Thinking and computing valence bond in organic chemistry

Abstract

This paper presents a short survey of some recent ab initio valence bond methods and their applications, and is aimed at justifying and encouraging a valence bond view of organic chemistry, as complementary to the molecular orbital approach. In the first section, the qualitative VB description of the elementary interactions is recalled and compared to the MO model. It is shown that the VB picture is fundamentally correct, even for the well-known cases of the low-lying states of dioxygen and the 4 n/4 n+2 aromaticity rule. The second section briefly discusses the classical VB method, which deals with atomic orbitals that are optimized for the free atoms and kept unchanged in molecules, then describes modern ab initio VB methods that all perform orbital optimization in molecular calculations. The generalized valence bond and spin-coupled theories both provide a one-configuration wavefunction. While the former is generally used with some time-saving restrictions such as the strong-orthogonality restriction and the perfect-pairing approximation, the latter releases any orthogonality constraint and allows all possible spin couplings. Multiconfiguration methods are also discussed, as well as methods using different orbitals for different structures. Some computational applications of these methods are presented in the last section. It is shown that if given full freedom to optimize its shape with the variational principle as a unique criterion, a one-configuration wavefunction spontaneously takes the form of a VB wavefunction displaying localized orbitals, and presents a picture in terms of hybrid orbitals and/or resonance between limiting structures, very close to the traditional qualitative picture. The concept of hybridization is firmly supported, as the unique outcome of the highest computational level still compatible with the orbital picture. The description of conjugated systems in terms of resonating Kekulé structures is also fully justified and shown to be the best framework for discussing questions such as the distortive tendencies of conjugated π-electronic systems, or violations of Hund's rules. The ab initio VB approach can be used for quantifying some traditional paradigms such as the role of the delocalization energy in the acidity of carboxylic acids and enols, or in the properties of the amide/thioamide functional group. It is also shown to be an elegant solution to some difficult computational problems like the symmetry-breaking artefact or the inclusion of dynamical correlation in the description of the chemical bond. Lastly, some of the methods presented here are shown to be appropriate for the calculation of diabatic potential surfaces, with applications to the Shaik–Pross reactivity model of the VB curve-crossing correlation diagrams.