Theoretical Structures for the Phenyl and Benzyl Radicals

Although the spectroscopy, chemistry, and quantum chemistry of phenyl1-3 and benzyl4-6 radicals have received much attention, the equilibrium geometry of the ground state of the benzyl radical has not been reported. Johnson1 performed a series of interesting calculations on the ground and several excited states of the phenyl radical; an optimized structure was reported for the ground state but a C2v symmetry constraint and an STO-3G basis set were used. Since radical structure calculations using a STO-3G basis sets are only relatively reliable (for example, the methyl radical7 was found to be nonplanar when an STO-3G basis set was used) and Johnson1 restricted the geometry optimization by starting with a C2v symmetry, we decided to refine the phenyl ground-state geometry by using a better basis set and a lower initial symmetry. In addition, we computed second derivatives in order to confirm that the optimized structures correspond to minima on the energy surface. We also investigated the ground-state structure for benzene by using ab initio methods; a comparison of the theoretical benzene structure with the accepted experimental structure8 provides a useful benchmark for the quality of the calculated equilibrium geometries for the ground states of the phenyl and benzyl radicals. © 1986, American Chemical Society. All rights reserved.