Exploring the Reaction Paths on the Potential Energy Surfaces of the S1 and T1 States in Methylenecyclopropane

The reaction paths of methylenecyclopropane 1 on the potential energy surfaces (PESs) of the lowest triplet (T1) state and the lowest excited singlet (S1) state, as well as that of the ground state (S0), 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 S1 PES were found along the reaction path to produce a carbene, cyclobutylidene 2. All of these transition states are lower in energy than the S1 state produced by vertical excitation at the S0 energy minimum in 1. Fast transition is predicted to occur from the T1 state or from the S1 state to the S0 state due to strong spin‐orbit coupling or nonadiabatic coupling in the geometrical vicinity of 2. On the MRMP2 S0 PES, the energy barriers of 5.0, 10.3 and 13.5 kcal mol−1 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. Methylenecyclopropane 1 is promoted to the lowest excited singlet state S1 by flash photolysis. Experimental results suggest that cyclobutylidene 2 is produced as an intermediate. The present theoretical investigation was performed for exploring the reaction paths on the potential energy surfaces of the S1 state, as well as that of the lowest triplet state T1. In order to estimate the rate constants for electronic transitions, the spin‐orbit couplings between S1 and T1 and between T1 and S0 were calculated by using the multiconfiguration self‐consistent field wave functions, in addition to the calculation of the nonadiabatic coupling between S1 and S0.