Bonding situation and N–O-bond strengths in amine-N-oxides—a combined experimental and theoretical study†

Abstract

The bonding situation and energetics of the N–O bond in a series of amine-N-oxides, Ph_x(CH₃)_3?xN–O, where x = 0–3, were analyzed experimentally and theoretically. There is a notable nearly linear decrease of the N–O bond dissociation energies (BDEs) for this series with an increasing number of phenyl groupsx. This was investigated experimentally by X-ray high angle multipole refinement techniques in combination with subsequent topological analysis of the electron density for the representative (CH₃)₂PhN–O, 2, and complementary theoretical calculations at the DFT and multireference CASSCF and MR-perturbation theory (MCQDPT2) levels. Both the theoretical and experimental results unambiguously revealed a polar covalent σ-bond for the N–O bond with an essentially identical bonding situation for all amine-N-oxides studied. This apparent disparity between the bonding situation and the trend of BDEs is attributed to the large differences of the relaxation energies of the corresponding amines Ph_x(CH₃)_3?xN, (x = 0–3), respectively, the required preparation energies (ΔE_prep) for the reverse N–O bond forming process. The detailed theoretical analysis of the amines allowed us to trace the trend of larger values of ΔE_prep for a higher number of phenyl groupsx to an increase of n(N) → π*(C–C) delocalization interactions.