Concluding remarks: Faraday Discussion on astrochemistry at high resolution

Fifty years on from the first detailed chemical kinetic modelling of astronomical sources, I provide some introductory comments on the history of astrochemistry, summarise some personal views on the topics covered in this discussion meeting, and conclude with some thoughts on its future development. I have left out the jokes.

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H

The reaction of CH radicals with H₂ has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298-748 K at pressures of ∼5-100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (Brownsword, R. A.; J. Chem. Phys. 1997, 106, 7662-7677). The rate coefficient k₁ shows complex behavior: at low pressures, the main route for the CH₃* formed from the insertion of CH into H₂ is the formation of ³CH₂ + H, and as the pressure is increased, CH₃* is increasingly stabilized to CH₃. The kinetic data on CH + H₂ have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, ³CH₂ + H → CH + H₂. The resulting parametrization is k_CH2+H(T) = (1.69 ± 0.11) × 10^-10 × (T/298 K)^{(0.05±0.010)} cm³ molecule^-1 s^-1, where the errors are 1σ.

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.

Product Detection of the CH(X2Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

The reaction of the methylidyne radical (CH(X²Π)) with cyclopentadiene (c-C₅H₆) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C₆H₆ + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C₆H₆ isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C₆H₆ isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C₆H₇ potential energy surface are performed to explore the competition between CH cycloaddition to a C═C bond vs CH insertion into C-H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C-H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C-H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene

We report the detection for the first time in space of three new pure hydrocarbon cycles in TMC-1: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6 (cyclopentadiene) and c-C9H8 (indene). We derive a column density of 3.1 × 1011 cm-2 for the former cycle and similar values, in the range (1-2) × 1013 cm-2, for the two latter molecules. This means that cyclopentadiene and indene, in spite of their large size, are exceptionally abundant, only a factor of five less abundant than the ubiquitous cyclic hydrocarbon c-C3H2. The high abundance found for these two hydrocarbon cycles, together with the high abundance previously found for the propargyl radical (CH2CCH) and other hydrocarbons like vinyl and allenyl acetylene (Agúndez et al. 2021; Cernicharo et al. 2021a,b), start to allow us to quantify the abundant content of hydrocarbon rings in cold dark clouds and to identify the intermediate species that are probably behind the in situ bottom-up synthesis of aromatic cycles in these environments. While c-C3HCCH is most likely formed through the reaction between the radical CCH and c-C3H2, the high observed abundances of cyclopentadiene and indene are difficult to explain through currently proposed chemical mechanisms. Further studies are needed to identify how are five- and six-membered rings formed under the cold conditions of clouds like TMC-1.

Discovery of CH2CHCCH and detection of HCCN, HC4N, CH3CH2CN, and, tentatively, CH3CH2CCH in TMC-1

We present the discovery in TMC-1 of vinyl acetylene, CH2CHCCH, and the detection, for the first time in a cold dark cloud, of HCCN, HC4N, and CH3CH2CN. A tentative detection of CH3CH2CCH is also reported. The column density of vinyl acetylene is (1.2±0.2)×1013 cm-2, which makes it one of the most abundant closed-shell hydrocarbons detected in TMC-1. Its abundance is only three times lower than that of propylene, CH3CHCH2. The column densities derived for HCCN and HC4N are (4.4±0.4)×1011 cm-2 and (3.7±0.4)×1011 cm-2, respectively. Hence, the HCCN/HC4N abundance ratio is 1.2±0.3. For ethyl cyanide we derive a column density of (1.1 ±0.3)×1011 cm-2. These results are compared with a state-of-the-art chemical model of TMC-1, which is able to account for the observed abundances of these molecules through gas-phase chemical routes.

Low-temperature gas-phase formation of indene in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecular building blocks of fullerenes and carbonaceous nanostructures in the interstellar medium and in combustion systems. However, an understanding of the formation of aromatic molecules carrying five-membered rings-the essential building block of nonplanar PAHs-is still in its infancy. Exploiting crossed molecular beam experiments augmented by electronic structure calculations and astrochemical modeling, we reveal an unusual pathway leading to the formation of indene (C₉H₈)-the prototype aromatic molecule with a five-membered ring-via a barrierless bimolecular reaction involving the simplest organic radical-methylidyne (CH)-and styrene (C₆H₅C₂H₃) through the hitherto elusive methylidyne addition-cyclization-aromatization (MACA) mechanism. Through extensive structural reorganization of the carbon backbone, the incorporation of a five-membered ring may eventually lead to three-dimensional PAHs such as corannulene (C₂₀H₁₀) along with fullerenes (C₆₀, C₇₀), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy.

