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 +• .