A Combined Crossed Beam and Ab Initio Investigation of the Gas Phase Reaction of Dicarbon Molecules (C₂; X¹Σ_g⁺/a³Π_u) with Propene (C₃H₆; X¹A′): Identification of the Resonantly Stabilized Free Radicals 1- and 3‑Vinylpropargyl

The crossed molecular beam reactions of dicarbon, C₂(X¹Σ_g⁺, a³Π_u), with propene (C₃H₆; X¹A′) and with the partially deuterated D3 counterparts (CD₃CHCH₂, CH₃CDCD₂) were conducted at collision energies of about 21 kJ mol^–1 under single collision conditions. The experimental data were combined with ab initio and statistical (RRKM) calculations to reveal the underlying reaction mechanisms. Both on the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the addition of the dicarbon reactant to the carbon–carbon double bond of propene. These initial addition complexes rearrange via multiple isomerization steps leading ultimately via atomic hydrogen elimination from the former <i>methyl</i> and <i>vinyl</i> groups to the formation of 1-vinylpropargyl and 3-vinylpropargyl. Both triplet and singlet methylbutatriene species were identified as important reaction intermediates. On the singlet surface, the unimolecular decomposition of the reaction intermediates was found to be barrier-less, whereas on the triplet surface, tight exit transition states were involved. In combustion flames, both radicals can undergo a hydrogen-atom assisted isomerization leading ultimately to the thermodynamically most stable cyclopentadienyl isomer. Alternatively, in a third body process, a subsequent reaction of 1-vinylpropargyl or 3-vinylpropargyl radicals with the propargyl radical might yield to the formation of styrene (C₆H₅C₂H₃) in <i>an entrance barrier-less</i> reaction under combustion-like conditions. This presents a strong alternative to the formation of styrene via the reaction of phenyl radicals with ethylene, which is affiliated with an entrance barrier of about 10 kJ mol^–1

A Combined Experimental and Theoretical Study on the Formation of the 2‑Methyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reactions of the Silylidyne Radical (SiH; X²Π) with Methylacetylene (CH₃CCH; X¹A₁) and D4-Methylacetylene (CD₃CCD; X¹A₁)

The bimolecular gas-phase reactions of the ground-state silylidyne radical (SiH; X²Π) with methylacetylene (CH₃CCH; X¹A₁) and D4-methylacetylene (CD₃CCD; X¹A₁) were explored at collision energies of 30 kJ mol^–1 under single-collision conditions exploiting the crossed molecular beam technique and complemented by electronic structure calculations. These studies reveal that the reactions follow indirect scattering dynamics, have no entrance barriers, and are initiated by the addition of the silylidyne radical to the carbon–carbon triple bond of the methylacetylene molecule either to one carbon atom (C1; [i1]/[i2]) or to both carbon atoms concurrently (C1–C2; [i3]). The collision complexes [i1]/[i2] eventually isomerize via ring-closure to the c-SiC₃H₅ doublet radical intermediate [i3], which is identified as the decomposing reaction intermediate. The hydrogen atom is emitted almost perpendicularly to the rotational plane of the fragmenting complex resulting in a sideways scattering dynamics with the reaction being overall exoergic by −12 ± 11 kJ mol^–1 (experimental) and −1 ± 3 kJ mol^–1 (computational) to form the cyclic 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC₃H₄; <b>p1</b>). In line with computational data, experiments of silylidyne with D4-methylacetylene (CD₃CCD; X¹A₁) depict that the hydrogen is emitted solely from the silylidyne moiety but not from methylacetylene. The dynamics are compared to those of the related D1-silylidyne (SiD; X²Π)–acetylene (HCCH; X¹Σ_g⁺) reaction studied previously in our group, and from there, we discovered that the methyl group acts primarily as a spectator in the title reaction. The formation of 2-methyl-1-silacycloprop-2-enylidene under single-collision conditions via a bimolecular gas-phase reaction augments our knowledge of the hitherto poorly understood silylidyne (SiH; X²Π) radical reactions with small hydrocarbon molecules leading to the synthesis of organosilicon molecules in cold molecular clouds and in carbon-rich circumstellar envelopes

