New benzene dimers: an MRMP2 study

Abstract

The reaction energies of one cis-fused and four new trans-fused benzene dimers have been studied with state-specific multi-reference Møller-Plesset perturbation theory of the second order (MRMP2) based on reference CASSCF(12,12) wavefunctions. The cis-fused dimer 1, the product of symmetry-allowed [4 + 2] cycloaddition process, has been previously characterized experimentally and theoretically and, in this study, was used as reference. Dimer 1 lies 45 kcal/mol above two isolated benzene molecules, and its retro-dimerization activation energy is ~ 8 kcal/mol. In contrast, the [4 + 2] formation of trans-fused dimer 2 is symmetry-forbidden with an energetic driving force (E_rel) of 57.6 kcal/mol. An activation barrier for corresponding retro-dimerization calculated to be 31.0 kcal/mol. The thermal [2 + 2] formation of dimers 3 and 4 are also symmetry-forbidden. They have been calculated to be 79.0 kcal/mol and 105.1 kcal/mol higher in energy than two isolated benzene molecules, respectively. However, the activation energies of 23.2 and 20.5 kcal/mol could allow for kinetic persistence of these compounds. Dimer 5 is the only symmetry-allowed [2 + 2] cycloaddition product. With E_rel of 67.5 kcal/mol and E_a for retro-dimerization reaction of 19.2 kcal/mol, it shows high similarity with the symmetry-forbidden dimers. The wave function for each of the dimers at their local minimum geometry contains greater than 80% of the configuration $\sigma^{4} \pi^{8} \sigma^{*0} \pi^{*0}$. The transition state for the retro-dimerization of 1 (1TS) also contains more than 80% of the closed-shell configuration $\sigma^{4} \pi^{8} \sigma^{*0} \pi^{*0}$. For 2TS–5TS systems, the wavefunctions exhibit significant biradical character (20%, 20%, 14% and 26%, respectively).