MCSCF study of the thermal isomerization of highly-strained polycyclic hydrocarbons.

摘要

The thermal isomerization of tricyclo[4.1.0.0 2,7]heptane, tricyclo[5.1.0.02 2,8]octane, (E,E)-1,3-cycloheptadiene, and quadricyclane have been studied using ab initio calculations at the multiconfiguration self-consistent, field level. Single point energy corrections were also performed using the MCQDPT2 and CCSD(T) methods. Two tricyclic compounds are shown to thermolyze through the (E,Z)-1,3-cyclodiene intermediate via an allowed, ansynchronous, conrotatory pathway. This is the first definitive study on the role of the intermediate in such a reaction process. The energy barriers were found to be 40 kcal·mol−1 for the seven carbon structure and 43 kcal·mol−1 for the eight one. Starting from the intermediate, two pathways exist: one is electrocyclic ring closure to bicycloalkene, and the second is trans double bond rotation to the cis structure. The former pathway has a lower energy barrier, which makes bicyclo[3.2.0]hept-6-ene and bicyclo[4.2.0]oct 2-ene dominant in the product mixtures, respectively. As the size of the intermediate ring increases, the barrier for trans double bond rotation increases from 3 kcal·mol−1 for the six carbon, to 20 kcal·mol−1 for the seven carbon, to 33 kcal·mol−1{09}for the eight carbon. This is explained in terms of ring strain due to the trans double bond. (E,E)-1,3-cycloheptadiene, the smallest ring with two conjugated trans double bonds, is reported for the first time. The barrier for its rotation to a trans cis structure is 7 kcal·mol−1, while the barrier to form trans-bicyclo[3.2.0]hept-6-ene is 13 kcal·mol −1. The thermal isomerization of quadricyclane to norbornadiene was calculated to have a 33 kcal·mol−1 barrier, very close to the 34 kcal·mol−1 experimental value. The transition state is almost a pure biradical and the reaction follows an asynchronous forbidden pathway. All pathways were verified by intrinsic reaction coordinate calculations. The degree of configuration mixing in the wavefunetion has been examined, showing the necessary of a multiconfigurational method to properly describe the potential energy surface.