Benzene cation radical in mesoporous silicate: EPR detection of unusually stabilized Jahn–Teller distortion

Abstract

It has been found that the benzene cation radical (Bz⁺) is produced by radiolysis of a phenyl-substituted mesoporous silicate MCM-41 (Ph-MCM), and it shows a static Jahn–Teller distortion even at 77 K. The distorted structure deduced from the electron paramagnetic resonance (EPR) spectrum is almost equal to that reported for the cation radical observed in CFCl₃ at 4.2 K, the singly occupied molecular orbital (SOMO) of which has been assigned to Ψ_s. On the other hand, Bz⁺ produced by radiolysis of benzene molecules adsorbed on Ph-MCM as well as on MCM-41 give dynamically averaged EPR spectra at 77 K, although the dynamics are frozen at 4.2 K. On radiolysis of "‘as-synthesized’' phenyl-MCM, not Bz⁺ but C₆H₅⁺–Si–R is generated. The trapping sites of Bz⁺ in the mesoporous silicates and the mechanism of stabilization of the distorted form are discussed.