Exploring the Chemical Dynamics of Phenylethynyl Radical (C6H5CC; X2A1) Reactions with Allene (H2CCCH2; X1A1) and Methylacetylene (CH3CCH; X1A1)

The bimolecular gas-phase reactions of the phenylethynyl radical (C₆H₅CC, X²A₁) with allene (H₂CCCH₂), allene-d₄ (D₂CCCD₂), and methylacetylene (CH₃CCH) were studied under single-collision conditions utilizing the crossed molecular beams technique and merged with electronic structure and statistical calculations. The phenylethynyl radical was found to add without an entrance barrier to the C1 carbon of the allene and methylacetylene reactants, resulting in doublet C₁₁H₉ collision complexes with lifetimes longer than their rotational periods. These intermediates underwent unimolecular decomposition via atomic hydrogen loss through tight exit transition states in facile radical addition─hydrogen atom elimination mechanisms forming predominantly 3,4-pentadien-1-yn-1-ylbenzene (C₆H₅CCCHCCH₂) and 1-phenyl-1,3-pentadiyne (C₆H₅CCCCCH₃) in overall exoergic reactions (-110 kJ mol^-1 and -130 kJ mol^-1) for the phenylethynyl-allene and phenylethynyl-methylacetylene systems, respectively. These barrierless reaction mechanisms mirror those of the ethynyl radical (C₂H, X²Σ⁺) with allene and methylacetylene forming predominantly ethynylallene (HCCCHCCH₂) and methyldiacetylene (HCCCCCH₃), respectively, suggesting that in the aforementioned reactions the phenyl group acts as a spectator. These molecular mass growth processes are accessible in low-temperature environments such as cold molecular clouds (TMC-1) or Saturn's moon Titan, efficiently incorporating a benzene ring into unsaturated hydrocarbons.

Synthesis of interstellar propen-2-ol (CH3C(OH)CH2) - the simplest enol tautomer of a ketone

Enols - tautomers of ketones or aldehydes - are anticipated to be ubiquitous in the interstellar medium and play a key role in the formation of complex organic molecules in deep space, but their fundamental formation mechanisms have remained largely elusive as of now. Here we present a combined experimental and computational study demonstrating the first preparation of propen-2-ol (CH₃C(OH)CH₂) and its isomer methyl vinyl ether (CH₃OCHCH₂) in low-temperature acetone (CH₃COCH₃) ices upon exposure to energetic electrons. Propen-2-ol is the simplest enol tautomer of a ketone. Exploiting tunable vacuum ultraviolet photoionization in conjunction with reflectron time-of-flight mass spectrometry, propen-2-ol and methyl vinyl ether were monitored in the gas phase upon sublimation during the temperature-programmed desorption process suggesting that both isomers are promising candidates for future astronomical searches such as via the James Webb Space Telescope. Electronic structure calculations reveal that the barrier of keto-enol tautomerization can be reduced by more than a factor of two (162 kJ mol^-1) through the involvement of solvating water molecules under realistic conditions on interstellar grains. The implicit solvent effects, i.e., the influences of the solvent dipole field on the barrier height are found to be minimal and do not exceed 10 kJ mol^-1. Our findings signify a crucial step toward a better understanding of the enolization of ketones in the interstellar medium thus constraining the molecular structures and complexity of molecules that form in extraterrestrial ices - ketones - through non-equilibrium chemistry.

Quantum Tunneling Mediated Low-Temperature Synthesis of Interstellar Hemiacetals

Sugars and sugar-related molecules are ubiquitous in carbonaceous meteorites and in star-forming regions, but the underlying mechanisms of their formation have remained largely elusive. Herein, we report an unconventional synthesis of the hemiacetal, (R/S)-1-methoxyethanol (CH₃OCH(OH)CH₃), through quantum tunneling mediated reactions in low-temperature interstellar model ices composed of acetaldehyde (CH₃CHO) and methanol (CH₃OH). The detection of racemic 1-methoxyethanol through a bottom-up synthesis from simple, abundant precursor molecules within interstellar ices represents a vital starting point to the formation of complex interstellar hemiacetals. Once synthesized, hemiacetals may act as possible precursors to interstellar sugars and sugar-related molecules in deep space.

Unconventional gas-phase preparation of the prototype polycyclic aromatic hydrocarbon naphthalene (C10H8) via the reaction of benzyl (C7H7) and propargyl (C3H3) radicals coupled with hydrogen-atom assisted isomerization

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium and in meteorites such as Murchison and Allende and signify the missing link between resonantly stabilized free radicals and carbonaceous nanoparticles (soot particles, interstellar grains). However, the predicted lifetime of interstellar PAHs of some 10⁸ years imply that PAHs should not exist in extraterrestrial environments suggesting that key mechanisms of their formation are elusive. Exploiting a microchemical reactor and coupling these data with computational fluid dynamics (CFD) simulations and kinetic modeling, we reveal through an isomer selective product detection that the reaction of the resonantly stabilized benzyl and the propargyl radicals synthesizes the simplest representative of PAHs - the 10π Hückel aromatic naphthalene (C₁₀H₈) molecule - via the novel Propargyl Addition-BenzAnnulation (PABA) mechanism. The gas-phase preparation of naphthalene affords a versatile concept of the reaction of combustion and astronomically abundant propargyl radicals with aromatic radicals carrying the radical center at the methylene moiety as a previously passed over source of aromatics in high temperature environments thus bringing us closer to an understanding of the aromatic universe we live in.

