A Crossed Molecular Beam and Ab-Initio Investigation of the Reaction of Boron Monoxide (BO; XΣ) with Methylacetylene

The gas-phase reaction of boron monoxide (BO; XΣ) with methylacetylene (CH3CCH; XA1) was investigated experimentally using crossed molecular beam technique at a collision energy of 22.7 kJ mol−1 and theoretically using state of the art electronic structure calculation, 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-derWaals complex (BOC3H4) followed by isomerization via the addition of BO(XΣ) to the C1 and/or C2 carbon atom of methylacetylene through submerged barriers. The resulting BOC3H4 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 BO(XΣ) 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 (CH3CC BO) and propadienyl boron monoxide (CH2CCH BO), respectively. Addition of BO(XΣ) to the C1 of methylacetylene followed by the migration of the boronyl group to the C2 carbon atom and/or an initial addition of BO(XΣ) 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 (HCCBO) in an overall exoergic reaction (78 ± 23 kJ mol−1). The branching ratios of these channels forming CH2CCH BO, CH3CC BO, and HCCBO 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.