Two-Bond 15N-19F Spin-Spin Coupling Constants (2hJN-F) across N-H+...F Hydrogen Bonds
Two-bond 15N−19F NMR spin−spin coupling constants (2hJN-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. 2hJN-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 2hJN-F for complexes with sp and sp2 nitrogens as proton donors are considerably greater than 2hJN-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 2hJN-F and the N−F distance. Small perturbations of the N−H+···F hydrogen bond from linearity are associated with only small decreases in 2hJN-F.