Tetra-radical and ionic S 1/S 0 conical intersections of cyclobutadiene

Abstract

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 (CI ionic), 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 (CI tetra), which was located by exploring the S 1/S 0 degeneracy space. While CI ionic is only involved in the automerization of CBD, CI tetra 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.