Electronic spectra of phenylcyclopropane and cumene cation radicals: Interplay of experiment and theory

The structures of phenylcyclopropane (1^+.) and cumene (2^+.) cation radicals were calculated using both CASSCF and B3LYP computational methods. Both methods predict that 1^+. adopts a delocalized, bisected structure and that the barrier to phenyl group rotation is substantial (11-14 kcal/mol). In contrast, the spin and charge in 2^+. is largely localized in the phenyl ring and there is no strongly preferred ground state conformation. The CASPT2 method was used to predict the electronic spectra of 1^+. and 2^+.. The large differences in the spectra of 1^+. and 2^+. can be traced to significant σ-π interaction in 1^+.. The results are rationalized in terms of a simple valence bond configuration mixing model. In general, the calculated transition energies were in good accord with the experimental values; however, the relative UV-vis peak intensities for 1^+. were not well reproduced if a static structure was assumed. Better agreement was obtained by taking into account libration of the phenyl group in 1^+..