A combined crossed molecular beams and computational study on the formation of distinct resonantly stabilized C5H3 radicals via chemically activated C5H4 and C6H6 intermediates

A Combined Crossed Molecular Beam and Theoretical Investigation of the Elementary Reaction of Tricarbon (C3(X1Σg+)) with Diacetylene (C4H2(X1Σg+)): Gas Phase Formation of the Heptatriynylidyne Radical (l-C7H(X2Π))

An elucidation of the underlying formation pathways to acyclic hydrocarbons such as polyynes (CnH2), cumulenes (CnH2), and linear resonantly stabilized linear radicals (l-CnH) is indispensable to understand the hydrocarbon chemistry in extreme low- and high-temperature environments. In this study, we exploited the crossed molecular beam technique to investigate the reaction of tricarbon C3(X1Σg+) with diacetylene (butadiyne; HCCCCH; X1Σg+) at a collision energy of 47 ± 1 kJ mol-1. The experimental data were merged with ab initio calculations of the singlet C7H2 potential energy surface (PES) revealing that the reaction is initiated via the formation of an initial van der Waals reactant complex in the entrance channel. Subsequent rearrangements lead to various carbene-type and cyclic intermediates via ring-opening, ring-closure, and hydrogen migration processes, eventually forming acyclic C7H2 isomers prior to their barrierless unimolecular decomposition to the most stable linear isomer, heptatriynylidyne (C7H, X2Π) in an overall endoergic reaction (+57 kJ mol-1). The reaction exhibits strong similarities to the tricarbon-acetylene (C3-C2H2). The significant energy threshold suggests that the tricarbon reaction with (poly)acetylenes forming resonantly stabilized linear radicals is open in high-temperature environments such as combustion flames and circumstellar envelopes of carbon stars and planetary nebulae as their descendants; however, these reactions are closed in low-temperature environments as in cold molecular clouds and hydrocarbon-rich atmospheres of planets and their moons such as in Titan.

Identification of the Elusive Methyl-Loss Channel in the Crossed Molecular Beam Study of Gas-Phase Reaction of Dicarbon Molecules (C2; X1Σg+/a3Πu) with 2-Methyl-1,3-butadiene (C5H8; X1A')

The crossed molecular beam technique was utilized to explore the reaction of dicarbon C2 (X1Σg+/a3Πu) with 2-methyl-1,3-butadiene (isoprene, CH2C(CH3)CHCH2; X1A') at a collision energy of 28 ± 1 kJ mol-1 using a supersonic dicarbon beam generated via photolysis (248 nm) of helium-seeded tetrachloroethylene (C2Cl4). Experimental data combined with previous ab initio calculations provide evidence of the detection of the hitherto elusive methyl elimination channels leading to acyclic resonantly stabilized hexatetraenyl radicals: 1,2,4,5-hexatetraen-3-yl (CH2CC•CHCCH2) and/or 1,3,4,5-hexatetraen-3-yl (CH2CHC•CCCH2). These pathways are exclusive to the singlet potential energy surface, with the reaction initiated by the barrierless addition of a dicarbon to one of the carbon-carbon double bonds in the diene. In combustion systems, both hexatetraenyl radicals can isomerize to the phenyl radical (C6H5) through a hydrogen atom-assisted isomerization─the crucial reaction intermediate and molecular mass growth species step toward the formation of polycyclic aromatic hydrocarbons and soot.

Spiers Memorial Lecture: New directions in molecular scattering

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

Crossed Beam Experiments and Computational Studies of Pathways to the Preparation of Singlet Ethynylsilylene (HCCSiH; X1A'): The Silacarbene Counterpart of Triplet Propargylene (HCCCH; X3B)

Ethynylsilylene (HCCSiH; X¹A') has been prepared in the gas phase through the elementary reaction of singlet dicarbon (C₂) with silane (SiH₄) under single-collision conditions. Electronic structure calculations reveal a barrierless reaction pathway involving 1,1-insertion of dicarbon into one of the silicon-hydrogen bonds followed by hydrogen migration to form the 3-sila-methylacetylene (HCCSiH₃) intermediate. The intermediate undergoes unimolecular decomposition through molecular hydrogen loss to ethynylsilylene (HCCSiH; C_s; X¹A'). The dicarbon-silane system defines a benchmark to explore the consequence of a single collision between the simplest "only carbon" molecule (dicarbon) with the prototype of a closed-shell silicon hydride (silane) yielding a nonclassical silacarbene, whose molecular geometry and electronic structure are quite distinct from the isovalent triplet propargylene (HCCCH; C₂; ³B) carbon-counterpart. These organosilicon transients cannot be prepared through traditional organic, synthetic methods, thus opening up a versatile path to access the previously largely elusive class of silacarbenes.

