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

It has been found that the benzene cation radical (Bz(+)) is produced by ra diolysis of a phenyl-substituted mesoporous silicate MCM-41 (Ph-MCM), and i t shows a static Jahn-Teller distortion even at 77 K. The distorted structu re deduced from the electron paramagnetic resonance (EPR) spectrum is almos t 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 Psi(s). On the other hand, Bz(+) produced by radiolysis of benzene molecul es adsorbed on Ph-MCM as well as on MCM-41 give dynamically averaged EPR sp ectra 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 tra pping sites of Bz(+) in the mesoporous silicates and the mechanism of stabi lization of the distorted form are discussed.