Modeling photochemical (4 + 4) cycloadditions: conical intersections located with CASSCF for butadiene + butadiene

Ab initio molecular orbital calculations were performed to study the (4 + 4) photocycloaddition of two butadienes. Specifically, the minima, transition structures and conical intersections were determined at the CASSCF /4-31G level. The resulting structure, which underscored the need to modify the general model for (4 + 4) cycloadditions based on H4 potential energy surfaces, was found to be similar to those obtained from the addition of ethylene to benzene and methyl migration to but-1-ene.