Abstract: The Jahn-Teller distorted states, ²B_2g and ²B_1g of the benzene radical cation have been studied by using ab initio MPn and SDCImethods. The calculations indicate that ²B_2g is the ground state of the benzene cation and that ²B_1g is higher in energy than₂ B 2g and represents a saddle point. The calculated isotropic and anisotropic hyperfine coupling constants for the ²B_2g state are in excellent agreement with those obtained from the ESRexperiments. It is concluded that the benzene cation is in the ²B_2g Jahn-Teller distorted state.

Key words: Benzene cation, Jahn-Teller effct, ab initio MPn, Hyperfine structure