Orbital deletion procedure and its applications

The orbital deletion procedure is introduced, which is suited to quantitatively investigating the electronic delocalization effect in carbocations and boranes. While the routineab initio molecular orbital methods can generate wavefunctions for real systems where all electrons are delocalized, the present orbital deletion procedure can generate wavefunctions for hypothetical reference molecules where electronic delocalization effect is deactivated. The latter wave-function normally corresponds to the most stable resonance structure in terms of the resonance theory. By comparing and analyzing the delocalized and the localized wavefunctions, one can obtain a quantitative and instinct picture to show how electronic delocalization inside a molecule affects the molecular structure, energy as well as other physical properties. Two examples are detailedly discussed. The first is related to the hyperconjugation of alkyl groups in carbocations and a comparison of the order of stability of carbocations is made. The second concerns the Lewis acidity of boron trihalides where the conjugation effect among the doubly-occupied π atomic orbitals on the halide atoms and the vacant π atomic rbital on the boron atom plays a dominant role in determining the relative acceptor properties. The results demonstrate that the orbital deletion procedure can be used to very successfully interpret some traditional chemical intuitions and concepts in a quantitative way.