Revisiting the Intriguing Electronic Features of the BeOBeC Carbyne and Some Isomers: A Quantum-Chemical Assessment

Extensive high-level quantum-chemical calculations reveal that the rod-shaped molecule BeOBeC, which was recently generated in matrix experiments, exists in two nearly isoenergetic states, the 5 Σ quintet (5 6) and the 3 Σ triplet (3 6). Their IR features are hardly distinguishable at finite temperature. The major difference concerns the mode of spin coupling between the terminal beryllium and carbon atoms. Further, the ground-state potential-energy surface of the [2Be,C,O] system at 4 K is presented and differences between the photochemical and thermal behaviors are highlighted. Finally, a previously not considered, so far unknown C2v -symmetric rhombus-like four-membered ring 3 [Be(O)(C)Be] (3 5) is predicted to represent the global minimum on the potential-energy surface.

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

Through reaction of beryllium dimers with carbon monoxide, a carbonyl complex BeBeCO is formed in solid neon. Upon visible light excitation, the BeBeCO complex rearranges to a BeCOBe isomer, which further isomerizes to a low-energy BeOBeC species under UV-visible light excitation. These species are identified on the basis of infrared absorption spectroscopy with isotopic substitutions and quantum chemical studies. The BeOBeC molecule is characterized to be a multiple radical species having an electronic quintet ground state featuring an unusual quartet carbyne unit with three unpaired electrons on the carbon center. Bonding analysis indicates that the strong Pauli repulsion between carbon 2s lone pair electrons and the σ electrons of the BeOBe fragment significantly weakens the Be-C bonding and destabilizes the triplet state of the BeOBeC radical with a doublet carbyne unit. The three-center π-bonding of BeOBe is also found to play a role in stabilizing the quartet carbyne.

Gas-Phase Formation of 1-Methylcyclopropene and 3-Methylcyclopropene via the Reaction of the Methylidyne Radical (CH; X2Π) with Propylene (CH3CHCH2; X1A')

The crossed molecular beam reactions of the methylidyne radical (CH; X²Π) with propylene (CH₃CHCH₂; X¹A') along with (partially) substituted reactants were conducted at collision energies of 19.3 kJ mol^-1. Combining our experimental data with ab initio electronic structure and statistical calculations, the methylidyne radical is revealed to add barrierlessly to the carbon-carbon double bond of propylene reactant resulting in a cyclic doublet C₄H₇ intermediate with a lifetime longer than its rotation period. These adducts undergo a nonstatistical unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming the cyclic products 1-methylcyclopropene and 3-methylcyclopropene with overall reaction exoergicities of 168 ± 25 kJ mol^-1. These C₄H₆ isomers are predicted to exist even in low-temperature environments such as cold molecular clouds like TMC-1, since the reaction is barrierless and exoergic, all transition states are below the energy of the separated reactants, and both the methylidyne radical (CH; X²Π) and propylene reactant were detected in cold molecular clouds such as TMC-1.

On the formation and the isomer specific detection of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), and 1,3-butadiene (CH2CHCHCH2) from interstellar methane ice analogues

Pure methane (CH4) ices processed by energetic electrons under ultra-high vacuum conditions to simulate secondary electrons formed via galactic cosmic rays (GCRs) penetrating interstellar ice mantles have been shown to produce an array of complex hydrocarbons with the general formulae: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-6 (n = 6-7). By monitoring the in situ chemical evolution of the ice combined with temperature programmed desorption (TPD) studies and tunable single photon ionization coupled to a reflectron time-of-flight mass spectrometer, specific isomers of C3H4, C3H6, C4H4, and C4H6 were probed. These experiments confirmed the synthesis of methylacetylene (CH3CCH), propene (CH3CHCH2), cyclopropane (c-C3H6), vinylacetylene (CH2CHCCH), 1-butyne (HCCC2H5), 2-butyne (CH3CCCH3), 1,2-butadiene (H2CCCH(CH3)), and 1,3-butadiene (CH2CHCHCH2) with yields of 2.17 ± 0.95 × 10-4, 3.7 ± 1.5 × 10-3, 1.23 ± 0.77 × 10-4, 1.28 ± 0.65 × 10-4, 4.01 ± 1.98 × 10-5, 1.97 ± 0.98 × 10-4, 1.90 ± 0.84 × 10-5, and 1.41 ± 0.72 × 10-4 molecules eV-1, respectively. Mechanistic studies exploring the formation routes of methylacetylene, propene, and vinylacetylene were also conducted, and revealed the additional formation of the 1,2,3-butatriene isomer. Several of the above isomers, methylacetylene, propene, vinylacetylene, and 1,3-butadiene, have repeatedly been shown to be important precursors in the formation of polycyclic aromatic hydrocarbons (PAHs), but until now their interstellar synthesis has remained elusive.

