An MC-SCF study of the thermal and photochemical cycloaddition of Dewar benzene

The thermal and photochemical transformation of Dewar benzene (1) to prismane (2) is studied using MC-SCF methods with a 4-31G basis. The thermal synchronous [2s + 2s] process does not exist because the "antiaromatic" transition state has two imaginary frequencies. Rather, the preferred thermal mechanism corresponds to a cisoid quasi one-step asynchronous process (transition state, diradical minimum, transition state separated by less than 2 kcal mol-1). A pathway leading to a transoid diradical intermediate does exist but has an energy that is so high that it is of no chemical significance. This diradical is a nonreactive diradical since the radical centers cannot close via rotation about the cross bond. However this path is photochemically accessible. The "classical" [2s + 2s] structure on the excited-state surface that has a geometry similar to the "anti-aromatic" transition state on the ground state is not an avoided crossing minimum but rather a transition state. The central feature of the photochemical mechanism is a conical intersection between ground and excited-state surfaces that hes between Dewar benzene and prismane. The system undergoes a fully efficient return to the ground state via this conical intersection and can proceed either to prismane or to Dewar benzene. The factors that control the topology of the ground- and excited-state potential surfaces have been analyzed using a VB model. The exchange energy shows clearly the origin of the conical intersection. The constraint of the cagelike framework of the molecule manifests itself in the Coulomb energy. This fact may explain why only the adiabatic photochemical reaction is observed in 1,4-Dewar naphthalene where the sigma-bond framework cannot distort (without breaking the aromaticity of the second ring) to the conical intersection geometry observed in the [2 + 2] photochemical transformation of Dewar benzene to prismane.