Theoretical Study of the Reaction of the Methylidyne Radical (CH; X2Π) with 1-Butyne (CH3CH2CCH; X1A')

Ab initio CCSD(T)-F12/cc-pVTZ-f12//ωB97X-D/6-311G(d,p) + ZPE[ωB97X-D/6-311G(d,p)] calculations were carried out to unravel the area of the C₅H₇ potential energy surface accessed by the reaction of the methylidyne radical with 1-butyne. The results were utilized in Rice-Ramsperger-Kassel-Marcus calculations of the product branching ratios at the zero pressure limit. The preferable reaction mechanism has been shown to involve (nearly) instantaneous decomposition of the initial reaction adducts, whose structures are controlled by the isomeric form of the C₄H₆ reactant. If CH adds to the triple C≡C bond in the entrance reaction channel, the reaction is predicted to predominantly form the methylenecyclopropene + methyl (CH₃) and cyclopropenylidene + ethyl (C₂H₅) products roughly in a 2:1 ratio. CH insertion into a C-H bond in the methyl group of 1-butyne is anticipated to preferentially form ethylene + propargyl (C₃H₃) by the C-C bond β-scission in the initial complex, whereas CH insertion into C-H of the CH₂ group would predominantly produce vinylacetylene + methyl (CH₃) also by the C-C bond β-scission in the adduct. The barrierless and highly exoergic CH + 1-butyne reaction, facile in cold molecular clouds, is not likely to lead to the carbon skeleton molecular growth but generates C₄H₄ isomers methylenecyclopropene, vinylacetylene, and 1,2,3-butatriene and smaller C₂ and C₃ hydrocarbons such as methyl, ethyl, and propargyl radicals, ethylene, and cyclopropenylidene.

Reaction mechanisms of C(3PJ) and C+(2PJ) with benzene in the interstellar medium from quantum mechanical molecular dynamics simulations

While spectroscopic data on small hydrocarbons in interstellar media in combination with crossed molecular beam (CMB) experiments have provided a wealth of information on astrochemically relevant species, much of the underlying mechanistic pathways of their formation remain elusive. Therefore, in this work, the chemical reaction mechanisms of C(3PJ) + C6H6 and C+(2P) + C6H6 systems using the quantum mechanical molecular dynamics (QMMD) technique at the PBE0-D3(BJ) level of theory is investigated, mimicking a CMB experiment. Both the dynamics of the reactions as well as the electronic structure for the purpose of the reaction network are evaluated. The method is validated for the first reaction by comparison to the available experimental data. The reaction scheme for the C(3PJ) + C6H6 system covers the literature data, e.g. the major products are the 1,2-didehydrocycloheptatrienyl radical (C7H5) and benzocyclopropenyl radical (C6H5-CH), and it reveals the existence of less common pathways for the first time. The chemistry of the C+(2PJ) + C6H6 system is found to be much richer, and we have found that this is because of more exothermic reactions in this system in comparison to those in the C(3PJ) + C6H6 system. Moreover, using the QMMD simulation, a number of reaction paths have been revealed that produce three distinct classes of reaction products with different ring sizes. All in all, at all the collision energies and orientations, the major product is the heptagon molecular ion for the ionic system. It is also revealed that the collision orientation has a dominant effect on the reaction products in both systems, while the collision energy mostly affects the charged system. These simulations both prove the applicability of this approach to simulate crossed molecular beams, and provide fundamental information on reactions relevant for the interstellar medium.

Gas-Phase Synthesis of 3-Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X2 Π) with 1,3-Butadiene (CH2 CHCHCH2 ; X1 Ag )

The crossed molecular beam reactions of the methylidyne radical (CH; X² Π) with 1,3-butadiene (CH₂ CHCHCH₂ ; X¹ A_g ) along with their (partially) deuterated counterparts were performed at collision energies of 20.8 kJ mol^-1 under single collision conditions. Combining our laboratory data with ab initio calculations, we reveal that the methylidyne radical may add barrierlessly to the terminal carbon atom and/or carbon-carbon double bond of 1,3-butadiene, leading to doublet C₅ H₇ intermediates with life times longer than the rotation periods. These collision complexes undergo non-statistical unimolecular decomposition through hydrogen atom emission yielding the cyclic cis- and trans-3-vinyl-cyclopropene products with reaction exoergicities of 119±42 kJ mol^-1 . Since this reaction is barrierless, exoergic, and all transition states are located below the energy of the separated reactants, these cyclic C₅ H₆ products are predicted to be accessed even in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and cold molecular clouds such as TMC-1.

Gas Phase Formation of the Interstellar Molecule Methyltriacetylene

Methyltriacetylene - the largest methylated polyacetylene detected in deep space - has been synthesized in the gas phase via the bimolecular reaction of the 1-propynyl radical with diacetylene under single-collision conditions. The barrier-less route to methyltriacetylene represents a prototype of a polyyne chain extension through a radical substitution mechanism and provides a novel low temperature route, in which the propynyl radical piggybacks a methyl group to be incorporated into methylated polyynes. This mechanism overcomes a key obstacle in previously postulated reactions of methyl radicals with unsaturated hydrocarbon, which fail the inclusion of methyl groups into hydrocarbons due to insurmountable entrance barriers thus providing a fundamental understanding on the electronic structure, chemical bonding, and formation of methyl-capped polyacetylenes. These species are key reactive intermediates leading to carbonaceous nanostructures in molecular clouds like TMC-1.