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

Abstract

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 computationallysno 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)sare 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.

A Crossed Molecular Beam Study on the Reaction of Boron Atoms with Methylacetylene and Partially Deuterated Methylacetylene

“…[28] Finally, the formation of p3 requires an initial insertion of atomic boron into a carbon-hydrogen bond of the [D3]methyl group followed by a deuterium atom emission. As shown previously in the crossed beam reactions of boron atoms with benzene, [25][26][27] dimethylacetylene, [28] ethylene, [22] and acetylene, [23,24] insertion reactions do not take place. Likewise, we conducted experiments of atomic boron with methane at a collision energy of about 25 kJ mol…”

“…The third boron reaction studied in crossed beams experiments was that with benzene (C 6 H 6 ) and [D6]benzene (C 6 D 6 ). [25][26][27] After successive isomerization and rearrangements of the initial addition complex, an atomic hydrogen/deuterium elimination yielded the [D5]benzoborirene molecule [ Figure 1; (3)]. Again, an insertion into a carbon-hydrogen bond was not feasible.…”

On the Directed Gas Phase Synthesis of the Imidoborane Molecule (HNBH) — An Isoelectronic Molecule of Acetylene (HCCH)

“…This finding likely indicates geometrical constraints of the decomposing complex, here a preferential hydrogen atom ejection parallel to the total angular momentum vector and perpendicularly to the plane defined by the rotating complex. 37 The above characteristics are also revealed in the flux contour map (Figure 4). …”

An LIF characterization of supersonic BO (X2Σ+) and CN (X2Σ+) radical sources for crossed beam studies

“…Taking into account its stability under combustion conditions, the reactions of boron monoxide with combustion species such as hydrocarbons deserve considerable attention. Although the reaction dynamics of boron atoms with hydrocarbon molecules 2 such as acetylene (C 2 H 2 ), 3-5 ethylene (C 2 H 4 ), 6, 7 benzene (C 6 H 6 ), 8,9 allene (C 3 H 2 ), 10 dimethylacetylene (CH 3 C 2 CH 3 ), 11 and methylacetylene (CH 3 C 2 H) (Ref. 12) were unraveled utilizing the crossed molecular beam approach, the reaction dynamics of boron monoxide (BO; X 2 + ) with hydrocarbons have neither been investigated experimentally nor theoretically to date.…”