Effect of lone-pair orientation on nuclear spin couplings

The 2 J nh and 1 J ch couplings involving 15N and 13C in oximes have been studied theoretically by the finite perturbation method employing CNDO/2 and INDO wave functions. The couplings are found to be significantly influenced by the orientation of the nitrogen lone-pair orbital. It has been shown that the dihedral-angle dependence of vicinal H-H coupling in methylamine is also influenced by the orientation of the lone-pair on nitrogen, leading to deviations in the couplings predicted by a Karplus-type equation. The localized molecular orbitals and the hybridization and orientation of the nitrogen lone-pair in these molecules have been obtained by the Edmiston-Ruedenberg transformation of the semi-empirical SCF molecular orbitals.