Theoretical study of the thermal isomerization of fulvene to benzene

The potential energy surface for the thermal isomerization of fulvene to benzene was studied by modified Gaussian-2 (G2M) and the bond additivity-corrected fourth-order perturbation Møller-Plesset (BAC-MP4) methods. Three isomerization pathways were investigated. One involves the intermediate prefulvene by a concerted mechanism, which has a significantly higher barrier. The second, also involving prefulvene and cyclopenta-1,3-dienylcarbene intermediates, has a barrier of 84·0 kcal mol^-1. The third, a multi-step pathway, includes bicyclo[3.1.0]hexa-1,3-diene and cyclohexadiene carbene intermediates. The activation energy of the multi-step pathway was calculated to be 74·3 kcal mol^-1, which is 7-11 kcal mol^-1 higher than the experimental value obtained by a brief very low-pressure pyrolysis (VLPP) study. RRKM calculations were performed on the multi-step pathway in order to determine the rate of isomerization. These theoretical results cast doubt on the validity of the VLPP data.