Exploring the Reaction Paths on the Potential Energy Surfaces of the S and T States in Methylenecyclopropane.

The reaction paths of methylenecyclopropane 1 on the potential energy surfaces (PESs) of the lowest triplet (T ) state and the lowest excited singlet (S ) state, as well as that of the ground state (S ), were explored by using the nudged elastic band method at the MRMP2//MCSCF/6-31++G(d,p) and DFT(B3LYP)/6-31++G(d,p) levels of theory. After vertical excitation of 1, three transition states on the PES of the lowest triplet state and one transition state on the S PES were found along the reaction path to produce a carbene, cyclobutylidene 2. All of these transition states are lower in energy than the S state produced by vertical excitation at the S energy minimum in 1. Fast transition is predicted to occur from the T state or from the S state to the S state due to strong spin-orbit coupling or nonadiabatic coupling in the geometrical vicinity of 2. On the MRMP2 S PES, the energy barriers of 5.0, 10.3 and 13.5 kcal mol were obtained for C migration reaction (backward reaction), 1,2-H migration reaction to cyclobutene 3, and 1,3-H migration reaction to bicyclopropane 4, respectively, started at 2. The introduction of phenyl groups makes the energy barriers smaller due to the π conjugation between the carbene center and phenyl groups.