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

Abstract

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