Formation of the 2,3-Dimethyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reaction of the Silylidyne Radical (SiH; X²Π) with Dimethylacetylene (CH₃CCCH₃; X¹A_1g)

We carried out crossed molecular beam experiments and electronic structure calculations to unravel the chemical dynamics of the reaction of the silylidyne­(-<i>d</i>₁) radical (SiH/SiD; X²Π) with dimethylacetylene (CH₃CCCH₃; X¹A_1g). The chemical dynamics were indirect and initiated by the barrierless addition of the silylidyne radical to both carbon atoms of dimethylacetylene forming a cyclic collision complex 2,3-dimethyl-1-silacyclopropenyl. This complex underwent unimolecular decomposition by atomic hydrogen loss from the silicon atom via a loose exit transition state to form the novel 2,3-dimethyl-1-silacycloprop-2-enylidene isomer in an overall exoergic reaction (experimentally: −29 ± 21 kJ mol^–1; computationally: −10 ± 8 kJ mol^–1). An evaluation of the scattering dynamics of silylidyne with alkynes indicates that in each system, the silylidyne radical adds barrierlessly to one or to both carbon atoms of the acetylene moiety, yielding an acyclic or a cyclic collision complex, which can also be accessed via cyclization of the acyclic structures. The cyclic intermediate portrays the central decomposing complex, which fragments via hydrogen loss almost perpendicularly to the rotational plane of the decomposing complex exclusively from the silylidyne moiety via a loose exit transition state in overall weakly exoergic reaction leading to ((di)­methyl-substituted) 1-silacycloprop-2-enylidenes (−1 to −13 kJ mol^–1 computationally; −12 ± 11 to −29 ± 21 kJ mol^–1 experimentally). Most strikingly, the reaction dynamics of the silylidyne radical with alkynes are very different from those of C1–C4 alkanes and C2–C4 alkenes, which do not react with the silylidyne radical at the collision energies under our crossed molecular beam apparatus, due to either excessive entrance barriers to reaction (alkanes) or overall highly endoergic reaction processes (alkenes). Nevertheless, molecules carrying carbon–carbon double bonds could react, if the carbon–carbon double bond is either consecutive like in allene (H₂CCCH₂) or in conjugation with another carbon–carbon double bond (conjugated dienes) as found, for instance, in 1,3-butadiene (H₂CCHCHCH₂)

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

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

Combined Crossed Molecular Beam and Ab Initio Investigation of the Reaction of Boron Monoxide (BO; X²Σ⁺) with 1,3-Butadiene (CH₂CHCHCH₂; X¹A_g) and Its Deuterated Counterparts

The reactions of the boron monoxide (¹¹BO; X²Σ⁺) radical with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g) and its partially deuterated counterparts, 1,3-butadiene-<i>d</i>₂ (CH₂CDCDCH₂; X¹A_g) and 1,3-butadiene-<i>d</i>₄ (CD₂CHCHCD₂; X¹A_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 ¹¹BOC₄H₆ doublet radical intermediates via the barrierless addition of the ¹¹BO radical to the terminal carbon atom (C1/C4) and/or the central carbon atom (C2/C3) of 1,3-butadiene. The resulting long-lived ¹¹BOC₄H₆ intermediate(s) underwent isomerization and/or unimolecular decomposition involving eventually at least two distinct atomic hydrogen loss pathways to 1,3-butadienyl-1-oxoboranes (CH₂CHCHCH¹¹BO) and 1,3-butadienyl-2-oxoboranes (CH₂C (¹¹BO)­CHCH₂) in overall exoergic reactions via tight exit transition states. Utilizing partially deuterated 1,3-butadiene-<i>d</i>₂ and -<i>d</i>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%; these data agree nicely with the theoretically predicted branching ratio of 80% versus 19%