Formation of Thioformic Acid (HCOSH)─The Simplest Thioacid─in Interstellar Ice Analogues

Since the observation of the first sulfur-containing molecule, carbon monosulfide (CS), in the interstellar medium (ISM) half a century ago, sulfur-bearing species have attracted great attention from the astrochemistry, astrobiology, and planetary geology communities. Nevertheless, it is still not clear in which forms most of the sulfur resides in molecular clouds, an unsolved problem referred to as "sulfur depletion". Reported herein is the formation of thioformic acid (HCOSH)─the simplest thioacid─in interstellar ice analogues containing carbon monoxide (CO) and hydrogen sulfide (H₂S) at 5 K. Utilizing single photoionization reflectron time-of-flight mass spectrometry and isotopically labeled molecules, thioformic acid molecules were selectively photoionized in the temperature-programmed desorption phase. These studies unravel a key reaction pathway to thioformic acid, an organic molecule recently detected toward the giant molecular cloud G+0.693-0.027 and the hot core G31.41+0.31, thus shedding light on interstellar sulfur chemistry.

Gas-phase synthesis of racemic helicenes and their potential role in the enantiomeric enrichment of sugars and amino acids in meteorites

The molecular origins of homochirality on Earth is not understood well, particularly how enantiomerically enriched molecules of astrobiological significance like sugars and amino acids might have been synthesized on icy grains in space preceding their delivery to Earth. Polycyclic aromatic hydrocarbons (PAHs) identified in carbonaceous chondrites could have been processed in molecular clouds by circularly polarized light prior to the depletion of enantiomerically enriched helicenes onto carbonaceous grains resulting in chiral islands. However, the fundamental low temperature reaction mechanisms leading to racemic helicenes are still unknown. Here, by exploiting synchrotron based molecular beam photoionization mass spectrometry combined with electronic structure calculations, we provide compelling testimony on barrierless, low temperature pathways leading to racemates of [5] and [6]helicene. Astrochemical modeling advocates that gas-phase reactions in molecular clouds lead to racemates of helicenes suggesting a pathway for future astronomical observation and providing a fundamental understanding for the origin of homochirality on early Earth.

Gas-Phase Preparation of Subvalent Germanium Monoxide (GeO, X1+) via Non-Adiabatic Reaction Dynamics in the Exit Channel

The subvalent germanium monoxide (GeO, X¹Σ⁺) molecule has been prepared via the elementary reaction of atomic germanium (Ge, ³P_j) and molecular oxygen (O₂, X³Σ_g^-) with each reactant in its electronic ground state by means of single-collision conditions. The merging of electronic structure calculations with crossed beam experiments suggests that the formation of germanium monoxide (GeO, X¹Σ⁺) commences on the singlet surface through unimolecular decomposition of a linear singlet collision complex (GeOO, i1, C_∞v, ¹Σ⁺) via intersystem crossing (ISC) yielding nearly exclusively germanium monoxide (GeO, X¹Σ⁺) along with atomic oxygen in its electronic ground state [p1, O(³P)]. These results provide a sophisticated reaction mechanism of the germanium-oxygen system and demonstrate the efficient "heavy atom effect" of germanium in ISC yielding (nearly) exclusive singlet germanium monoxide and triplet atomic oxygen compared to similar systems (carbon dioxide and dinitrogen monoxide), in which non-adiabatic reaction dynamics represent only minor channels.

Gas-Phase Study of the Elementary Reaction of the D1-Ethynyl Radical (C2D; X2Σ+) with Propylene (C3H6; X1A') under Single-Collision Conditions

The bimolecular gas-phase reactions of the D1-ethynyl radical (C₂D; X²Σ⁺) with propylene (C₃H₆; X¹A') and partially substituted D3-3,3,3-propylene (C₂H₃CD₃; X¹A') were studied under single collision conditions utilizing the crossed molecular beams technique. Combining our laboratory data with electronic structure and statistical calculations, the D1-ethynyl radical is found to add without barrier to the C1 and C2 carbons of the propylene reactant, resulting in doublet C₅H₆D intermediate(s) with lifetime(s) longer than their rotational period(s). These intermediates undergo isomerization and unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming predominantly cis/trans-3-penten-1-yne ((HCC)CH═CH(CH₃)) and, to a minor amount, 3-methyl-3-buten-1-yne ((HCC)C(CH₃)═CH₂) via overall exoergic reactions. Although the title reaction does not lead to the cyclopentadiene molecule (c-C₅H₆, X¹A₁), high-temperature environments can convert the identified acyclic C₅H₆ isomers through hydrogen atom assisted isomerization to cyclopentadiene (c-C₅H₆, X¹A₁). Since both the ethynyl radical and propylene reactants have been observed in cold interstellar environments such as TMC-1 and the reaction is exoergic and all barriers lie below the energy of the separated reactants, these C₅H₆ product isomers are predicted to form in those low-temperature regions.

