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

Abstract

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