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.