Combined Crossed Molecular Beam and ab Initio Investigation of the Multichannel Reaction of Boron Monoxide (BO; X super(2) capital sigma super(+)) with Propylene (CH sub(3)CHCH sub(2); X super(1)A'): Competing Atomic Hydrogen and Methyl Loss Pathways

The reaction dynamics of boron monoxide ( super(11)BO; X super(2) capital sigma super(+)) with propylene (CH sub(3)CHCH sub(2); X super(1)A') were investigated under single collision conditions at a collision energy of 22.5 plus or minus 1.3 kJ mol super(-1). The crossed molecular beam investigation combined with ab initio 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 super(11)BOC sub(3)H sub(6) intermediate(s). The long-lived super(11)BOC sub(3)H sub(6) 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 cis-/trans-1-propenyl-oxo-borane (CH sub(3)CHCH super(11)BO), 3-propenyl-oxo-borane (CH sub(2)CHCH sub(2) super(11)BO), and ethenyl-oxo-borane (CH sub(2)CH super(11)BO), respectively. Utilizing partially deuterated propylene (CD sub(3)CHCH sub(2) and CH sub(3)CDCD sub(2)), we reveal that the loss of a vinyl hydrogen atom is the dominant hydrogen elimination pathway (85 plus or minus 10%) forming cis-/trans-1-propenyl-oxo-borane, compared to the loss of a methyl hydrogen atom (15 plus or minus 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 plus or minus 8%:5 plus or minus 3%:69 plus or minus 15%, respectively; these data correlate nicely with the branching ratios calculated via RRKM theory of 19%:5%:75%, respectively.