The electronic structure of methane and its derivatives: apparent versus effective symmetry control of sigma bonding

Abstract

Resonance theory is one major conceptual basis of chemistry. However, it has been shown that resonance theory is “VB theory without symmetry control”. Hence, many qualitative concepts of chemistry must either be incorrect or accidentally correct. Molecular orbital valence bond (MOVB) theory is “VB theory over canonical fragment MOs” or “VB theory with symmetry control made clear” and it is ideally suited for explaining old facts and for designing new chemistry. In this paper, a systematic presentation is given of central problems which are now recognized to have no satisfactory solution. The MOVB solution is put forward, and the problems with current interpretations are explained. The electronic structures of methane and fluoromethane are discussed in order to illustrate the following general MOVB concept: the nature of the AOs of an atom creates a distinction between apparent and effective molecular symmetry. When the constituent atoms are all first-row non-metal atoms the former is different from the latter. For example, the apparent symmetry of fluoromethane is C 3v, but the effective symmetry is T d, i.e., the σ bonds are made as if the true molecular symmetry were T d (methane-type bonds). By contrast, the apparent and effective symmetries of, for example, chlorostannane are the same, C 3v. This difference between C and Sn is a result of the different absolute and relative radial extensions of the valence AOs of the central atom.