Unconventional excited-state dynamics in the concerted benzyl (C7H7) radical self-reaction to anthracene (C14H10)

Polycyclic aromatic hydrocarbons (PAHs) are prevalent in deep space and on Earth as products in combustion processes bearing direct relevance to energy efficiency and environmental remediation. Reactions between hydrocarbon radicals in particular have been invoked as critical molecular mass growth processes toward cyclization leading to these PAHs. However, the mechanism of the formation of PAHs through radical – radical reactions are largely elusive. Here, we report on a combined computational and experimental study of the benzyl (C₇H₇) radical self-reaction to phenanthrene and anthracene (C₁₄H₁₀) through unconventional, isomer-selective excited state dynamics. Whereas phenanthrene formation is initiated via a barrierless recombination of two benzyl radicals on the singlet ground state surface, formation of anthracene commences through an exotic transition state on the excited state triplet surface through cycloaddition. Our findings challenge conventional wisdom that PAH formation via radical-radical reactions solely operates on electronic ground state surfaces and open up a previously overlooked avenue for a more “rapid” synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

The reaction of benzyl radical self-reaction to anthracene opens-up a previously overlooked avenue for a more efficient synthesis of aromatic, multi-ringed structures via excited state dynamics in the gas phase.

Formation of Benzene and Naphthalene through Cyclopentadienyl-Mediated Radical-Radical Reactions

Resonantly stabilized free radicals (RSFRs) have been contemplated as fundamental molecular building blocks and reactive intermediates in molecular mass growth processes leading to polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanoparticles on Earth and in deep space. By combining molecular beams and computational fluid dynamics simulations, we provide compelling evidence on the formation of benzene via the cyclopentadienyl-methyl reaction and of naphthalene through the cyclopentadienyl self-reaction, respectively. These systems offer benchmarks for the conversion of a five-membered ring to the 6π-aromatic (benzene) and the generation of the simplest 10π-PAH (naphthalene) at elevated temperatures. These results uncover molecular mass growth processes from the "bottom up" via RSFRs in high temperature circumstellar environments and combustion systems expanding our fundamental knowledge of the organic, hydrocarbon chemistry in our universe.

A chemical dynamics study of the reaction of the methylidyne radical (CH, X2Π) with dimethylacetylene (CH3CCCH3, X1A1g)

The gas-phase reaction of the methylidyne (CH; X²Π) radical with dimethylacetylene (CH₃CCCH₃; X¹A_1g) was studied at a collision energy of 20.6 kJ mol^-1 under single collision conditions with experimental results merged with ab initio calculations of the potential energy surface (PES) and ab initio molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C₅H₇ reaction intermediate(s) ultimately dissociating to C₅H₆ isomers along with atomic hydrogen with atomic hydrogen predominantly released from the methyl groups as verified by replacing the methylidyne with the D1-methylidyne reactant. AIMD simulations reveal that the reaction dynamics are statistical leading predominantly to p28 (1-methyl-3-methylenecyclopropene, 13%) and p8 (1-penten-3-yne, 81%) plus atomic hydrogen with a significant amount of available energy being channeled into the internal excitation of the polyatomic reaction products. The dynamics are controlled by addition to the carbon-carbon triple bond with the reaction intermediates eventually eliminating a hydrogen atom from the methyl groups of the dimethylacetylene reactant forming 1-methyl-3-methylenecyclopropene (p28). The dominating pathways reveal an unexpected insertion of methylidyne into one of the six carbon-hydrogen single bonds of the methyl groups of dimethylacetylene leading to the acyclic intermediate, which then decomposes to 1-penten-3-yne (p8). Therefore, the methyl groups of dimethylacetylene effectively 'screen' the carbon-carbon triple bond from being attacked by addition thus directing the dynamics to an insertion process as seen exclusively in the reaction of methylidyne with ethane (C₂H₆) forming propylene (CH₃C₂H₃). Therefore, driven by the screening of the triple bond, one propynyl moiety (CH₃CC) acts in four out of five trajectories as a spectator thus driving an unexpected, but dominating chemistry in analogy to the methylidyne - ethane system.

