Combined Crossed Molecular Beam and Ab Initio Investigation of the Reaction of Boron Monoxide (BO; X2Σ+) with 1,3-Butadiene (CH2CHCHCH2; X1Ag) and Its Deuterated Counterparts

The reactions of the boron monoxide ((BO)-B-11; X-2 Sigma(+)) radical with 1,3-butadiene (CH2CHCHCH2; X(1)A(g)) and its partially deuterated counterparts, D2-1,3-butadiene (CH2CDCDCH2; X1Ag) and D4-1,3-butadiene (CD2CHCHCD2; X(1)A(g)), 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 (BOC4H6)-B-11 doublet radical intermediates via the barrierless addition of the (BO)-B-11 radical to the terminal carbon atom (C1/C4) and/or the central carbon atom (C2/C3) of 1,3-butadiene. The resulting long lived (BOC4H6)-B-11 intermediate(s) under-went isomerization and/or unimolecular decomposition involving eventually at least two distinct atomic hydrogen loss pathways to 1,3-butadienyl-1-oxoboranes ((CH2CHCHCHBO)-B-11) and 1,3-butadienyl-2-oxoboranes (CH2C ((BO)-B-11)CHCH2) in overall exoergic reactions via tight exit transition states. Utilizing partially deuterated 1,3-butadiene-d(2)- and d(4), we revealed that the hydrogen loss from the methylene moiety (CH2) dominated with 70 +/- 10% compared to an atomic hydrogen loss from the methylidyne group (CH) of only 30 +/- 10%; this data agrees nicely with the theoretically predicted branching ratio of 80 % versus 19%.