Nitrogen-15 Magnetic Resonance Spectroscopy. II. Coupling Constants

Coupling constants have been measured between nitrogen-15 and directly bonded hydrogen or carbon-13, and between nitrogen-15 and hydrogen separated by two or more bonds. The data may generally be interpreted in terms of the Fermi contact coupling mechanism except for those cases where the electron distribution around the nitrogen atom is highly anisotropic. Empirical relationships have been developed which relate the observed J^(15)NH and J^(15)N^(13)C to the hybridization of the orbitals involved. These relationships fail for J^(16)NH in diphenylketimine and for J^(15)N^(13)C in benzalmethylamine and acetonitrile, probably because of contributions to the nuclear spin-spin coupling from electron orbital motion. The data are consistent with the interpretation that nitrogen in ammonia forms nearly sp^3 bonds to hydrogen, rather than p bonds as suggested by Pauling.