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus - Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C₄ H₄ to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals. The o-C₆ H₄ +C₄ H₄ reaction is highly exothermic, by ∼143 kcal/mol to form naphthalene and by 31-32 kcal mol^-1 to produce naphthyl radicals plus H, but features relatively high entrance barriers of 9-11 kcal mol^-1 . Although the reaction is rather slow, much slower than the reaction of phenyl radical with vinylacetylene, it forms naphthalene and 1- and 2-naphthyl radicals directly, with their relative yields controlled by the temperature and pressure, and thus represents a viable source of the naphthalene core under conditions where ortho-benzyne and vinylacetylene are available.

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.

Ozone destruction due to the recombination of oxygen atoms

Kinetics of ozone destruction due to the recombination of oxygen atoms produced by pulsed 266 nm laser photolysis of O₃/M (M = CO₂ and/or N₂) mixtures was studied using the absorption and emission spectroscopy to follow time evolutions of O₃ and electronically excited molecules O₂* formed in the recombination process 2O(³P) + M → O₂* + M. An unexpected high ozone destruction rate was observed when O₂* was present in the system. The kinetic model developed for the oxygen nightglow on the terrestrial planets was adapted to interpret the detected temporal profiles of the ozone number density and the O₂* emission intensities. It was deduced that the vibrationally excited singlet delta oxygen molecule O₂(a¹Δ, υ) formed in the secondary processes reacts efficiently with ozone in the process O₂(a¹Δ, υ ≥ 3) + O₃ → 2O₂ + O, and the rate constant of this process was estimated to be 3 × 10^-11 cm³ s^-1. Ab initio calculations at the CASPT2(14, 12)/cc-pVTZ/UωB97XD/cc-pVTZ level of theory were applied to find the reaction pathway from the reactants to products on the O₅ potential energy surface. These calculations revealed that the O₂(a¹Δ) + O₃ reaction is likely to proceed via singlet-triplet intersystem crossing exhibiting an energy barrier of 9.6 kcal/mol, which lies between two and three quanta of vibrational excitation of O₂(a¹Δ), and hence, O₂(a¹Δ, υ) with υ ≥ 3 could rapidly react with ozone.

A molecular beam and computational study on the barrierless gas phase formation of (iso)quinoline in low temperature extraterrestrial environments

Despite remarkable progress toward the understanding of the formation pathways leading to polycyclic aromatic hydrocarbons (PAHs) in combustion systems and in deep space, the complex reaction pathways leading to nitrogen-substituted PAHs (NPAHs) at low temperatures of molecular clouds and hydrocarbon-rich, nitrogen-containing atmospheres of planets and their moons like Titan have remained largely obscure. Here, we demonstrate through laboratory experiments and computations that the simplest prototype of NPAHs - quinoline and isoquinoline (C₉H₇N) - can be synthesized via rapid and de-facto barrier-less reactions involving o-, m- and p-pyridinyl radicals (C₅H₄N˙) with vinylacetylene (C₄H₄) under low-temperature conditions.

Directed Gas-Phase Formation of Aminosilylene (HSiNH2; 1A'): The Simplest Silicon Analogue of an Aminocarbene, under Single-Collision Conditions

The aminosilylene molecule (HSiNH₂, X¹A')-the simplest representative of an unsaturated nitrogen-silylene-has been formed under single collision conditions via the gas phase elementary reaction involving the silylidyne radical (SiH) and ammonia (NH₃). The reaction is initiated by the barrierless addition of the silylidyne radical to the nonbonding electron pair of nitrogen forming an HSiNH₃ collision complex, which then undergoes unimolecular decomposition to aminosilylene (HSiNH₂) via atomic hydrogen loss from the nitrogen atom. Compared to the isovalent aminomethylene carbene (HCNH₂, X¹A'), by replacing a single carbon atom with silicon, a profound effect on the stability and chemical bonding of the isovalent methanimine (H₂CNH)-aminomethylene (HNCH₂) and aminosilylene (HSiNH₂)-silanimine (H₂SiNH) isomer pairs is shown; i.e., thermodynamical stabilities of the carbene versus silylene are reversed by 220 kJ mol^-1. Hence, the isovalency of the main group XIV element silicon was found to exhibit little similarities with the atomic carbon revealing a remarkable effect not only on the reactivity but also on the thermochemistry and chemical bonding.

Combined Crossed Molecular Beams and Ab Initio Study of the Bimolecular Reaction of Ground State Atomic Silicon (Si; 3 P) with Germane (GeH4 ; X1 A1 )

The chemical dynamics of the elementary reaction of ground state atomic silicon (Si; ³ P) with germane (GeH₄ ; X¹ A₁ ) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol^-1 exploiting the crossed molecular beams technique contemplated with electronic structure calculations. The reaction follows indirect scattering dynamics and is initiated through an initial barrierless insertion of the silicon atom into one of the four chemically equivalent germanium-hydrogen bonds forming a triplet collision complex (HSiGeH₃ ; ³ i1). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH₃ ; ¹ i1). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H₃ SiGeH isomer i5. This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly-structured isomer ¹ p1 (Si(μ-H₂ )Ge) plus molecular hydrogen through a tight exit transition state. Alternatively, up to two subsequent hydrogen shifts to i6 and i7, followed by fragmentation of each of these intermediates, could also form ¹ p1 (Si(μ-H₂ )Ge) along with molecular hydrogen. The overall non-adiabatic reaction dynamics provide evidence on the existence of exotic dinuclear hydrides of main group XIV elements, whose carbon analog structures do not exist.

