A Crossed Molecular Beam and Ab-Initio Investigation of the Reaction of Boron Monoxide (BO; X²Σ⁺) with Methylacetylene (CH₃CCH; X¹A₁): Competing Atomic Hydrogen and Methyl Loss Pathways

The gas-phase reaction of boron monoxide (¹¹BO; X²Σ⁺) with methylacetylene (CH₃CCH; X¹A₁) was investigated experimentally using crossed molecular beam technique at a collision energy of 22.7 kJ mol^–1 and theoretically <i>using state of the art electronic structure calculation</i>, for the first time. The scattering dynamics were found to be indirect (complex forming reaction) and the reaction proceeded through the barrier-less formation of a van-der-Waals complex (¹¹BOC₃H₄) followed by isomerization via the addition of ¹¹BO­(X²Σ⁺) to the C1 and/or C2 carbon atom of methylacetylene through submerged barriers. The resulting ¹¹BOC₃H₄ doublet radical intermediates underwent unimolecular decomposition involving three competing reaction mechanisms via two distinct atomic hydrogen losses and a methyl group elimination. Utilizing partially deuterated methylacetylene reactants (CD₃CCH; CH₃CCD), we revealed that the initial addition of ¹¹BO­(X²Σ⁺) to the C1 carbon atom of methylacetylene was followed by hydrogen loss from the acetylenic carbon atom (C1) and from the methyl group (C3) leading to 1-propynyl boron monoxide (CH₃CC¹¹BO) and propadienyl boron monoxide (CH₂CCH¹¹BO), respectively. Addition of ¹¹BO­(X²Σ⁺) to the C1 of methylacetylene followed by the migration of the boronyl group to the C2 carbon atom and/or an initial addition of ¹¹BO­(X²Σ⁺) to the sterically less accessible C2 carbon atom of methylacetylene was followed by loss of a methyl group leading to the ethynyl boron monoxide product (HCC¹¹BO) in an overall exoergic reaction (78 ± 23 kJ mol^–1). The branching ratios of these channels forming CH₂CCH¹¹BO, CH₃CC¹¹BO, and HCC¹¹BO were derived to be 4 ± 3%, 40 ± 5%, and 56 ± 15%, respectively; these data are in excellent agreement with the calculated branching ratios using statistical RRKM theory yielding 1%, 38%, and 61%, respectively

Combined Crossed Molecular Beam and ab Initio Investigation of the Multichannel Reaction of Boron Monoxide (BO; X²Σ⁺) with Propylene (CH₃CHCH₂; X¹A′): Competing Atomic Hydrogen and Methyl Loss Pathways

The reaction dynamics of boron monoxide (¹¹BO; X²Σ⁺) with propylene (CH₃CHCH₂; X¹A′) were investigated under single collision conditions at a collision energy of 22.5 ± 1.3 kJ mol^–1. The crossed molecular beam investigation combined with <i>ab initio</i> electronic structure and statistical (RRKM) calculations reveals that the reaction follows indirect scattering dynamics and proceeds via the barrierless addition of boron monoxide radical with its radical center located at the boron atom. This addition takes place to either the terminal carbon atom (C1) and/or the central carbon atom (C2) of propylene reactant forming ¹¹BOC₃H₆ intermediate(s). The long-lived ¹¹BOC₃H₆ doublet intermediate(s) underwent unimolecular decomposition involving at least three competing reaction mechanisms via an atomic hydrogen loss from the vinyl group, an atomic hydrogen loss from the methyl group, and a methyl group elimination to form <i>cis</i>-/<i>trans</i>-1-propenyl-oxo-borane (CH₃CHCH¹¹BO), 3-propenyl-oxo-borane (CH₂CHCH₂¹¹BO), and ethenyl-oxo-borane (CH₂CH¹¹BO), respectively. Utilizing partially deuterated propylene (CD₃CHCH₂ and CH₃CDCD₂), we reveal that the loss of a vinyl hydrogen atom is the dominant hydrogen elimination pathway (85 ± 10%) forming <i>cis</i>-/<i>trans</i>-1-propenyl-oxo-borane, compared to the loss of a methyl hydrogen atom (15 ± 10%) leading to 3-propenyl-oxo-borane. The branching ratios for an atomic hydrogen loss from the vinyl group, an atomic hydrogen loss from the methyl group, and a methyl group loss are experimentally derived to be 26 ± 8%:5 ± 3%:69 ± 15%, respectively; these data correlate nicely with the branching ratios calculated via RRKM theory of 19%:5%:75%, respectively

Combined Crossed Molecular Beam and Ab Initio Investigation of the Reaction of Boron Monoxide (BO; X²Σ⁺) with 1,3-Butadiene (CH₂CHCHCH₂; X¹A_g) and Its Deuterated Counterparts

The reactions of the boron monoxide (¹¹BO; X²Σ⁺) radical with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g) and its partially deuterated counterparts, 1,3-butadiene-<i>d</i>₂ (CH₂CDCDCH₂; X¹A_g) and 1,3-butadiene-<i>d</i>₄ (CD₂CHCHCD₂; X¹A_g), were investigated under single collision conditions exploiting a crossed molecular beams machine. The experimental data were combined with the state-of-the-art ab initio electronic structure calculations and statistical RRKM calculations to investigate the underlying chemical reaction dynamics and reaction mechanisms computationally. Our investigations revealed that the reaction followed indirect scattering dynamics through the formation of ¹¹BOC₄H₆ doublet radical intermediates via the barrierless addition of the ¹¹BO radical to the terminal carbon atom (C1/C4) and/or the central carbon atom (C2/C3) of 1,3-butadiene. The resulting long-lived ¹¹BOC₄H₆ intermediate(s) underwent isomerization and/or unimolecular decomposition involving eventually at least two distinct atomic hydrogen loss pathways to 1,3-butadienyl-1-oxoboranes (CH₂CHCHCH¹¹BO) and 1,3-butadienyl-2-oxoboranes (CH₂C (¹¹BO)­CHCH₂) in overall exoergic reactions via tight exit transition states. Utilizing partially deuterated 1,3-butadiene-<i>d</i>₂ and -<i>d</i>₄, we revealed that the hydrogen loss from the methylene moiety (CH₂) dominated with 70 ± 10% compared to an atomic hydrogen loss from the methylidyne group (CH) of only 30 ± 10%; these data agree nicely with the theoretically predicted branching ratio of 80% versus 19%