NF₂H:^-O₂PH₂. Three-bond coupling constants can be appreciable when the phosphorus is P(V), but are negligible with P(III). ^3hJ_N-P values in complexes with cyclic or open structures are less than 1 Hz, a consequence of the nonlinear arrangement of N, H, O, and P atoms. For complexes with these structures, ^3hJ_N-P may not be experimentally measurable. In contrast, complexes in which the N, H, O, and P atoms are collinear or nearly collinear have larger values of ^3hJ_N-P, even though the N−P distances are longer than N−P distances in cyclic and open structures. In linear complexes, ^3hJ_N-P is dominated by the Fermi-contact term, which is distance dependent. Therefore, N−P (and hydrogen-bonding N−O) distances in these complexes can be determined from experimentally measured ¹⁵N−³¹P coupling constants.

DOI: 10.1021/jp021159w

Ab Initio Study of the Structural, Energetic, Bonding, and IR Spectroscopic Properties of Complexes with Dihydrogen Bonds 

The results of an ab initio study of complexes with X−H···H−M dihydrogen bonds are presented. The proton donors include HCCH and its derivatives HCCF, HCCCl, and HCCCN; HCN and its derivatives HCNLi⁺ and HCNNa⁺; CNH, and H₂O, and the proton acceptor is LiH. For comparison, selected complexes with NaH as the proton acceptor have also been investigated. The structures, binding energies and harmonic vibrational frequencies of all complexes were obtained at the MP2/aug‘-cc-pVTZ level of theory. The most stable complexes with C−H groups as proton donors are the cationic complexes NaNCH⁺:HLi and LiNCH⁺:HLi. These complexes exhibit very short H····H distances and are prototypical of dihydrogen-bonded complexes that may dissociate by eliminating H₂. The calculated binding energies correlate with the H···H distance, the elongation of the C−H donor bond, the amount of charge transfer into the H····H bonding region, and the charge density at the H···H bond critical point. As in conventional hydrogen-bonded complexes, the elongation of the proton donor C−H group correlates with the strength of the interaction, and with the red shift of the C−H stretching frequency. Although changes in the Li−H bond length do not follow a simple pattern, the Li−H stretching frequency is blue-shifted in the complexes.

doi:10.1016/S0022-2860(01)00759-1

1-Benzoylazoles: an experimental (NMR and crystallography) and theoretical study

Five N-benzoylazoles (imidazole, pyrazole, indole, benzimidazole and carbazole) have been prepared following modified literature procedures. Their NMR spectra in solution (¹H, ¹³C and ¹⁵N) have been measured. The crystal structures of 1-benzoylindole and 9-benzoylcarbazole have been determined by X-ray crystallography and the corresponding ¹³C NMR spectra in the solid state have been measured by the CPMAS technique. Whereas 1-benzoylindole presents a standard behaviour, 9-benzoylcarbazole shows an unexpected ¹³C CPMAS spectrum with additional splittings. In order to understand this fact, the ¹H and ¹³C NMR spectra in dimethylether at −143°C (130 K) have been recorded and ab initio calculations (RHF/6-311G**) carried out. The corresponding absolute shieldings (GIAO/ RHF/6-311G**) together with the X-ray structure and the ¹³C chemical shifts at low temperature have been used to discuss the CPMAS spectrum. We propose that the supplementary splittings of this spectrum are due to its conglomerate structure.

DOI: 10.3987/COM-01-9407

Synthesis, 1H and 13C NMR study of pyrazoles derived from chiral cyclohexanones (3-methylcyclohexanone, menthone, pulegone, dihydrocarvone and carvone)

The ¹H and ¹³C chemical shifts of four tetrahydroindazoles (two of them existing as diastereomeric mixtures) and one aldazine were measured and assigned. These compounds were obtained from monoterpenic ketones (R)-(+)-3-methylcyclohexanone, (2S,5R)-(-)-menthone, (R)-(+)-pulegone, (5R)-(+)-dihydrocarvone, and (R)-(-)-carvone in a two-step procedure. The annular tautomerism in CDCl₃solution was calculated and compared with ab initio calculations (B3LYP/6-31G*).

