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

The known lone pair orientation effect of an a-nitrogen atom on (1)J(C,C) c ouplings was studied using the natural J coupling (NJC) dissection method f or the Fermi contact (FC) term. This dissection was performed at the DFT-B3 LYP level using three different basis sets. It was verified that non-contac t 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 o xime (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 con taining the coupled carbon atoms and the carbon inner core orbitals contrib utions. In 3 the angular dependence of (1)J(C,C) vs the amine group conform ation 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.