Modern valence bond theory

The spin-coupled (SC) theory of molecular electronic structure is introduced as the modern development of classical valence bond (VB) theory. Various important aspects of the SC wave function are described. Attention is particularly focused on the construction and properties of different sets of N-electron spin function in different spin bases, such as the Kotani, Rumer and Serber. Applications of the SC description to a range of different kinds of chemical problems are presented, beginning with simple examples: the H2 and CH4 molecules. This is followed by the description offered by the SC wave function of more complex situations such as the insertion reaction of H2 into CH2(1A1), the phenomenon of hypervalence as displayed by molecules such as diazomethane, CH2N2, SF6 and XeF2. The SC description of the ground and excited states of benzene is briefly surveyed. This is followed by the SC description of antiaromatic systems such as C4H4 and related molecules.