Orbital deletion procedure and its applications

The orbital deletion procedure is introduced, which is suited to quantitati vely investigating the electronic delocalization effect in carbocations and boranes. While the routine ab initio molecular orbital methods can generat e warefunctions for real systems where all electrons are delocalized, the p resent orbital deletion procedure can generate wavefunctions for hypothetic al reference molecules where electronic delocalization effect is deactivate d. The latter wave-function normally corresponds to the most stable resonan ce structure in terms of the resonance theory. By comparing and analyzing t he delocalized and the localized wavefunctions, one can obtain a quantitati ve and instinct picture to show how electronic delocalization inside a mole cule affects the molecular structure, energy as well as other physical prop erties. Two examples are detailedly discussed. The first is related to the hyperconjugation of alkyl groups in carbocations and a comparison of the or der of stability of carbocations is made. The second concerns the Lewis aci dity of boron trihalides where the conjugation effect among the doubly-occu pied pi atonic orbitals on the halide atoms and the vacant pi atomic orbita l on the boron atom plays a dominant role in determining the relative accep tor properties. The results demonstrate that the orbital deletion procedure can be used to very successfully interpret some traditional chemical intui tions and concepts in a quantitative way.