MODERN VALENCE-BOND DESCRIPTION OF CHEMICAL-REACTION MECHANISMS - DIELS-ALDER REACTION

The electronic mechanism for the gas-phase Diels-Alder cycloaddition r eaction is studied through a combination of modern valence-bond (VB) t heory in its spin-coupled (SC) form and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field ( CASSCF) wave function. Throughout the reaction, the nonorthogonal SC o rbitals resemble well-localized sp(x) hybrids, each of which remains p ermanently attached to a single carbon atom. The changes in the shapes of these SC orbitals, together with the variations of the overlaps be tween neighboring orbitals, produce a lucid picture of the parallel br eaking of the butadiene and ethene pi bonds and of the formation of th e two new sigma bonds, closing the ring, and of the cyclohexene pi bon d. The analogue of classical VB resonance, namely, the active-space sp in-coupling pattern within the SC wave function, shows no resonance we ll before and well after the transition structure (TS). At and around the TS, this pattern is dominated by two Kekule Rumer spin functions o f comparable weight. This and other resemblances to the well-known SC description of benzene (similar orbital shapes, equalization of the ov erlaps between neighboring orbitals) indicate clearly that the Diels-A lder reaction passes through a geometry, very close to the TS, at whic h it is aromatic. The visual changes in the SC wave function as the sy stem follows the reaction path strongly suggest that the best schemati c representation of the Diels-Alder reaction is through a ''homolytic' ' mechanism, in which six half-arrows indicate the simultaneous breaki ng of the three pi bonds on the reactants and formation of the three n ew bonds, two sigma and one pi, in the product.