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¹³C and ¹⁵N NMR shieldings of 1,2,4-diazaphospholes in the solid state and in solution

The solid state ¹³C and ¹⁵N CPMAS NMR spectra of 3,5-di-tert-butyl-1,2,4-diazaphosphole 4 and 3,5-diphenyl-1,2,4-diazaphosphole 5 have been recorded. The X-ray structure of the first compound was already known (it is a cyclic dimer with localized N–H protons) while the structure of the second cannot be determined due to the difficulty to grow suitable single crystals. NMR results pointed out that 4 is a “classical” compound while 5 is probably a tetramer showing Intermolecular Solid-State Proton Transfer (ISSPT). GIAO/ab initio calculations have been carried out to estimate the absolute ¹H, ¹³C and ¹⁵N shieldings. The agreement with the experimental chemical shifts is good enough to assign the signals of carbons C-3 and C-5.

DOI:10.1016/S0022-2860(01)00486-0

A 13C NMR study of the structure of four cinnamic acids and their methyl esters

The ¹³C NMR spectra, both in DMSO solution and in the solid state of four cinnamic acids (p-methoxy, p-hydroxy, p-methyl, p-chloro) and their corresponding methyl esters have been recorded. The two main results in the solid state are: (i) the only significant difference between acids and esters chemical shifts concerns the CO group which, on average, appears at 173 ppm in the acids and 168 ppm in the esters; (ii) the signals of the ortho and meta carbons both in the acids and the esters are splitted. The two ‘anomalies’ disappear in DMSO solution. These observations can be rationalized using simple GIAO/B3LYP/6-31G^∗ calculations.

DOI: 10.1002/jhet.5570380221

Ab initio study of azolides: energetics and spectroscopic properties

We report the ab initio study of twenty-four azolides derived from pyrrole, imidazole, pyrazole, both triazoles, tetrazole, pentazole, indole and carbazole bearing at the nitrogen atom the groups COMe, CHO, COCF₃ and CO₂Me. Theoretical values (isomerism, barriers, dipole moments, C=O stretching) are compared with experimental ones, when available, and also internally compared. A special effort has been devoted to the calculation of the absolute shieldings for the different nuclei present in azolides. At the level of calculation used (RHF/6-311G**) the results are satisfactory. To complete the nmr data from the literature, some ¹H, ¹³C, ¹⁵N, ¹⁷O and 19F chemical shifts have been determined.

DOI:10.1023/A:1012276824422

A theoretical and experimental study of the interaction of C6F6 with electron donors

The NMR effects produced on the nitrogen absolute shieldings in a series of electron donors when they interact with hexafluorobenzene, C₆F₆, have been theoretically studied. The complexes have been optimized at the B3LYP/6-311++G** level and the NMR shieldings have been calculated using the GIAO method. The results obtained have allowed devising an experiment (C₆F₆···NCCH₃complex) that is compatible with the theoretical calculations.

DOI:10.1016/S0040-4020(01)00585-3

An ab initio study of the NMR properties (absolute shieldings and NICS) of a series of significant aromatic and antiaromatic compounds

The absolute shieldings of protons and the nuclear independent chemical shifts of six aromatic and antiaromatic hydrocarbons have been calculated at the GIAO/B3LYP/6-31G^∗ level. In general, the results are in agreement with the experiment ones except in the case of 1,5,9-tridehydro[12]annulene 7. Three possible explanations have been tested for this compound concluding that the anomaly (∼14 ppm!) probably lies on the use of a DFT approach.

DOI:10.1016/S0022-328X(00)00808-1

Aminopropylsilanes versus silatranes: an experimental and theoretical study

The ¹H-, ¹³C-, ¹⁵N- and ²⁹Si-NMR spectra of 3-aminopropyltrimethoxysilane and N,N-dimethylamino-propyltrimethoxysilane have been recorded. An analysis of the spectra together with GIAO calculations leads to the conclusion that these compounds exist as open-chain structures and not as ring-closed silatranes. The N→Si interaction is important only when there is at least one cycle in the structure, although two or three cycles (silatranes) are a more favorable disposition.

DOI: 10.1021/jp001681n

^4h J(³¹ P- ³¹ P) Coupling Constants through N-H + -N Hydrogen Bonds: A Comparsion of Computed ab Initio and Experimental Data

EOM-CCSD calculations have been performed to evaluate the ³¹P−³¹P coupling constant (^4hJ_P-P) across an N−H⁺−N hydrogen bond in a model system. Computed ^4hJ_P-P values were obtained as a function of distance and are in agreement with an experimentally measured value of the ³¹P−³¹P coupling constant across an N−H⁺−N hydrogen bond.

