Reaction of Benzene and Boron Atom:  Mechanism of Formation of Benzoborirene and Hydrogen Atom

The reaction of C 6 D 6 + B( 2 P) is investigated by crossed molecular beam experiments at a collision energy of 5.5 kcal mol -1 and by electronic structure computations. The latter were performed employing hybrid Hartree−Fock/density functional theory (B3LYP), coupled cluster theory with single, double, and a perturbative estimate of triple excitations [CCSD(T)], complete active space self consistent field (CASSCF), and multiconfiguration quasi-degenerate perturbation theory to second order (MC-QDPT2) in conjunction with 6-31G*, 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 were characterized. One involves a weakly bound benzene−boron π complex and proceeds over a barrier, which lies below the energy of separated reactants, to an η 1 C 6 H 6 - B σ complex ( 4 ). The second channel is the symmetric attack (η 2 ) to a π bond of benzene. This addition mode following the 2 A‘ ‘ potential energy surface involves a valley ridge inflection (VRI) point and therefore results in 4 . This VRI point also makes the formation of the 7-boranorbornadiene-7-yl radical ( 6, 2 A‘) via nonadiabatic transition from 2 A‘ ‘ to 2 A‘ unlikely. The primary addition products 4 (and 6 ) can rearrange over barriers below the energy of separated reactants to finally reach the phenylboryl radical ( 10, 2 A‘). This is the most stable C 6 H 6 B species identified. Cleavage of an o -CH bond goes along with closure of a C−B bond and yields benzoborirene ( 1, 1 A 1 ) and hydrogen atom (−19.2 kcal mol -1; −16.7 kcal mol -1 for the [D 6 ]-benzene system). Abstraction of hydrogen by boron to produce phenyl radical ( 2 A 1 ) and borylene ( 1 Σ + ) is endoergic by +30.4 kcal mol -1 (C 6 D 5 + BD, +31.6 kcal mol -1 ) and is therefore not viable under our experimental conditions. The features of this C 6 D 6 B potential energy surface identified computationally no entrance barrier with respect to separated reactants, exoergicity of −16.7 kcal mol -1, transition state energy and structure in the exit channel (6.5 kcal mol -1 with respect to 1 + D) are in agreement with crossed-beam data (exoergicity −14.8 ± 1.2 kcal mol -1; exit channel barrier 2.4−4.8 kcal mol -1 ). Phenylborylene 2 and didehydroborepine 3 are alternative C 6 H 5 B species, but these are higher in energy than 1 by 32 and 43 kcal mol -1 and are therefore not formed in the crossed-beam experiment.