title: 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)(CH(3))(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 (CH(3))(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 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)(CH(3))(3-x)N, (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 groups x to an increase of n(N) → π*(C-C) delocalization interactions.
authors.affiliation: Institut für Anorganische und Angewandte Chemie, Universität Hamburg, 20146 Hamburg, Germany.
authors.forename: Andrey Yu
authors.initials: AY
authors.lastname: Rogachev
authors.forename: Peter
authors.initials: P
authors.lastname: Burger
mesh_terms: D000588:Amines / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008962:Models, Theoretical; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D010087:Oxides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D000588:Amines; D010087:Oxides; D009584:Nitrogen; D010100:Oxygen