Theoretical Investigations on the Reactivity of Methylidyne Radical toward 2,3,7,8-Tetrachlorodibenzo-p-Dioxin: A DFT and Molecular Dynamics Study

To explore the potential reactivity of the methylidyne radical (CH) toward 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the reaction mechanism between them has been systematically investigated employing the density functional theory (DFT) and ab initio molecular dynamics simulations. The relevant thermodynamic and kinetic parameters in the possible reaction pathways have been discussed as well as the IR spectra and hyperfine coupling constants (hfcc's) of the major products. Different from the reaction of the CH radical with 2,3,7,8-tetrachlorodibenzofuran, CH radical can attack all the C-C bonds of TCDD to form an initial intermediate barrierlessly via the cycloaddition mechanism. After then, the introduced C-H bond can be further inserted into the C-C bond of TCDD, resulting in the formation of a seven-membered ring structure. The whole reactions are favorable thermodynamically and kinetically. Moreover, the major products have been verified by ab initio molecular dynamics simulations. The distinct IR spectra and hyperfine coupling constants of the major products can provide some help for their experimental detection and identification. In addition, the reactivity of the CH radical toward the F- and Br-substituted TCDDs has also been investigated. Hopefully, the present findings can provide new insights into the reactivity of the CH radical in the transformation of TCDD-like dioxins.

Theoretical insights into the reaction mechanisms between 2,3,7,8-tetrachlorodibenzofuran and the methylidyne radical

To explore the potential role of the methylidyne radical (CH) in the transformation of 2,3,7,8-tetrachlorodibenzofuran (TCDF), in this study, the detailed reaction mechanisms between TCDF and CH radical have been systematically investigated employing the B3LYP method of density functional theory (DFT) in combination with the atoms in molecules (AIM) theory and ab initio molecular dynamics. It was found that the title reaction is a multi-channel reaction, i.e., the CH radical can attack the C-X (X = C, Cl, H, O) bonds of TCDF via the insertion modes, resulting in the formation of 13 products. Thermodynamically, the whole reaction processes are exothermic and spontaneous since all the enthalpy and Gibbs free energy changes are negative values in the formation processes. Moreover, the thermodynamic stability of the products is controlled by the distribution of the single unpaired electron. Kinetically, the most favorable reaction channel is the insertion of the CH radical into the C-C bond except for the C atoms attached to the chlorine atom. Moreover, the dominant products have been further confirmed by the molecular dynamics. Meanwhile, the IR spectra and hyperfine coupling constants of the dominant products have been investigated to provide helpful information for their identification experimentally. In addition, the reactivity of the CH radical toward the F- and Br-substituted TCDFs has also been investigated. Expectedly, the present findings can enable us to better understand the reactivity of the CH radical toward organic pollutants analogous to TCDF in the atmosphere.

Isomer-specific detection in the UV photodissociation of the propargyl radical by chirped-pulse mm-wave spectroscopy in a pulsed quasi-uniform flow

Isomer-specific detection and product branching fractions in the UV photodissociation of the propargyl radical is achieved through the use of chirped-pulse Fourier-transform mm-wave spectroscopy in a pulsed quasi-uniform flow (CPUF). Propargyl radicals are produced in the 193 nm photodissociation of 1,2-butadiene. Absorption of a second photon leads to H atom elimination giving three possible C₃H₂ isomers: singlets cyclopropenylidene (c-C₃H₂) and propadienylidene (l-C₃H₂), and triplet propargylene (³HCCCH). The singlet products and their appearance kinetics in the flow are directly determined by rotational spectroscopy, but due to the negligible dipole moment of propargylene, it is not directly monitored. However, we exploit the time-dependent kinetics of H-atom catalyzed isomerization to infer the branching to propargylene as well. We obtain the overall branching among H loss channels to be 2.9% (+1.1/-0.5) l-C₃H₂ + H, 16.8% (+3.2/-1.3) c-C₃H₂ + H, and 80.2 (+1.8/-4.2) ³HCCCH + H. Our findings are qualitatively consistent with earlier RRKM calculations in that the major channel in the photodissociation of the propargyl radical at 193 nm is to ³HCCCH + H; however, a greater contribution to the energetically most favorable isomer, c-C₃H₂ + H is observed in this work. We do not detect the predicted HCCC + H₂ channel, but place an upper bound on its yield of 1%.

