Two-Bond 13C-15N Spin-Spin Coupling Constants (2hJC-N) Across C-H-N Hydrogen Bonds
Ab initio EOM-CCSD calculations have been performed to determine 13C−15N spin−spin coupling constants (2hJC-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 13C−15N spin−spin coupling constants (2hJC-N) are negligible, so 2hJC-N is determined solely by the Fermi contact term, which is distance-dependent. 2hJC-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 2hJC-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 2hJC-N.