Tetra-radical and ionic S1/S0 conical intersections of cyclobutadiene

We have located two conical intersections between the first singlet excited (S-1) and singlet ground (S-0) states of cyclobutadiene (CBD) using the complete active space self-consistent field (CASSCF) method. One is the ionic-structure S-1/S-0 conical intersection (CIionic), which was located by carrying out a minimum-energy-path calculation from the Franck-Condon point of the HOMO to LUMO double-electron excited state, and the other is the tetra-radical S-1/S-0 conical intersection (CItetra), which was located by exploring the S-1/S-0 degeneracy space. While CIionic is only involved in the automerization of CBD, CItetra is involved in not only the automerization but also the criss-cross reaction. It is possible for one of the highest constrained compounds, tetrahedrane, to be produced if S-1 excited CBD undergoes a transition to the S-0 state via the tetra-radical S-1/S-0 conical intersection. In this paper, we discuss the possibility that unsubstituted tetrahedrane can be produced by irradiating CBD. (C) 2010 Elsevier B.V. All rights reserved.