An EPR, ENDOR and ESEEM study of the benzene radical cation in CFCl3 matrix: isotopic substitution effects on structure and dynamics

Abstract

A combination of EPR lineshapes, ENDOR and ESEEM was performed to investigate possible Jahn–Teller effects in mono- and perdeuterated benzene radical cation in a polycrystalline CFCl₃ matrix. Replacement of one proton by a deuteron was utilized to exclude significantly fast quantum pseudorotation and overall rotary motion below 77 K. Clear evidence was obtained for static Jahn–Teller distortion at temperatures up to 30 K, with major spin densities on two para positions (1 and 4) in agreement with a b_2g(χ_s⁺) localized orbital. From ENDOR measurements of C₆H₅D⁺, the isotropic and dipolar coupling constants for protons occupying both high and low spin density positions were accurately measured. Additional ESEEM experiments and simulations confirmed the results, indicating further that deuteron-isotope substitution does not disturb appreciably the fundamental dynamics of the benzene radical benzene. The hyperfine tensors of C₆D₆⁺ in the Jahn–Teller distorted configuration obtained by ESEEM were analogous to those of fully protonated compounds, but the components were scaled by the magnetic moment ratio of deuterons to protons.