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.

Computational Study of Selected Amine and Lactam N-Oxides Including Comparisons of N-O Bond Dissociation Enthalpies with Those of Pyridine N-Oxides

“…This is simply another means to display the experimental discrepancy between the five DFT calculations and the experimental data published for Equation (1). It is worth noting an ambitious study employing calculations of TMAO and the three molecules derived by sequentially replacing methyl with phenyl substituents, employing DFT, multireference CASSCF, and the MR-perturbation theory (MCQDPT2) [ 22 ]. The N-O BDE value for TMAO was compared with referenced experimental data.…”

Polycentric binding in complexes of trimethylamine-N-oxide with dihalogens

“…The energy difference for these two minima, global and local, was estimated to be 3. 63 The results of these two procedures (inversion and turn of the dichlorine fragment) demonstrate the signicance of the role of the C-H/Cl hydrogen bond in the whole stability of the halogen-bonded complex. We did not nd the C-H/I bond critical points for the hydrogen-iodine shortened contacts, although the experimental structure of the TMAO/ICl adduct unequivocally suggests the presence of this bonding.…”

“…X ¼ I, Y ¼ Cl for 2a and 3a; X ¼ I, Y ¼ Br for 2b and 3b. 63 1.348 1.487 conformity gives hope to nd credible structures for complexes of TMAO and halogens. TMAO-halogen complexes.…”

Analysis of Oxygen–Pnictogen Bonding with Full Bond Path Topological Analysis of the Electron Density

“…The nature of the O-E bonding in R3EO compounds, which include such commonly encountered species as trimethylamine N-oxide and triphenylphosphine oxide, has long served as a point of discussion. [16][17][18][19][20][21][22][23][24] The amine oxides are conventionally depicted with a polar single bond (O --N + ) but the heavier congeners are conventionally shown with a double bond (O=E). It is now well established, however, that the d orbitals of these heavier pnictogens do not participate in bonding.…”