Gas-phase synthesis of benzene via the propargyl radical self-reaction

Polycyclic aromatic hydrocarbons (PAHs) have been invoked in fundamental molecular mass growth processes in our galaxy. We provide compelling evidence of the formation of the very first ringed aromatic and building block of PAHs-benzene-via the self-recombination of two resonantly stabilized propargyl (C₃H₃) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations. Along with benzene, three other structural isomers (1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene) and o-benzyne are detected, and their branching ratios are quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations. These results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments but also in combustion systems, which help us gain a better understanding of the hydrocarbon chemistry of our universe.

Theoretical Study of the Phenoxy Radical Recombination with the O(3P) Atom, Phenyl plus Molecular Oxygen Revisited

Quantum chemical calculations of the C₆H₅O₂ potential energy surface (PES) were carried out to study the mechanism of the phenoxy + O(³P) and phenyl + O₂ reactions. CASPT2(15e,13o)/CBS//CASSCF(15e,13o)/DZP multireference calculations were utilized to map out the minimum energy path for the entrance channels of the phenoxy + O(³P) reaction. Stationary points on the C₆H₅O₂ PES were explored at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311++G** level for the species with a single-reference character of the wave function and at the CASPT2(15e,13o)/CBS//B3LYP/6-311++G** level of theory for the species with a multireference character of the wave function. Conventional, variational, and variable reaction coordinate transition-state theories were employed in Rice-Ramsperger-Kassel-Marcus master equation calculations to assess temperature- and pressure-dependent phenomenological rate constants and product branching ratios. The main bimolecular product channels of the phenoxy + O(³P) reaction are concluded to be para/ortho-benzoquinone + H, 2,4-cyclopentadienone + HCO and, at high temperatures, also phenyl + O₂. The main bimolecular product channels of the phenyl + O₂ reaction include 2,4-cyclopentadienone + HCO at lower temperatures and phenoxy + O(³P) at higher temperatures. For both the phenoxy + O(³P) and phenyl + O₂ reactions, the collisional stabilization of peroxybenzene at low temperatures and high pressures competes with the bimolecular product channels.

Mechanism and kinetics of the oxidation of 1,3-butadien-1-yl (n-C4H5): a theoretical study

Ab initio CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the C4H5O2 potential energy surface have been combined with Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of temperature- and pressure-dependent rate constants and product branching ratios to unravel the mechanism and kinetics of the n-C4H5 + O2 reaction. The results indicate that the reaction is fast, with the total rate constant being in the range of 3.4-5.6 × 10-11 cm3 molecule-1 s-1. The main products include 1-oxo-n-butadienyl + O and acrolein + HCO, with their cumulative yield exceeding 90% at temperatures above 1500 K. Two conformers of 1-oxo-n-butadienyl + O are formed via a simple mechanism of O2 addition to the radical site of n-C4H5 followed by the cleavage of the O-O bond proceeding via a van der Waals C4H5OO complex. Alternatively, the pathways leading to acrolein + HCO involve significant reorganization of the heavy-atom skeleton either via formal migration of one O atom to the opposite end of the molecule or its insertion into the C1-C2 bond. Not counting thermal stabilization of the initial peroxy adducts, which prevails at low temperatures and high pressures, all other products share a minor yield of under 5%. Rate constants for the significant reaction channels have been fitted to modified Arrhenius expressions and are proposed for kinetic modeling of the oxidation of aromatic molecules and 1,3-butadiene. As a secondary reaction, n-C4H5 + O2 can be a source for the formation of acrolein observed experimentally in oxidation of the phenyl radical at low combustion temperatures, whereas another significant (secondary) product of the C6H5 + O2 reaction, furan, could be formed through unimolecular decomposition of 1-oxo-n-butadienyl. Both the n-C4H5 + O2 reaction and unimolecular decomposition of its 1-oxo-n-butadienyl primary product are shown not to be a substantial source of ketene.