The growth of carbon chains in IRC +10216 mapped with ALMA

Linear carbon chains are common in various types of astronomical molecular sources. Possible formation mechanisms involve both bottom-up and top-down routes. We have carried out a combined observational and modeling study of the formation of carbon chains in the C-star envelope IRC +10216, where the polymerization of acetylene and hydrogen cyanide induced by ultraviolet photons can drive the formation of linear carbon chains of increasing length. We have used ALMA to map the emission of λ 3 mm rotational lines of the hydrocarbon radicals C2H, C4H, and C6H, and the CN-containing species CN, C3N, HC3N, and HC5N with an angular resolution of ~1″. The spatial distribution of all these species is a hollow, 5-10″ wide, spherical shell located at a radius of 10-20″ from the star, with no appreciable emission close to the star. Our observations resolve the broad shell of carbon chains into thinner sub-shells which are 1-2″ wide and not fully concentric, indicating that the mass loss process has been discontinuous and not fully isotropic. The radial distributions of the species mapped reveal subtle differences: while the hydrocarbon radicals have very similar radial distributions, the CN-containing species show more diverse distributions, with HC3N appearing earlier in the expansion and the radical CN extending later than the rest of the species. The observed morphology can be rationalized by a chemical model in which the growth of polyynes is mainly produced by rapid gas-phase chemical reactions of C2H and C4H radicals with unsaturated hydrocarbons, while cyanopolyynes are mainly formed from polyynes in gas-phase reactions with CN and C3N radicals.

Insights into gas-phase reaction mechanisms of small carbon radicals using isomer-resolved product detection

Gas-phase radical reactions of CN and CH with small hydrocarbons are overviewed with emphasis on isomer-resolved product detection.

Formation of C3H2, C5H2, C7H2, and C9H2 from reactions of CH, C3H, C5H, and C7H radicals with C2H2

A three-dimensional velocity distribution contour of C_2n+1H₂ produced from the reaction of C_2n−1H (n = 1–4) with C₂H₂ in crossed molecular beams.

Tuning spin-states of carbynes and silylynes: a long jump with one leg

The challenge motivating this paper is to induce, by chemical substitution, a silylyne, SiR, or a congeneric carbyne, CR, to adopt the high-spin quartet rather than the low-spin doublet as its ground state. The difficulty is seen in the preference for the doublet of the parent SiH (doublet-quartet energy difference ∼39 kcal/mol, favoring the doublet) or CH (∼17 kcal/mol). Strategies for having high-spin ground state parallel those for silylenes and carbenes: greater electropositivity (σ-donation) and π-acceptance of the single substituent favor the high-spin state. The electronegativity trend can be understood from an ions in molecules way of thinking already present in the literature in the works of Boldyrev and Simons, and of Mavridis and Harrison; i.e., the quartet ground state spin of some CR/SiR species is largely determined by the ground state spin of C(-)/Si(-). In this study, we provide a diabatization analysis that solidly confirms the ions in molecules picture and explains the difference in the equilibrium internuclear distances for the two spin states. In general, electronegativity dominates the ordering of spin states. π-Acceptors also help to lower the quartet state energy of the many carbynes (silylynes) examined, whose range of doublet-quartet differences calculated is impressive, 120 (100) kcal/mol. The qualitative understanding gained leads to the prediction of some quartet-ground state carbynes (CMgH, CAlH2, CZnH, CSiH3, CSiF3, etc.) and a smaller number of silylynes (SiMgH, SiMgF, SiBeH, etc.). A beginning is made on the energetics of approach geometries of the fragments in the highly exoergic dimerization of CH to acetylene; it should proceed for the ground state doublet CH through C2h-like trajectories, with no activation energy.

Formation of Carbon-Carbon Triply Bonded Molecules from Two Free Carbyne Radicals via a Conical Intersection

The recent proposal (Bogoslavsky, B.; Levy, O.; Kotlyar, A.; Salem, M.; Gelman, F.; Bino, A. Angew. Chem., Int. Ed.2012, 51, 90-94) that metallo-alkylidyne complexes decompose in aqueous solution and give rise to free carbynes, which couple to yield acetylenes, is examined here theoretically. On the basis of the known marker reactions of carbynes in the doublet and quartet state, it is concluded that most of the reactivity patterns observed in the Bino experiment arose from quartet carbynes. Indeed, theory shows that quartet carbynes can be funneled to acetylene via a conical intersection. Moreover, many of the minor products are also identified as markers of the quartet carbynes. Carbynes formation in their doublet state is a minor channel that branches from the conical intersection and leads to the formation of dienes and olefins in the Bino experiment. Thus, we show that conical intersections are important also in thermally initiated reactions. Coupled to the experimental approach, the study opens a window to studies of carbynes under mild conditions.

Formation of benzene in the interstellar medium

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block--the aromatic benzene molecule--has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C(6)H(6)) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C(2)H + H(2)CCHCHCH(2) → C(6)H(6) + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.