Jahn-Teller Effect of the Benzene Radical Cation: A Direct ab Initio Molecular Dynamics Study

The benzene radical cation (Bz⁺) is a typical model molecule of the Jahn-Teller (J-T) active species. Bz⁺ has two structural forms due to the J-T effect. These are the compressed and elongated forms, expressed as Bz⁺(comp) and Bz⁺(elong), respectively. In Bz⁺(comp), the hexagonal structure of the benzene ring is compressed up and down, and in Bz⁺(elong), it is pulled up and down. From electron spin resonance experiments, it was found that Bz⁺ takes a compressed form in low-temperature Freon matrices (CF₃Cl and CF₂ClCFCl₂), whereas the elongated form was found in argon matrices. However, the selectivity of these structural forms is still unclear. In this study, the ionization dynamics of isolated benzene (Bz) and benzene-M complexes (where M denotes counter-molecules, M = NH₃, H₂O, CF₃Cl, CH₄, CH₃OH, Ar, SH₂, ammonia dimer, or water dimer) have been investigated by means of the direct ab initio molecular dynamics (AIMD) method in order to shed light on the Bz⁺ formation mechanism. The static ab initio calculations showed that Bz⁺(comp) is slightly more energetically stable than Bz⁺(elong), although the energy difference was only 0.1 kcal/mol at the CCSD/6-311++G(d,p) level. The direct AIMD calculations indicated that Bz⁺(comp) was formed from the Bz-M complexes when M was NH₃, CF₃Cl, or an ammonia dimer, whereas the ionization of Bz-M when M was H₂O, CH₄, CH₃OH, SH_2, or a water dimer formed Bz⁺(elong). In the case of complexes with an argon dimer, Bz(Ar)₂, both forms were obtained from a slight orientation change of Ar on Bz. A selective rule is discussed on the basis of the calculated results.