Product Distributions from Molecular Mechanics−Valence Bond Dynamics: Modeling Photochemical [4 + 4] Cycloadditions

The purpose of this paper is to study the model [4 + 4] photocycloaddition of butadiene + butadiene by using direct dynamics calculations with no geometric constraints to describe motion along excited-state reaction paths and subsequent decay to the ground state. We use the molecular mechanics−valence bond (MMVB) potential, which is calibrated against previous CASSCF calculations for this system (Bearpark, M. J.; Deumal, M.; Robb, M. A.; Vreven, T.; Yamamoto, N.; Olivucci, M.; Bernardi, F. J. Am. Chem. Soc. 1997, 119, 709−718). Our dynamics calculations show that efficient nonradiative decay of butadiene + butadiene in the presence of two different S 1 / S 0 conical intersections can account for the formation of many products. The major product predicted by MMVB is consistent with the limited experimental data available.