Ab initio study of the thermal isomerization of quadricyclane to norbornadiene

CASSCF calculations have been performed to understand the thermal isomerization of quadricyclane (Q) to norbornadiene (N) which occurs via a Woodward-Hoffmann forbidden pathway. The activation barrier is calculated to be 34 kcal mol−1 at the MCQDPT2/6-31G(d,p)//MCSCF(8,8)/6-31G(d,p) level, consistent with the 33 kcal mol−1 experimental value of Frey [H.M. Frey, J. Am. Chem. Soc. (1964) 365]. The transition state has high biradical character and the structure belongs to point group C1, whereas both Q and N belong to point group C2V. The reaction is found to be highly asynchronous with one bond cleaving entirely before the transition state and the second bond cleaving afterward on the intrinsic reaction coordinate.