GIAO, DFT, AIM and NBO analysis of the N-H···O intramolecular hydrogen‐bond influence on the1J(N,H) coupling constant in push–pull diaminoenones

Abstract

In the series of diaminoenones, large high‐frequency shifts of the¹H NMR of the NH group in thecis‐position relative to the carbonyl group suggests strong NH···O intramolecular hydrogen bonding comprising a six‐membered chelate ring. The NH···O hydrogen bond causes an increase of the¹J(N,H) coupling constant by 2–4 Hz and high‐frequency shift of the¹⁵N signal by 9–10 ppm despite of the lengthening of the relevant NH bond. These experimental trends are substantiated by gauge‐independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3‐bis(isopropylamino)‐1‐(aryl)prop‐2‐en‐1‐one (12) for conformations with theZ‐ andE‐orientations of the carbonyl group relative to the NH group. The effects of the NH···O hydrogen‐bond on the NMR parameters are analyzed with the atoms‐in‐molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the NH···O hydrogen bond as compared with that of 1,1‐di(pyrrol‐2‐yl)‐2‐formylethene (13) where NH···O hydrogen bridge establishes a seven‐membered chelate ring, and the corresponding¹J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) →σ*_NHhyperconjugative interaction is weakened on going from the six‐membered chelate ring to the seven‐membered one due to a more bent hydrogen bond in the former case. A dominating effect of the NH bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the¹J(N,H) value as a consequence of the NH···O hydrogen bonding in the studied diaminoenones. Copyright © 2010 John Wiley & Sons, Ltd.