Ab initio study of the thermal isomerization of quadricyclane to norbornadiene

Abstract

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 C 1, whereas both Q and N belong to point group C 2V. 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.