Directed Gas Phase Formation of the Elusive Silylgermylidyne Radical (H3 SiGe, X2 A'')

The previously unknown silylgermylidyne radical (H₃ SiGe; X² A'') was prepared via the bimolecular gas phase reaction of ground state silylidyne radicals (SiH; X² Π) with germane (GeH₄ ; X¹ A₁ ) under single collision conditions in crossed molecular beams experiments. This reaction begins with the formation of a van der Waals complex followed by insertion of silylidyne into a germanium-hydrogen bond forming the germylsilyl radical (H₃ GeSiH₂ ). A hydrogen migration isomerizes this intermediate to the silylgermyl radical (H₂ GeSiH₃ ), which undergoes a hydrogen shift to an exotic, hydrogen-bridged germylidynesilane intermediate (H₃ Si(μ-H)GeH); this species emits molecular hydrogen forming the silylgermylidyne radical (H₃ SiGe). Our study offers a remarkable glance at the complex reaction dynamics and inherent isomerization processes of the silicon-germanium system, which are quite distinct from those of the isovalent hydrocarbon system (ethyl radical; C₂ H₅ ) eventually affording detailed insights into an exotic chemistry and intriguing chemical bonding of silicon-germanium species at the microscopic level exploiting crossed molecular beams.

Gas-Phase Formation of C5H6 Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,2-Butadiene (CH3CHCCH2; X1A')

The bimolecular gas-phase reaction of the methylidyne radical (CH; X²Π) with 1,2-butadiene (CH₂CCHCH₃; X¹A') was investigated at a collision energy of 20.6 kJ mol^-1 under single collision conditions. Combining our laboratory data with high-level electronic structure calculations, we reveal that this bimolecular reaction proceeds through the barrierless addition of the methylidyne radical to the carbon-carbon double bonds of 1,2-butadiene leading to doublet C₅H₇ intermediates. These collision adducts undergo a nonstatistical unimolecular decomposition through atomic hydrogen elimination to at least the cyclic 1-vinyl-cyclopropene (p5/p26), 1-methyl-3-methylenecyclopropene (p28), and 1,2-bis(methylene)cyclopropane (p29) in overall exoergic reactions. The barrierless nature of this bimolecular reaction suggests that these cyclic C₅H₆ isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.

Gas-phase synthesis of corannulene – a molecular building block of fullerenes

Corannulene can be formed through molecular mass growth processes in circumstellar envelopes.

Directed Gas Phase Formation of Silene (H2 SiCH2 )

The silene molecule (H₂ SiCH₂ ; X¹ A₁ ) has been synthesized under single collision conditions via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with silane (SiH₄ ). Exploiting crossed molecular beams experiments augmented by high-level electronic structure calculations, the elementary reaction commenced on the doublet surface through a barrierless insertion of the methylidyne radical into a silicon-hydrogen bond forming the silylmethyl (CH₂ SiH₃ ; X² A') complex followed by hydrogen migration to the methylsilyl radical (SiH₂ CH₃ ; X² A'). Both silylmethyl and methylsilyl intermediates undergo unimolecular hydrogen loss to silene (H₂ SiCH₂ ; X¹ A₁ ). The exploration of the elementary reaction of methylidyne with silane delivers a unique view at the widely uncharted reaction dynamics and isomerization processes of the carbon-silicon system in the gas phase, which are noticeably different from those of the isovalent carbon system thus contributing to our knowledge on carbon silicon bond couplings at the molecular level.

Gas Phase Formation of Methylgermylene (HGeCH3)

The methylgermylene species (HGeCH₃ ; X¹ A') has been synthesized via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with germane (GeH₄ ) under single collision conditions in crossed molecular beams experiments. Augmented by electronic structure calculations, this elementary reaction was found to proceed through barrierless insertion of the methylidyne radical in one of the four germanium-hydrogen bonds on the doublet potential energy surface yielding the germylmethyl (CH₂ GeH₃ ; X² A') collision complex. This insertion is followed by a hydrogen shift from germanium to carbon and unimolecular decomposition of the methylgermyl (GeH₂ CH₃ ; X² A') intermediate by atomic hydrogen elimination leading to singlet methylgermylene (HGeCH₃ ; X¹ A'). Our investigation provides a glimpse at the largely unknown reaction dynamics and isomerization processes of the carbon-germanium system, which are quite distinct from those of the isovalent carbon system thus providing insights into the intriguing chemical bonding of organo germanium species on the most fundamental, microscopic level.

A Free-Radical Prompted Barrierless Gas-Phase Synthesis of Pentacene

A representative, low-temperature gas-phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C22 H14 ) along with its benzo[a]tetracene isomer (C22 H14 ) is unraveled by probing the elementary reaction of the 2-tetracenyl radical (C18 H11 . ) with vinylacetylene (C4 H4 ). The pathway to pentacene-a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes-is facilitated by a barrierless, vinylacetylene mediated gas-phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low-temperature pathway can launch isomer-selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free-radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe.

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 cyclopentanaphthalene (benzindene) isomers via reactions of 5- and 6-indenyl radicals with vinylacetylene

The reaction of indenyl radicals with vinylacetylene leads to cyclopentanaphthalene at low temperature.

Gas phase formation of phenalene via 10π-aromatic, resonantly stabilized free radical intermediates

1H-Phenalene can be synthesized via the reaction of the 1-naphthyl radical with methylacetylene and allene under high temperature conditions prevalent in carbon-rich circumstellar environments and combustion systems.

