Ab initio studies of the ring-opening reaction of the cyclobutene radical cation

The ring-opening reaction of the cyclobutene radical cation was studied by using Hartree-Fock and density functional theory as well as various correlated MO methods. The species involved in three different mechanism-a concerted C-2-symmetric, a concerted C-1-symmetric, and a stepwise C-1-symmetric pathway- have been considered. It is shown that any symmetric pathway is dominated by state symmetry election rules which determine the outcome of the reaction. The concerted C-2-symmetric pathway is subject to a pseudo Jahn-Teller distortion and leads to trans-butadiene. This pathway is approximately 5 kcal/mol higher in energy than the concerted C-1-symmetric pathway, leading to cis-butadiene. Based on the results presented here, the stepwise C-1-symmetric or ''nonelectrocyclic'' pathway discussed in the literature is an artifact of an overestimation of the radical stability by MP2 and minimum basis set ab initio methods. It is therefore concluded that the ring opening of the cyclobutene radical cation proceeds via a concerted, unsymmetric pathway to cis-butadiene. The results from Becke3LYP calculations for geometries, energies, and isotropic hyperfine coupling constants are in good agreement with the available experimental data and with QCISD(T)/QCISD calculations. The results from the different methods are discussed with respect to the experimental data, and the implications of the data presented here for future computational studies of radical ion reactions are considered.