Combined Experimental and Theoretical Study on the Formation of the Elusive 2‑Methyl-1-silacycloprop-2-enylidene Molecule under Single Collision Conditions via Reactions of the Silylidyne Radical (SiH; X²Π) with Allene (H₂CCCH₂; X¹A₁) and D4-Allene (D₂CCCD₂; X¹A₁)

The crossed molecular beam reactions of the ground-state silylidyne radical (SiH; X²Π) with allene (H₂CCCH₂; X¹A₁) and D4-allene (D₂CCCD₂; X¹A₁) were carried out at collision energies of 30 kJ mol^–1. Electronic structure calculations propose that the reaction of silylidyne with allene has no entrance barrier and is initiated by silylidyne addition to the π electron density of allene either to one carbon atom (C1/C2) or to both carbon atoms simultaneously via indirect (complex forming) reaction dynamics. The initially formed addition complexes isomerize via two distinct reaction pathways, both leading eventually to a cyclic SiC₃H₅ intermediate. The latter decomposes through a loose exit transition state via an atomic hydrogen loss perpendicularly to the plane of the decomposing complex (sideways scattering) in an overall exoergic reaction (experimentally: −19 ± 13 kJ mol^–1; computationally: −5 ± 3 kJ mol^–1). This hydrogen loss yields the hitherto elusive 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC₃H₄), which can be derived from the closed-shell cyclopropenylidene molecule (c-C₃H₂) by replacing a hydrogen atom with a methyl group and the carbene carbon atom by the isovalent silicon atom. The synthesis of the 2-methyl-1-silacycloprop-2-enylidene molecule in the bimolecular gas-phase reaction of silylidyne with allene enriches our understanding toward the formation of organosilicon species in the gas phase of the interstellar medium in particular via exoergic reactions of no entrance barrier. This facile route to 2-methyl-1-silacycloprop-2-enylidene via a silylidyne radical reaction with allene opens up a versatile approach to form hitherto poorly characterized silicon-bearing species in extraterrestrial environments; this reaction class might represent the missing link, leading from silicon-bearing radicals via organosilicon chemistry eventually to silicon–carbon-rich interstellar grains even in cold molecular clouds where temperatures are as low as 10 K

Gas-Phase Synthesis of Phenyl Oxoborane (C₆H₅BO) via the Reaction of Boron Monoxide with Benzene

Organyl oxoboranes (RBO) are valuable reagents in organic synthesis due to their role in Suzuki coupling reactions. However, organyl oxoboranes (RBO) are only found in trimeric forms (RBO₃) commonly known as boronic acids or boroxins; obtaining their monomers has proved a complex endeavor. Here, we demonstrate an oligomerization-free formation of organyl oxoborane (RBO) monomers in the gas phase by a radical substitution reaction under single-collision conditions in the gas phase. Using the cross molecular beams technique, phenyl oxoborane (C₆H₅BO) is formed through the reaction of boronyl radicals (BO) with benzene (C₆H₆). The reaction is indirect, initially forming a van der Waals complex that isomerizes below the energy of the reactants and eventually forming phenyl oxoborane by hydrogen emission in an overall exoergic radical–hydrogen atom exchange mechanism

Formation of 6‑Methyl-1,4-dihydronaphthalene in the Reaction of the <i>p</i>‑Tolyl Radical with 1,3-Butadiene under Single-Collision Conditions

Crossed molecular beam reactions of <i>p</i>-tolyl (C₇H₇) plus 1,3-butadiene (C₄H₆), <i>p</i>-tolyl (C₇H₇) plus 1,3-butadiene-<i>d</i>₆ (C₄D₆), and <i>p</i>-tolyl-<i>d</i>₇ (C₇D₇) plus 1,3-butadiene (C₄H₆) were carried out under single-collision conditions at collision energies of about 55 kJ mol^–1. 6-Methyl-1,4-dihydronaphthalene was identified as the major reaction product formed at fractions of about 94% with the monocyclic isomer (<i>trans</i>-1-<i>p</i>-tolyl-1,3-butadiene) contributing only about 6%. The reaction is initiated by <i>barrierless</i> addition of the <i>p</i>-tolyl radical to the terminal carbon atom of the 1,3-butadiene via a van der Waals complex. The collision complex isomerizes via cyclization to a bicyclic intermediate, which then ejects a hydrogen atom from the bridging carbon to form 6-methyl-1,4-dihydronaphthalene through a tight exit transition state located about 27 kJ mol^–1 above the separated products. This is the dominant channel under the present experimental conditions. Alternatively, the collision complex can also undergo hydrogen ejection to form <i>trans</i>-1-<i>p</i>-tolyl-1,3-butadiene; this is a minor contributor to the present experiment. The de facto barrierless formation of a methyl-substituted aromatic hydrocarbons by dehydrogenation via a single event represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated analogues in combustion flames and the interstellar medium

