Valence-Bond Description of Gas-Phase Pericyclic Reactions

Modern Valence Bond Description of Gas-Phase Pericyclic Reactions 

David L. Coopera, Peter B. Karadakovb and Thorstein Thorsteinssona 

The combination of the spin-coupled formulation of modem valence bond theory with intrinsic reaction coordinate calculations provides easy-to-interpret models for the electronic rearrangements that occur along reaction pathways. We survey here the information revealed by such studies of the mechanisms of various gas-phase six-electron pericyclic reactions the Diels-Alder reaction between butadiene and ethene, the electrocyclization of cis-l,3,5-hexatriene, the 1,3-dipolar cycloaddition between fulminic acid and ethyne, and the 1,3-dipolar cycloaddition of diazomethane. The fully-variational CASVB strategy proves particularly efficient for such studies. 

A second, equally important, aspect of the theoretical modelling of chemical reactions is related to the elucidation of the often radical changes in the electronic structure of a reacting system as it evolves from reactants, through one or more transition structures and/or reaction intermediates, to one or more sets of products. There is a well-recognized need to develop qualitative models based on quantitative wavefunctions but highlighting general features and tendencies in chemical structure and reactivity this can be a very difficult task, especially if 

Most chemists still tend to think about the structure and reactivity of atomic and molecular species in qualitative terms that are related to electron pairs and to unpaired electrons. Concepts utilizing these terms such as, for example, the Lewis theory of valence, have had and still have a considerable impact on many areas of chemistry. They are particularly useful when it is necessary to highlight the qualitative similarities between the structure and reactivity of molecules containing identical functional groups, or within a homologous series. Many organic chemistry textbooks continue to use full and half-arrows to indicate the supposed movement of electron pairs or single electrons in the description of reaction mechanisms. Such concepts are closely related to classical valence-bond (VB) theory which, however, is unable to compete with advanced molecular orbital (MO) approaches in the accurate calculation of the quantitative features of the potential surface associated with a chemical reaction. 

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D. L. Cooper, P. B. Karadakov, T. Thorsteinsson, Modem Valence-Bond Description of Gas-Phase Pericyclic Reactions. In Theoretical and Computational Chemistry, volume 10, p. 41, D. L. Cooper, ed., Elsevier, Amsterdam (2002)... 

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