An ESR study of benzene radical cation in an argon matrix: evidence for favourable stabilization of 2B1g rather than 2B2g state

Benzene radical cations were generated in an argon matrix by fast electron irradiation at 16 K. The ESR spectrum measured immediately after irradiation results probably from the mixture of the two distorted states; however, it converts irreversibly to the spectrum of the 2B1g state upon annealing. The latter spectrum exhibits major hyperfine coupling with four of six protons (the isotropic coupling constant was estimated to be 0.64 mT). The role of basic Jahn–Teller distortion and matrix-assisted effects for benzene radical cations trapped in low-temperature matrices is discussed.