Modern Valence-Bond Description of Chemical Reaction Mechanisms: Diels−Alder Reaction

Abstract

The electronic mechanism for the gas-phase Diels−Alder cycloaddition reaction is studied through a combination of modern valence-bond (VB) theory 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 orbitals resemble well-localized spx hybrids, each of which remains permanently attached to a single carbon atom. The changes in the shapes of these SC orbitals, together with the variations of the overlaps between neighboring orbitals, produce a lucid picture of the parallel breaking of the butadiene and ethene π bonds and of the formation of the two new σ bonds, closing the ring, and of the cyclohexene π bond. The analogue of classical VB resonance, namely, the active-space spin-coupling pattern within the SC wave function, shows no resonance well before and well after the transition structure (TS). At and around the TS, this pattern is dominated by two Kekule ...