A General Approach to Organic Reactivity: The Configuration Mixing Model

The aim of this chapter is to present in some detail a simple, qualitative framework for understanding the factors which go into the creation of a reaction profile. Because the question raised is so fundamental —“What determines the barrier in any chemical reaction?”—the model encompasses within its single structure reactions as different as electron-transfer reactions. It brings under one roof both thermal and photochemical processes. The theory termed as the “configuration mixing (CM) model,” or when valence-bond configurations are solely utilized, the “valence-bond configuration mixing (VBCM) model” has been applied over recent years to many of these mentioned reaction types. In view of the fundamental role played by quantum theory in modern chemistry, it is not surprising that the theoretical basis of the CM model goes back to the earliest days of quantum chemistry. In fact, the basic concept of CM theory may be considered a theoretical precursor of resonance theory. The chapter concludes that the notion that a molecule “hesitates” between different structures, borrows its characteristics, and finally adopts a structure that is somewhat intermediate between them, is central to understanding “electronic behavior.” It is from this single yet powerful principle that CM theory borrows its utility and generality.