Bonding situation and NO-bond strengths in amine-N-oxidesa combined experimental and theoretical studyElectronic supplementary information (ESI) available. See DOI: 10.1039/c2cp22341d

The bonding situation and energetics of the NO bond in a series of amine- N -oxides, Ph x (CH 3 ) 3 x NO, where x = 03, were analyzed experimentally and theoretically. There is a notable nearly linear decrease of the NO 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 (CH 3 ) 2 PhNO, 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 -bond for the NO 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 (CH 3 ) 3 x N, ( x = 03), respectively, the required preparation energies ( E prep ) for the reverse NO bond forming process. The detailed theoretical analysis of the amines allowed us to trace the trend of larger values of E prep for a higher number of phenyl groups x to an increase of n (N) *(CC) delocalization interactions. The bonding situation and energetics of the NO bond in a series of amine- N -oxides, Ph x (CH 3 ) 3 x NO, where x = 03, were analyzed experimentally and theoretically.