Ground state potential energy surface between cyclobutadiene and tetrahedrane looked down from S1/S0 conical intersections

We have explored the singlet ground state potential energy surface (S0 PES) between cyclobutadiene (CBD) and tetrahedrane (THD) looked down from S1/S0 conical intersections through multi-configuration self-consistent field theory. On the basis of the obtained S0 PES, we propose the revised process of the THD to CBD symmetry-forbidden reaction. According to the present result, the THD to CBD rearrangement occurs via plural steps similarly to previous suggestions, but via a tetra-radical species (instead of an endo-species), which is considered for the first time in this paper. Since the endo-species is significantly destabilized when hydrogen atoms are replaced by bulky substituents (such as tert-butyl group), the present one, where endo-species are not involved, would be realized in actual systems having bulky substituents.