Two-bond 13C-15N spin-spin coupling constants (2hJC-N) across C-H-N hydrogen

Ab initio EOM-CCSD calculations have been performed to determine C-13-N-15 spin-spin coupling constants ((2h)J(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 C-13-N-15 spin-spin coupling constants ((2h)J(C-N)) are negligible, So (2h)J(C-N) is determined solely by the Fermi contact term, which is distance-dependent. (2h)J(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 (2h)J(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 (2h)J(C-N).