A crossed molecular beam and ab initio investigation of the exclusive methyl loss pathway in the gas phase reaction of boron monoxide (BO; X2Σ+) with dimethylacetylene (CH3CCCH3; X1A1g)

Abstract

The crossed molecular beam reaction of boron monoxide (¹¹BO; X²Σ⁺) with dimethylacetylene (CH₃CCCH₃; X¹A_1g) was investigated at a collision energy of 23.9 ± 1.5 kJ mol⁻¹. The scattering dynamics were suggested to be indirect (complex forming reaction) and were initiated by the addition of ¹¹BO(X²Σ⁺) with the radical center located at the boron atom to the π electron density at the acetylenic carbon–carbon triple bond without entrance barrier leading to cis–trans¹¹BOC₄H₆ doublet radical intermediates. cis-¹¹BOC₄H₆ underwent cis–trans isomerization followed by unimolecular decomposition via a methyl group (CH₃) loss forming 1-propynyl boron monoxide (CH₃CC¹¹BO) in an overall exoergic reaction (experimental: −91 ± 22 kJ mol⁻¹; theoretical: −105 ± 9 kJ mol⁻¹; NIST: −104 ± 12 kJ mol⁻¹) via a tight exit transition state; trans-¹¹BOC₄H₆ was found to lose a methyl group instantaneously. Neither atomic nor molecular hydrogen loss pathways were detectable. The experimental finding of an exclusive methyl loss pathway gains full support from our computational study predicting a methyl group versus atomic hydrogen loss branching ratio of 99.99% to 0.01% forming 1-propynyl boron monoxide (CH₃CC¹¹BO) and 1-methyl-propadienyl boron monoxide (CH₃(¹¹BO)CCCH₂), respectively.