Molecular orbital study of the catalytic activity of onium compounds in liquid-phase oxidation

Abstract

A theoretical study involving ASMO-SCF and extended Hückel methods including d-orbitals has been undertaken in an attempt to correlate the electronic properties of various onium compounds and their catalytic activities in the liquid-phase oxidation of hydrocarbons. First, the stable geometries of trimethylonium compounds (carbonium, oxonium, sulphonium, selenonium, and telluronium) were found to involve planar cations; whilst ammonium, phosphonium, and arsonium cations assume tetrahedral configurations. Secondly, the differences in the catalytic activities of second-row onium compounds may be ascribed to the differences in the AO populations of the p_z- orbital on their central atoms. Thirdly, the distinctive activities of sulphonium and phosphonium compounds are reasonably well explained by the contribution of the partially occupied sulphur or phosphorous 3d-orbitals, to the catalytic activity. Among 3d-orbitals, the d_xz(or d_yz)-orbital is most effective for interaction with the (1_πg)_z-orbital of molecular oxygen. Finally, the relatively low activities of arsonium, selenonium, and telluronium compounds are due to the vacant and diffuse 4d and 5d orbitals of the central atoms. The order of activity, arsonium ≃ telluronium < selenonium, is in reasonable accord with the AO populations of 4d or 5d orbitals of the central arsenic, selenium, or tellurium.