Two-Bond13C−15N Spin−Spin Coupling Constants (2hJC-N) Across C−H−N Hydrogen Bonds

Abstract

Ab initio EOM-CCSD calculations have been performed to determine 1 3 C- 1 5 N spin-spin coupling constants ( 2 h 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 1 3 C- 1 5 N spin-spin coupling constants ( 2 h J C - N ) are negligible, so 2 h J C - N is determined solely by the Fermi contact term, which is distance-dependent. 2 h 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 protondonor C-H group. Nevertheless, a single curve can be constructed from 2 h 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 2 h J C - N .