Natural J coupling (NJC) analysis of the electron lone pair effect on NMR couplings:: Part 1.: The lone pair orientation effect of an α-nitrogen atom on 1J(C,C) couplings

The known lone pair orientation effect of an a-nitrogen atom on (1)J(C,C) couplings was studied using the natural J coupling (NJC) dissection method for the Fermi contact (FC) term. This dissection was performed at the DFT-B3LYP level using three different basis sets. It was verified that non-contact contributions, which are much smaller than the FC term, do not depend on the orientation of the N lone pair. Acetone oxime (1), protonated acetone oxime (2) and ethylamine (3), were taken as model compounds to perform this study. In 1 the difference between (1)J(Z) and (1)J(E) is mainly determined by three NJC terms, i.e the nitrogen lone pair, the carbon-carbon bond containing the coupled carbon atoms and the carbon inner core orbitals contributions. In 3 the angular dependence of (1)J(C,C) vs the amine group conformation is dominated by the same three contributions that define the (1)J(Z) and (1)J(E) difference in 1. Copyright (C) 2001 John Wiley & Sons, Ltd.