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Synthesis of Nitrogen-Substituted PAHs in the Interstellar Medium and in Planetary Atmospheres
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Selected time-of-flight data for m/z = 63 (C4HN+) recorded at collision energies of 29.9 kJmol-1 at various laboratory angles. The circles indicate the experimental data, the solid lines the calculated fit. |
Resonantly stabilized free radicals (RSFRs) such as the propargyl (C3H3;X2B3) radical are believed to play a crucial role in the formation of polycyclic aromatic hydrocarbons (PAHs) and soot, in the combustion of fuels, and in the chemical processing of carbon rich circumstellar envelopes. Due to the delocalization of the unpaired electron, resonantly stabilized free hydrocarbon radicals are more stable than ordinary radicals and form weaker bonds with stable molecules such as molecular oxygen in combustion flames. These weakly bound complexes are not easily stabilized by collisions at high temperatures. Therefore, RSFRs are relatively unreactive and can reach high concentration in flames. These high concentrations and the relatively fast rates of the corresponding radical – radical reactions make them important in the mechanism of formation of complex hydrocarbons such as the very first aromatic ring species in flames and in the interstellar medium. For instance, Miller and Melius as well as Kern and Xie suggested odd-carbon-atom reaction pathways involving the recombination of two propargyl radicals. Electronic structure calculations imply that the initially formed acyclic collision complex(es) of the recombination of two propargyl radicals can isomerize via multiple steps to ultimately form benzene and/or the phenyl radical plus a hydrogen atom. An alternative even-odd-carbon-atom sequence could include the reaction of propargyl radicals (C3H3) with acetylene (C2H2) to yield the cyclopentadienyl radical (C5H5). The latter can react in multiple steps to benzene. |
Nevertheless, reactions of substituted propargyl radicals, which in turn could yield substituted benzene molecules and cyclopentadienyl radicals via reaction with a propargyl radical and acetylene molecule, respectively, have not been included yet in reaction models simulating hydrocarbon flames and the chemistry in circumstellar envelopes such as of IRC+10216. For instance, the cyano propargyl radical, i.e. a propargyl radical in which a hydrogen at the C1 or C3 position is replaced by a cyano (CN) group, could react with propargyl and acetylene to form cyano benzene (C6H5CN) and cyano cyclopentadienyl radicals (C5H4CN), respectively. However, to validate this proposed route, the first step is to elucidate possible formation routes to the cyano propargyl radical itself. A recent crossed beams study on the reaction of ground state carbon atoms, C(3Pj), with ethylene, C2H4(X1A1), suggested that the propargyl radical and atomic hydrogen are the dominating reaction products. Therefore, a reaction of carbon atoms with vinyl cyanide (cyano ethylene; C2H3CN(X1A’)), i.e. a substituted ethylene molecule, is expected to yield cyano propargyl radicals under single collision conditions. Very recently, Su et al. investigated this reaction computationally. The authors concluded that cyano propargyl radicals are the sole reaction products under single collision conditions. However, an experimental verification of these computations remains to be carried out. |
Structures of the 1- and 3-(substituted) propargyl radicals formed in the reactions of ground state carbon atoms with (substituted) ethylene molecules. The position of the radical center is defined as C1. |
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Here, the chemical dynamics of the reaction of ground state carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X1A’), were examined under single collision conditions at collision energies of 29.9 kJmol-1 and 43.9 kJmol-1 using the crossed molecular beams approach. Our investigations suggest that the reaction follows indirect scattering dynamics via addition of the carbon atom to the carbon-carbon double bond of the vinyl cyanide molecule yielding a cyano cyclopropylidene collision complex. The latter undergoes ring opening to form cis/trans triplet cyano allene which fragments predominantly to the 1-cyano propargyl radical via tight exit transition states; the 3-cyano propargyl isomer was inferred to be formed at least a factor of two less. The discovery of the cyano propargyl radical in the reaction of atomic carbon with vinyl cyanide under single collision conditions implies that this molecule can be an important reaction intermediate in combustion flames and also in extraterrestrial environments (cold molecular clouds, circumstellar envelopes of carbon stars) which could lead to the formation of cyano benzene (C6H5CN) upon reaction with a propargyl radical. |
It is interesting to compare the present findings with the reactions of ethylene, propylene, and 1,3-butadiene studied recently using crossed molecular beams. Here, the reaction dynamics and potential energy surfaces involved in the formation of (substituted) propargyl radicals show common features. All reactions are dictated by indirect scattering dynamics; about 30 – 35 % of the total available energy channels into the translational degrees of freedom. The carbon atom adds to the π electron density. This yields (substituted) cyclopropylidene intermediates which are stabilized by 210 – 220 kJmol-1 with respect to the reactants. These cyclic intermediates undergo ring opening through barriers of between 30 and 50 kJmol-1 to form (substituted) cis/trans allene intermediates on the triplet surface. The latter are bound by 340 – 380 kJmol-1 compared to the separated reactants. The fate of the triplet allene complexes is governed by hydrogen atom loss channels to (substituted) propargyl radicals via exit transition states located about 10 – 35 kJ mol-1 above the products. Most of the initial angular momentum channels into rotational excitation of the products. The existence of an exit barrier is documented in the center-of-mass translation energy distributions; here, all P(ET)s peak around 20 – 50 kJmol-1. Finally, the exoergicities to form the C3H2R (R = CH3, C2H3, H, CN) products are very similar and fall between 190 and 220 kJmol-1; in all reactions investigated, the 1-substituted propargyl radical was found to be formed systematically in higher yields compared to the 3-substituted isomer with ratios of 3:1 (propylene), 8:1 (1,3-butadiene), and 5:1 (vinyl cyanide). |
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Y. Guo, X. Gu, F. Zhang, A.H.H. Chang, R.I. Kaiser, A Crossed Beams Study of the Reaction of Carbon Atoms, C(3Pj), with Vinyl Cyanide, C2H3CN(X1A') - Investigating the Formation of Cyano Propargyl Radicals. Phys. Chem. Chem. Phys, 8, 5454 - 5461 (2006) (PDF file)
