The reaction of benzene with a ground state carbon atom, C(3Pj)

Abstract

The reaction between benzene and a single ground state carbon atom, C(3Pj), which yields a C7H5 radical without a barrier in the exit channel, has been studied using density functional theory (B3LYP), Møller–Plesset perturbation theory, and the G2(B3LYP/MP2) and complete basis set (CBS) model chemistries. Comparing the computed reaction energies for the formation of various C7H5 radicals with experimental data suggests that the 1,2-didehydrocycloheptatrienyl radical (15) is observed in crossed-beams experiments at collision energies between 2 and 12 kcal mol−1. The carbon atom attacks the π-electron density of benzene and forms without entrance barrier a Cs symmetric complex (17T) in which the carbon atom is bound to the edge of benzene. From 17T, the insertion of the C atom into a benzene CC bond to yield triplet cycloheptatrienylidene (9T) is associated with a much lower barrier than the insertion into a CH bond to give triplet phenylcarbene (7T). As both steps are strongly exothermic, high energy vinyl carbene rearrangements on the triplet C7H6 potential energy surface provide pathways between 9T and 7T below the energy of separated reactants. In addition, intersystem crossing in the vicinity of 17T and 9T might give rise to singlet cycloheptatetraene (12S). The monocyclic seven-membered ring compounds 9T or 12S are precursors of the 1,2-didehydrocycloheptatrienyl radical: the dissociation of a CH bond α to the divalent carbon atom proceeds without an exit barrier, in agreement with experiment. In contrast, a direct carbon–hydrogen exchange reaction pathway analogous to the aromatic electrophilic substitution followed by rearrangement of phenylcarbyne (13) to 15 involves high barriers (39 kcal mol−1 with respect to separated reactants) and is thus not viable under the experimental conditions.