Two-Bond ~(15)N-~(19)F Spin-Spin Coupling Constants (~(2h)J_(N-F)) across N-H~+…F Hydrogen Bonds

摘要

Two-bond ~(15)N-~(19)F NMR spin-spin coupling constants (~(2h)J_(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 sp~2 aromatic rings and imines,a nd protonated sp~3 bases derived from NH_3, with FH as the proton acceptor. ~(2h)J_(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 ~(2h)J_(N-F) for complexes with sp and sp~2 nitrogens as proton donors are considerably greater than ~(2h)J_(N-F) values for complexes with sp~3 nitrogens as donors. When the cationic complexes are grouped according to the hybridization of the nitrogen, good correlations are found between ~(2h)J_(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 ~(2h)J_(N-F).