| Abstract
| - The reaction of C6D6 + B(2P) is investigated by crossed molecular beam experiments at a collision energyof 5.5 kcal mol-1 and by electronic structure computations. The latter were performed employing hybridHartree−Fock/density functional theory (B3LYP), coupled cluster theory with single, double, and a perturbativeestimate of triple excitations [CCSD(T)], complete active space self consistent field (CASSCF), andmulticonfiguration quasi-degenerate perturbation theory to second order (MC-QDPT2) in conjunction with6-31G*, 6-311+G**, and cc-pVTZ basis sets. Final energies were obtained at the CCSD(T)/cc-pVTZ//B3LYP/6-311+G** + ZPVE level of theory. Two possible addition channels to the benzene π system werecharacterized. One involves a weakly bound benzene−boron π complex and proceeds over a barrier, whichlies below the energy of separated reactants, to an η1 C6H6-B σ complex (4). The second channel is thesymmetric attack (η2) to a π bond of benzene. This addition mode following the 2A‘ ‘ potential energy surfaceinvolves a valley ridge inflection (VRI) point and therefore results in 4. This VRI point also makes theformation of the 7-boranorbornadiene-7-yl radical (6, 2A‘) via nonadiabatic transition from 2A‘ ‘ to 2A‘ unlikely.The primary addition products 4 (and 6) can rearrange over barriers below the energy of separated reactantsto finally reach the phenylboryl radical (10, 2A‘). This is the most stable C6H6B species identified. Cleavageof an o-CH bond goes along with closure of a C−B bond and yields benzoborirene (1, 1A1) and hydrogenatom (−19.2 kcal mol-1; −16.7 kcal mol-1 for the [D6]-benzene system). Abstraction of hydrogen by boronto produce phenyl radical (2A1) and borylene (1Σ+) is endoergic by +30.4 kcal mol-1 (C6D5 + BD, +31.6kcal mol-1) and is therefore not viable under our experimental conditions. The features of this C6D6B potentialenergy surface identified computationallyno entrance barrier with respect to separated reactants, exoergicityof −16.7 kcal mol-1, transition state energy and structure in the exit channel (6.5 kcal mol-1 with respect to1 + D)are in agreement with crossed-beam data (exoergicity −14.8 ± 1.2 kcal mol-1; exit channel barrier2.4−4.8 kcal mol-1). Phenylborylene 2 and didehydroborepine 3 are alternative C6H5B species, but these arehigher in energy than 1 by 32 and 43 kcal mol-1 and are therefore not formed in the crossed-beam experiment.
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