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 on¹J(C,C) couplings

The known lone pair orientation effect of an α-nitrogen atom on ¹J(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 ¹J(Z) and ¹J(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 ¹J(C,C) vs the amine group conformation is dominated by the same three contributions that define the ¹J(Z) and ¹J(E) difference in 1.