ABSTRACT

Electronic structures of a series of substituted benzene radical cations produced in solid halocarbon matrices with ionizing irradiation were investigated by means of ESR and associated spectroscopic techniques. The precise hf coupling tensors of non-substituted benzene radical cation, C₆H₆⁺, with a ²B_2g state in a D_2h symmetry were obtained by ENDOR and single crystal ESR. The substitution of weakly perturbing deuterium for hydrogen, C₆H₅D⁺, showed that the deuterium does not considerably affect the electronic structure and dynamics of the benzene cation. Meanwhile, with strongly perturbing fluorine and methyl-group substituents removed the degeneracy (²E_1g) of the benzene cation to be either ²B_2g(Ψs) or ²B_1g(Ψ_a) types of electronic states. These effects were explained in terms of the electron repelling property of the fluorine and the methyl-group which induces π electron repulsion. The singly occupied electron orbitals (SOMO) of the cations were similar to the HOMO of the parent neutral molecules, indicating that the interaction between the substituents and the benzene cation is decisive for the electronic structures. When a relatively high amount of the solute is contained in the halocarbon matrices, dimer cation of benzene and toluene were also detected above 100 K. With the help of the density functional theory (DFT), structures of these cations were investigated based on ¹H isotropic and dipole couplings precisely determined by ENDOR. In ease of the toluene dimer cation, an anomalously small hf coupling of the methyl protons due to the interaction between the two equivalent partners was observed.