A Crossed Molecular Beam and Ab-Initio Investigation of the Reaction of Boron Monoxide (BO; X²Σ⁺) with Methylacetylene (CH₃CCH; X¹A₁): Competing Atomic Hydrogen and Methyl Loss Pathways

The gas-phase reaction of boron monoxide (¹¹BO; X²Σ⁺) with methylacetylene (CH₃CCH; X¹A₁) was investigated experimentally using crossed molecular beam technique at a collision energy of 22.7 kJ mol^–1 and theoretically <i>using state of the art electronic structure calculation</i>, for the first time. The scattering dynamics were found to be indirect (complex forming reaction) and the reaction proceeded through the barrier-less formation of a van-der-Waals complex (¹¹BOC₃H₄) followed by isomerization via the addition of ¹¹BO­(X²Σ⁺) to the C1 and/or C2 carbon atom of methylacetylene through submerged barriers. The resulting ¹¹BOC₃H₄ doublet radical intermediates underwent unimolecular decomposition involving three competing reaction mechanisms via two distinct atomic hydrogen losses and a methyl group elimination. Utilizing partially deuterated methylacetylene reactants (CD₃CCH; CH₃CCD), we revealed that the initial addition of ¹¹BO­(X²Σ⁺) to the C1 carbon atom of methylacetylene was followed by hydrogen loss from the acetylenic carbon atom (C1) and from the methyl group (C3) leading to 1-propynyl boron monoxide (CH₃CC¹¹BO) and propadienyl boron monoxide (CH₂CCH¹¹BO), respectively. Addition of ¹¹BO­(X²Σ⁺) to the C1 of methylacetylene followed by the migration of the boronyl group to the C2 carbon atom and/or an initial addition of ¹¹BO­(X²Σ⁺) to the sterically less accessible C2 carbon atom of methylacetylene was followed by loss of a methyl group leading to the ethynyl boron monoxide product (HCC¹¹BO) in an overall exoergic reaction (78 ± 23 kJ mol^–1). The branching ratios of these channels forming CH₂CCH¹¹BO, CH₃CC¹¹BO, and HCC¹¹BO were derived to be 4 ± 3%, 40 ± 5%, and 56 ± 15%, respectively; these data are in excellent agreement with the calculated branching ratios using statistical RRKM theory yielding 1%, 38%, and 61%, respectively

Are Nonadiabatic Reaction Dynamics the Key to Novel Organosilicon Molecules? The Silicon (Si(³P))–Dimethylacetylene (C₄H₆(X¹A_1g)) System as a Case Study

The bimolecular gas phase reaction of ground-state silicon (Si; ³P) with dimethylacetylene (C₄H₆; X¹A_1g) was investigated under single collision conditions in a crossed molecular beams machine. Merged with electronic structure calculations, the data propose nonadiabatic reaction dynamics leading to the formation of singlet SiC₄H₄ isomer(s) and molecular hydrogen (H₂) via indirect scattering dynamics along with intersystem crossing (ISC) from the triplet to the singlet surface. The reaction may lead to distinct energetically accessible singlet SiC₄H₄ isomers (^<b>1</b><b>p8</b>–^<b>1</b><b>p24</b>) in overall exoergic reaction(s) (−107_–20⁺¹² kJ mol^–1). All feasible reaction products are either cyclic, carry carbene analogous silylene moieties, or carry C–Si–H or C–Si–C bonds that would require extensive isomerization from the initial collision complex­(es) to the fragmenting singlet intermediate(s). The present study demonstrates the first successful crossed beams study of an exoergic reaction channel arising from bimolecular collisions of silicon, Si­(³P), with a hydrocarbon molecule

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%