Formations of C6H from reactions C3 + C3H2 and C3H + C3H and of C8H from reactions C4 + C4H2 and C4H + C4H

We interrogated C6H and C8H produced separately from the reactions C3 + C3H2/C3H + C3H/C3H2 + C3 → C6H + H and C4 + C4H2/C4H + C4H/C4H2 + C4 → C8H + H using product translational and photoionization spectroscopy. Individual contributions of the three reactions to the product C6H or C8H were evaluated with reactant concentrations. Translational-energy distributions, angular distributions, and photoionization efficiency curves of products C6H and C8H were unraveled. The product C6H (C8H) was recognized as the most stable linear isomer by comparing its photoionization efficiency curve with that of l-C6H (l-C8H), produced exclusively from the reaction C2 + C4H2 → l-C6H + H (C2 + C6H2 → l-C8H + H). The ionization threshold after deconvolution was determined to be 9.3 ± 0.1 eV for l-C6H and 8.9 ± 0.1 eV for l-C8H, which is in good agreement with theoretical values. Quantum-chemical calculations indicate that the reactions of C3 + C3H2 and C3H + C3H (C4 + C4H2 and C4H + C4H) incur no energy barriers that lie above the corresponding reactant and the most stable product l-C6H (l-C8H) with H on the lower-lying potential-energy surfaces. The theoretical calculation is in accord with the experimental observation. This work implies that the reactions of C3 + C3H2/C3H + C3H and C4 + C4H2/C4H + C4H need to be taken into account for the formation of interstellar C6H and C8H, respectively.

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.

Gas-phase kinetics of CH3CHO with OH radicals between 11.7 and 177.5 K

Gas-phase reactions in the interstellar medium (ISM) are a source of molecules in this environment. The knowledge of the rate coefficient for neutral-neutral reactions as a function of temperature, k(T), is essential to improve astrochemical models. In this work, we have experimentally measured k(T) for the reaction between the OH radical and acetaldehyde, both present in many sources of the ISM. Laser techniques coupled to a CRESU system were used to perform the kinetic measurements. The obtained modified Arrhenius equation is k(T = 11.7-177.5 K) = (1.2 ± 0.2) × 10-11 (T/300 K)-(1.8±0.1) exp-{(28.7 ± 2.5)/T} cm3 molecule-1 s-1. The k(T) value of the title reaction has been measured for the first time below 60 K. No pressure dependence of k(T) was observed at ca. 21, 50, 64 and 106 K. Finally, a pure gas-phase model indicates that the title reaction could become the main CH3CO formation pathway in dark molecular clouds, assuming that CH3CO is the main reaction product at 10 K.

The Chemistry of Cosmic Dust Analogues from C, C2, and C2H2 in C-Rich Circumstellar Envelopes

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H2 and CO. In a previous study, we addressed the chemistry of carbon (C and C2) with H2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C2) with C2H2. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp2 and sp3 hydrocarbons with amorphous morphology.

Formation of C9H2 and C10H2 from Reactions C3H + C6H2 and C4H + C6H2

The reactions of C₃H and C₄H radicals with C₆H₂ were investigated for the first time. Reactants C₃H, C₄H, and C₆H₂ were synthesized in two beams of C₂H₂ diluted with helium by pulsed high-voltage discharge. We measured translational-energy distributions, angular distributions, and photoionization-efficiency spectra of C₉H₂ and C₁₀H₂ produced from the title reactions in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet photoionization. The C₃H (C₄H) + C₆H₂ reaction releases 42% (33%) of available energy into the translational degrees of freedom of product C₉H₂ (C₁₀H₂) + H and scatters products into a nearly isotropic angular distribution. The photoionization-efficiency spectrum of C₉H₂ (C₁₀H₂) is in good agreement with that of C₉H₂ (C₁₀H₂) produced from the C₇H (C₈H) + C₂H₂ reaction. The ionization threshold, after deconvolution, was determined as 8.0 ± 0.1 eV for C₉H₂ and 8.8 ± 0.1 eV for C₁₀H₂. The combination of measurements of product translational-energy release and photoionization-efficiency spectra indicates productions of ³HC₉H/c-¹HC₃(C)C₅H/c-¹HC₇(C)CH + H and ¹HC₁₀H + H in the two title reactions, which are supported also by quantum-chemical calculations. Ratios branching to the three isomers of C₉H₂ remain unknown. This work demonstrates that long carbon-chain molecules (e.g., C₉H₂ and C₁₀H₂) can be synthesized from reactions of C_mH (e.g., m = 3 and 4) radicals with polyynes (e.g., HC₆H) and gives some valuable implications to planetary, interstellar, and combustion chemistry.

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.

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.

Formation of Polyynes C4H2, C6H2, C8H2, and C10H2 from Reactions of C2H, C4H, C6H, and C8H Radicals with C2H2

Some of the polyynes (HC2n+2H, 1 ≤ n ≤ 4) are observable in planetary atmospheres, interstellar space, and flames. Polyynes are proposed to play an important role in synthesis of large carbonaceous molecules. We explore the dynamics of reactions of C2nH (n = 1-4) radicals with C2H2 by interrogating time-of-flight spectra and photoionization efficiency spectra of products C2n+2H2. The reactions of n = 2-4 were investigated for the first time. The translational energy release is biased to low energy but extends to the energetic limit of product HC2n+2H + H, corresponding to a fraction of 0.34-0.36 on translational energy. Product C2n+2H2 has a deconvoluted ionization threshold in good agreement with the ionization energy of polyynes. The quantum chemical calculations support the experimental observations. This work verifies that the title reaction is an important source for formation of polyynes that have been observed in interstellar/circumstellar media and combustion processes.

OXIDATION MECHANISM OF THE BUTADIYNYL RADICAL, C4H: ANALOGUE OF C2H OR NOT?

Reactions of the carbon-chain radicals are of great importance in the combustion and astrophysical processes. The kinetics of the butadiynyl radical, C 4 H , has received recent attention. While there has been sufficient knowledge concerning the oxidation of the ethynyl radical, C 2 H , oxidation of the higher even-numbered members C 2n H (n > 1) is hardly known. In this paper, to enrich the C 4 H -chemistry, we report the first study of the oxidation mechanism of C 4 H . At the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(d,p)+ZPVE level, the potential energy surface (PES) survey is presented covering various product channels P1( CO + HC 3 O ) (-152.7 kcal/mol), P2( C 3 H + CO 2) (-117.9), P3( HCO + C 3 O ) (-108.5), P4( HC 4 O +3 O ) (-45.2), and P5( OH + C 4 O ) (-33.2) accompanied by the master equation rate constant calculations. Despite the similarity in the PES, the kinetics of C 4 H +3 O 2 differs dramatically from that of the analogous C 2 H +3 O 2 reaction. For the C 4 H +3 O 2 reaction, the O -abstraction product P4( HC 4 O +3 O ) is almost the exclusive product, whereas the lowest C , O -exchange product P1( CO + HC 3 O ) and other products have little importance. By contrast, the C 2 H +3 O 2 reaction favors the C , O -exchange product HCO + CO . Being overall barrierless and mainly associated with the molecular → atomic oxygen conversion, the C 4 H +3 O 2 reaction should play an important role in the soot formation and interstellar chemistry where C 4 H is involved.