Photocyclization paths of butadiene. An ab initio MO study

The potential energy surfaces for a cyclic reaction of s-cis-butadiene at the ground and excited states were calculated by the CAS-SCF molecular orbital method. The avoided crossing path produces s-cis-butadiene but does not lead to cyclobutene. For the conical intersection path, the stereo selectivity (disrotatory path for the exited reaction) is controlled on the potential energy surface of the excited state. The transition state at the ground state is located in the vicinity of the conical intersection, which connects to s-cis-butadiene and cyclobutene.