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

The bonding situation and energetics of the N-O bond in a series of amine-N-oxides, Ph-x(CH3)(3-x)N-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 groups x. 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)(2)PhN-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 sigma-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-x)N, (x = 0-3), respectively, the required preparation energies (Delta E-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 Delta E-prep for a higher number of phenyl groups x to an increase of n(N) -> pi*(C-C) delocalization interactions.