Ab initio study of the thermal isomerization of tricyclo[3.1.0.0^2,6]hexane to (Z,Z)-1,3-cyclohexadiene through the (E,Z)-1,3-cyclohexadiene intermediate

The isomerization of tricyclo[3.1.0.0^2,6]hexane to 1,3-cyclohexadiene was studied using ab initio calculations at the multiconfiguration and single-configuration levels of theory. Single-point energy calculations were also performed using the MCQDPT2 and CCSD(T) methods. The isomerization process was found to proceed through an (E,Z)-1,3-cyclohexadiene intermediate following a concerted, asynchronous pathway characterized as a conrotatory ring opening of the bicyclobutane moiety. The second step involves rotation about the trans double bond resulting in the (Z,Z)-1,3-cyclohexadiene product. The rate determining step is the first one, with an activation barrier of about 43 kcal mol^-1. The activation barrier of the second step was found to be only 3 kcal mol^-1. A second concerted pathway was found leading directly to (Z,Z)-1,3-cyclohexadiene but with an activation barrier of 54 kcal mol^-1. A multiconfiguration based wave function is necessary to properly describe the potential energy surface of the reaction.