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Solution and solid state (CPMAS) NMR studies of the tautomerism of six-membered heterocyclic compounds related to 2-pyridones

Several 13C and 15N chemical shifts of 2-pyridone (1), 4(3H)-pyrimidone (2), uracil (3) and cytosine (4) have been measured in solution and in the solid state. These data have been discussed in relation with the tautomerism of the four heterocycles. GIAO ab initio calculations of absolute shieldings have been carried out to identify the predominant tautomers in the case of compounds (1) and (2).

DOI: 10.3987/COM-98-8381

Fluoropyrazoles: an ab initio study

Quantum mechanic calculations have been done at the RHF and MP2 levels with the STO-3G, 6-31G**, 6-311G** basis sets on pyrazole itself and seven N-unsubstituted C-fluoropyrazoles. These calculations have been used to discuss the molecular structure of these compounds in relation to their aromaticity. The corresponding ¹H, ¹³C, ¹⁵N and ¹⁹F chemical shifts were calculated using the GIAO perturbation method.

DOI: 10.1039/A904537F 

Can aromaticity be described with a single parameter? Benzene vs cyclohexatriene

The problem of magnetic vs. geometric criteria of aromaticity is approached using NICS/GIAO/B3LYP/6-31G* calculations on benzene and distorted benzenes, the conclusion being that a relationship exists between these criteria but only for specific subsets.

DOI:10.1023/A:1022419030317

Ab initio (GIAO) calculations of absolute shieldings for representative compounds containing ¹⁽²⁾ H, ⁶⁽⁷⁾ Li, ¹¹B, ¹³C, ¹⁴⁽¹⁵⁾ N, ¹⁷O, ¹⁹F, ²⁹Si, ³¹P, ³³S and ³⁵Cl nuclei

Two kinds of ab initio calculations (B3LYP and HF) using the GIAO approach have been carried out for fifteen H, three Li, three B, eleven C, seven N, nine O, four F, three Si, four P, six S and five Cl containing derivatives. The calculated absolute nuclear shieldings have been compared with experimental absolute values (either measured or estimated from relative chemical shifts) when available or with relative chemical shifts (¹¹B, ³⁵Cl). The correlations range from good to excellent and only three compounds (PN, P₂H₂, SO₂) deviate significantly.

DOI: 10.1039/A801405A

The conformation of dibenzocyclooctadiene: an experimental and theoretical NMR study

Hybrid abinitio calculations (GIAO/B3LYP/6-31G*) together with new DNMR experiments (¹H and ¹³C) have been used to clarify the problem of the conformation of 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene. The carbon-13 chemical shifts have been assigned unambiguously to the chair C and twist-boat TB conformations. Lineshape analysis of the ¹H NMR spectra has been carried out at several temperatures using an estimated set of Karplus-type vicinal coupling constants.

DOI: 10.1039/A708743H

Ab initio hybrid DFT–GIAO calculations of the shielding produced by carbon–carbon bonds and aromatic rings in ¹H NMR spectroscopy

Theoretical calculations of ¹H shieldings by single, double and triple CC bonds as well as by aromatic rings (benzene, cyclopropenyl cation and hexafluorobenzene) have been performed using abinitio MO theory. As an illustration of the methodological approach, absolute chemical shieldings of ¹H-, ¹³C-, ¹⁷O- and ¹⁹F-containing molecules have been calculated. The results, both inter- and intramolecular, range from good to excellent. The relative chemical shifts of some large molecules having strongly shielded protons are conveniently reproduced. Calculsabinitio hybrides DFT-GIAO de l'effet d'e′cran produit par des liaisons carbone–carbone et par de noyaux aromatiques en RMN du ¹H. On rapporte les calculs the′oriques abinitio des blindages de protons par des liaisons CC simple, double et triple ainsi par des noyaux aromatiques (benzène, cation cycloprope′nyle et hexafluorobenzène). Pour illustrer l'approche utilise′e, les blindages des noyaux ¹H, ¹³C, ¹⁷O et 19^F ont e′te′ calcule′s. Les re′sultats tant inter- que intramole′culaires sont bons ou excellents. Les de′placements chimiques de quelques mole′cules de relativement grande taille sont reproduits de façon convenable.