Transversely optically pumped Ar:He laser with a pulsed-periodic discharge

Optically pumped rare gas lasers have the potential for scaling to high-power cw systems with good beam quality. Metastable atoms of heavier rare gases that are the lasing species are produced in an electric discharge at near atmospheric pressure. The key problem for this class of lasers at present is the development of a suitable discharge system. In this paper, we present the results of optimization of a pulsed discharge system with the goal of minimizing cathode sputtering and peak discharge current. The first demonstration of a transversely pumped system and measurements of the optical pumping threshold for the Ar:He laser are also presented.

Computational investigation of energy transfer and line broadening for Ar* + He collisions

Potential energy curves for all states arising from the interaction of He with the 3p⁶, 3p⁵4s, and 3p⁵4p configurations of Ar have been determined using high-level electronic structure calculations. The results have been used to examine collisional energy transfer probabilities and spectral line shape parameters (shifting and broadening rate coefficients). The main focus has been on states and transitions that are of relevance to optically pumped He/Ar^* laser systems. The line shape predictions were found to be in good agreement with experimental data, while there is notable disagreement for the energy transfer probabilities. The experimental data are found to be at variance with the predictions of standard two-state curve crossing models for energy transfer.

Reactivity of the Indenyl Radical (C9 H7 ) with Acetylene (C2 H2 ) and Vinylacetylene (C4 H4 )

The reactions of the indenyl radicals with acetylene (C₂ H₂ ) and vinylacetylene (C₄ H₄ ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉ H₇ ^. ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆ H₅ ^. ) and acetylene forming phenylacetylene (C₆ H₅ CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol^-1 ) and slow, contrary to the exoergic (-38 kJ mol^-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.

Gas phase synthesis of [4]-helicene

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C₁₄H₉]^•) with vinylacetylene (C₄H₄) yields [4]-helicene (C₁₈H₁₂) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C₁₈H₁₃). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

Helicenes represent key building blocks leading eventually to carbonaceous nanostructures. Here, exploiting [4]-helicene as a benchmark, the authors present a synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry with aryl radicals involving ring annulation.

How to add a five-membered ring to polycyclic aromatic hydrocarbons (PAHs) – molecular mass growth of the 2-naphthyl radical (C10H7) to benzindenes (C13H10) as a case study

The reaction of aryl radicals with allene/methylacetylene leads to five-membered ring addition in PAH growth processes.

Rate constants for collision-induced emission of O2(a1Δg) with He, Ne, Ar, Kr, N2, CO2 and SF6 as collisional partners

Rate constants for singlet oxygen collision induced emission of the a1Δg-X3Σ-g transition at 1.27 μm were measured for CO2, N2, SF6, and rare gases as collisional partners. Photolysis of ozone by 266 nm laser radiation produced singlet oxygen. We performed direct measurements of pressure dependences of the 1.27 μm emission intensity for partner gases. The measured rate constants kMa-X in the units of 10-24 cm3 s-1 are as follows: CO2 - 10 ± 2; N2 - 3.2 ± 0.6; SF6 - 7 ± 1; He - 1.1 ± 0.3; Ne - 1.3 ± 0.3; Ar - 2.8 ± 0.6; Kr - 6 ± 1. The measured values of kMa-X are close to the values calculated from absorption measurements. Considering the known rate constants kMb-a for the b1Σg+-a1Δg transition in the gas phase we found that the ratio kMa-X/kMb-a was constant and independent of a collisional partner according to the "spin-orbit based" mechanism of intensity borrowing proposed by Minaev (THEOCHEM, 1989, 183, 207). However, this ratio amounted to (1.3 ± 0.2) × 10-4, which is considerably lower than the theoretically predicted value of (3-6) × 10-4.

Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor

Computational fluid dynamics (CFD) simulations and isothermal approximation were applied for the interpretation of experimental measurements of the C₁₀H₇Br pyrolysis efficiency in the high-temperature microreactor and of the pressure drop in the flow tube of the reactor. Applying isothermal approximation allows the derivation of analytical relationships between the kinetic, gas flow, and geometrical parameters of the microreactor, which, along with CFD simulations, accurately predict the experimental observations. On the basis of the obtained analytical relationships, a clear strategy for measuring rate coefficients of (pseudo) first-order bimolecular and unimolecular reactions using the microreactor was proposed. The pressure- and temperature-dependent rate coefficients for the C₁₀H₇Br pyrolysis calculated using variable reaction coordinate transition state theory were invoked to interpret the experimental data on the pyrolysis efficiency.

