Boron oxides: Ab initio studies with natural bond orbital analysis

Abstract

We employ ab initio theory and natural bond orbital (NBO) analysis to describe the structure, energetics, vibrational properties, and bonding in small boron oxides, BmOn, supplementing recent studies on isovalent aluminum oxide clusters, Al2On, in order to extend the overview of bonding tendencies in group IIIA metal oxides. The comparison of analogous boron and aluminum species reveals many surprising differences, such as the V-shaped (M=B) vs linear (M=Al) geometry of M2O3, the altered tendency toward cyclic structures (higher for M2O2 with M=Al, for M3O3 with M=B), and the diminished role of electron correlation in the boron congeners. Correlation effects are examined by a recently introduced selective, localized multiconfigurational approach. Differences in bonding patterns are traced to basic hybridization and electronegativity shifts (reduced ionic character of B–O vs Al–O bond). Detailed comparisons with recent experimental BmOn data (including isotopomer ir shifts) provide additional support for theoretical assignments in corresponding aluminum species, where significant disagreements between theory and experiment persist.