Structure of dimeric radical cations of benzene and toluene in halocarbon matrices: an EPR, ENDOR and MO study

Abstract

The structure of dimeric cations of benzene and toluene formed in X-irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C₆H₆)₂⁺, exhibited hf couplings due to twelve equivalent protons and the isotropic coupling (a_iso) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl₃ matrix, whereas clear axially symmetric transitions with ∣A_‖∣<∣A_⊥∣ were obtained in CF₃CCl₃ even at 50 K. The rhombic dipolar coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH₃C₆H₅)₂⁺, ENDOR transitions of the CH₃ and H(4,4′) protons were observed. The isotropic hf coupling of the CH₃ protons deviated even more strongly from the half value, being rather close to one-third of the value of the monomer. The hf coupling of the H(4,4′) protons was almost half the coupling of the monomer. It was suggested that the anomalous hf couplings of the CH₃ protons were due to the interaction between the two rings through the σ-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH₃ protons hf coupling in the (CH₃C₆H₅)₂⁺ were successfully evaluated.