VUV Photoionization Study of the Formation of the Simplest Polycyclic Aromatic Hydrocarbon: Naphthalene (C10H8)

The formation of the simplest polycyclic aromatic hydrocarbon (PAH), naphthalene (C₁₀H₈), was explored in a high-temperature chemical reactor under combustion-like conditions in the phenyl (C₆H₅)-vinylacetylene (C₄H₄) system. The products were probed utilizing tunable vacuum ultraviolet light by scanning the photoionization efficiency (PIE) curve at a mass-to-charge m/ z = 128 (C₁₀H₈⁺) of molecules entrained in a molecular beam. The data fitting with PIE reference curves of naphthalene, 4-phenylvinylacetylene (C₆H₅CCC₂H₃), and trans-1-phenylvinylacetylene (C₆H₅CHCHCCH) indicates that the isomers were generated with branching ratios of 43.5±9.0 : 6.5±1.0 : 50.0±10.0%. Kinetics simulations agree nicely with the experimental findings with naphthalene synthesized via the hydrogen abstraction-vinylacetylene addition (HAVA) pathway and through hydrogen-assisted isomerization of phenylvinylacetylenes. The HAVA route to naphthalene at elevated temperatures represents an alternative pathway to the hydrogen abstraction-acetylene addition (HACA) forming naphthalene in flames and circumstellar envelopes, whereas in cold molecular clouds, HAVA synthesizes naphthalene via a barrierless bimolecular route.

O2(b1Σg+) Quenching by O2, CO2, H2O, and N2 at Temperatures of 300-800 K

Rate constants for the removal of O₂(b¹Σ_g⁺) by collisions with O₂, N₂, CO₂, and H₂O have been determined over the temperature range from 297 to 800 K. O₂(b¹Σ_g⁺) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b¹Σ_g⁺-X³Σ_g^- fluorescence. The removal rate constants for CO₂, N₂, and H₂O were not strongly dependent on temperature and could be represented by the expressions k_CO2 = (1.18 ± 0.05) × 10^-17 × T^1.5 × exp[Formula: see text], k_N2 = (8 ± 0.3) × 10^-20 × T^1.5 × exp[Formula: see text], and k_H2O = (1.27 ± 0.08) × 10^-16 × T^1.5 × exp[Formula: see text] cm³ molecule^-1 s^-1. Rate constants for O₂(b¹Σ_g⁺) removal by O₂(X), being orders of magnitude lower, demonstrated a sharp increase with temperature, represented by the fitted expression k_O2 = (7.4 ± 0.8) × 10^-17 × T^0.5 × exp[Formula: see text] cm³ molecule^-1 s^-1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.

Removal of Rb(6(2)P) by H(2), CH(4), and C(2)H(6)

The saturated hydrocarbons methane and ethane are often used as collisional energy transfer agents in diode-pumped alkali vapor lasers (DPALs). Problems are encountered because the hydrocarbons eventually react with the optically pumped alkali atoms, resulting in the contamination of the gas lasing medium and damage of the gas cell windows. The reactions require excitation of the more highly excited states of the alkali atoms, which can be generated in DPAL systems by energy pooling processes. Knowledge of the production and loss rates for the higher excited states is needed for a quantitative understanding of the photochemistry. In the present study, we have used experimental and theoretical techniques to characterize the removal of Rb(6P2) by hydrogen, methane, and ethane.

Luminescence of the (O2(a(1)Δ(g)))2 collisional complex in the temperature range of 90-315 K: Experiment and theory

Experimental and theoretical studies of collision induced emission of singlet oxygen molecules O2(a(1)Δg) in the visible range have been performed. The rate constants, half-widths, and position of peaks for the emission bands of the (O2(a(1)Δg))2 collisional complex centered around 634 nm (2) and 703 nm (3) have been measured in the temperature range of 90-315 K using a flow-tube apparatus that utilized a gas-liquid chemical singlet oxygen generator. The absolute values of the spontaneous emission rate constants k2 and k3 are found to be similar, with the k3/k2 ratio monotonically decreasing from 1.1 at 300 K to 0.96 at 90 K. k2 slowly decreases with decreasing temperature but a sharp increase in its values is measured below 100 K. The experimental results were rationalized in terms of ab initio calculations of the ground and excited potential energy and transition dipole moment surfaces of singlet electronic states of the (O2)2 dimole, which were utilized to compute rate constants k2 and k3 within a statistical model. The best theoretical results reproduced experimental rate constants with the accuracy of under 40% and correctly described the observed temperature dependence. The main contribution to emission process (2), which does not involve vibrational excitation of O2 molecules at the ground electronic level, comes from the spin- and symmetry-allowed 1(1)Ag←(1)B3u transition in the rectangular H configuration of the dimole. Alternatively, emission process (3), in which one of the monomers becomes vibrationally excited in the ground electronic state, is found to be predominantly due to the vibronically allowed 1(1)Ag←2(1)Ag transition induced by the asymmetric O-O stretch vibration in the collisional complex. The strong vibronic coupling between nearly degenerate excited singlet states of the dimole makes the intensities of vibronically and symmetry-allowed transitions comparable and hence the rate constants k2 and k3 close to one another.