Combined
Crossed Molecular Beam and Ab Initio Investigation
of the Reaction of Boron Monoxide (BO; X<sup>2</sup>Σ<sup>+</sup>) with 1,3-Butadiene (CH<sub>2</sub>CHCHCH<sub>2</sub>; X<sup>1</sup>A<sub>g</sub>) and Its Deuterated Counterparts
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Abstract
The reactions of the boron monoxide
(<sup>11</sup>BO; X<sup>2</sup>Σ<sup>+</sup>) radical with 1,3-butadiene
(CH<sub>2</sub>CHCHCH<sub>2</sub>; X<sup>1</sup>A<sub>g</sub>) and
its partially deuterated
counterparts, 1,3-butadiene-<i>d</i><sub>2</sub> (CH<sub>2</sub>CDCDCH<sub>2</sub>; X<sup>1</sup>A<sub>g</sub>) and 1,3-butadiene-<i>d</i><sub>4</sub> (CD<sub>2</sub>CHCHCD<sub>2</sub>; X<sup>1</sup>A<sub>g</sub>), were investigated under single collision conditions
exploiting a crossed molecular beams machine. The experimental data
were combined with the state-of-the-art ab initio electronic structure
calculations and statistical RRKM calculations to investigate the
underlying chemical reaction dynamics and reaction mechanisms computationally.
Our investigations revealed that the reaction followed indirect scattering
dynamics through the formation of <sup>11</sup>BOC<sub>4</sub>H<sub>6</sub> doublet radical intermediates via the barrierless addition
of the <sup>11</sup>BO radical to the terminal carbon atom (C1/C4)
and/or the central carbon atom (C2/C3) of 1,3-butadiene. The resulting
long-lived <sup>11</sup>BOC<sub>4</sub>H<sub>6</sub> intermediate(s)
underwent isomerization and/or unimolecular decomposition involving
eventually at least two distinct atomic hydrogen loss pathways to
1,3-butadienyl-1-oxoboranes (CH<sub>2</sub>CHCHCH<sup>11</sup>BO)
and 1,3-butadienyl-2-oxoboranes (CH<sub>2</sub>C (<sup>11</sup>BO)CHCH<sub>2</sub>) in overall exoergic reactions via tight exit transition
states. Utilizing partially deuterated 1,3-butadiene-<i>d</i><sub>2</sub> and -<i>d</i><sub>4</sub>, we revealed that
the hydrogen loss from the methylene moiety (CH<sub>2</sub>) dominated
with 70 ± 10% compared to an atomic hydrogen loss from the methylidyne
group (CH) of only 30 ± 10%; these data agree nicely with the
theoretically predicted branching ratio of 80% versus 19%