INDO Molecular Orbital Calculations of Nitrogen–Proton Spin–Spin Coupling Constants Over Two and Three Bonds. Effects of Lone-pair Orientation, of Dihedral Angles, and of Protonation

Abstract

Molecular orbital theory at the INDO level of approximation is used to calculate the Fermi contact contribution to two-bond and three-bond nitrogen–proton coupling constants for a wide variety of compounds. The dependence of calculated N,H coupling constants upon the spatial orientation of the nitrogen lone-pair is examined for selected molecules. For both saturated and unsaturated compounds the calculated 2J(15N,H) is large and negative when the lone-pair is oriented cis to the proton, and is small and of either sign when the lone-pair lies trans to the proton. Calculated 3J(N,H) depends both on the orientation of the nitrogen lone-pair and on the HCCN dihedral angle. Computed 2J(N,H) and 3J(N,H) generally follow the experimentally known substituent and structural effects. It is suggested that observed phosphorus–proton coupling constants in tervalent phosphorus compounds are dependent on lone-pair orientation in a manner analogous to the corresponding nitrogen–proton coupling constants. The influence of the nitrogen lone-pair orientation on geminal proton–proton coupling constants in methylamine is computed and compared with experiment. Calculated barriers to pyramidal inversion and methyl group rotation are in reasonable agreement with available experimental data.