Isotope effect on the J–T distortion of partially deuteriated benzene cation radicals: an experimental EPR and theoretical DFT study

D/H isotope-substitution effects on the Jahn–Teller (J–T) distortion of cation radicals of partially deuteriated benzenes; benzene-d5, benzene-1,2,4,5-d4, benzene-1,4-d2, and benzene-1,3,5-d3, were investigated by electron spin resonance (EPR) using frozen haloalkanes and porous silicas as matrices. Quantum-chemical calculations using the density functional theory (DFT) method were also performed. The way of distortion of the cation radicals of selectively deuteriated benzenes is unique relative to the deuteriation sites in all the matrices used. The ground state of the distorted cation is 2B2g of D2h symmetry, regardless of the way of isotope labeling. The undeuteriated C–H bonds occupy the specific positions in the distorted form where the higher spin density appears, in agreement with the result of theoretical calculations. The role of the matrix to stabilize the distorted form is also discussed.