Phenyl radical, cation, and anion. The triplet-singlet gap and higher excited states of the phenyl cation

Abstract

High-level ab initio molecular orbital calculations have been carried out for the phenyl cation (1), the phenyl radical (2), and the phenyl anion (3). Our best estimate for the heat of formation (ΔH(f298)) of the phenyl radical is 340 kJ mol-1, corresponding to 476 kJ mol-1 for the 298 K C-H bond dissociation energy in benzene. The calculated ΔH(f298) of the phenyl anion is 224 kJ mol-1, leading to an electron affinity for the phenyl radical of 116 kJ mol-1, and a gas-phase acidity for benzene of 1671 kJ mol-1. The ground state of the phenyl cation is found to be a singlet (1-1A1,ΔH(f298) = 1134 kJ mol-1), with the triplet (1-3B1, ΔH(f298) = 1237 kJ mol-1) lying significantly higher in energy (by 103 kJ mol-1). The energies of the 1-1A1 and 1-3B1 states of the phenyl cation at the phenyl radical geometry are much closer, but 1-1A1 remains the energetically lowest electronic state. The essentially isoenergetic 1-3A2 and 1-1A2 states lie about 25 kJ mol-1 higher in energy than 1-3B1, while 1-1B1 is nearly 60 kJ mol-1 higher in energy. The implications of these results with respect to recent spectroscopic studies of the phenyl radical and the role of the triplet phenyl cation in the dissociation of benzene cation are discussed.