Direct measurements of rate coefficients for the reaction of ethynyl radical (C₂H) with C₂H₂ at 90 and 120 K using a pulsed Laval nozzle apparatus

Abstract

Abstract. Rate coefficients for the reaction C2H + C2H2 -• C4H2 + H are measured at 90 and 120 K by using a new pulsed Laval nozzle apparatus equipped with laser ionization, time-of-flight mass spectrometric detection. The C2 H radicals are generated by 193 nm laser photolysis of C2H2, and the reaction product, C4H2, is directly detected by single-photon ionization at 118 nm (ls0.5 • eV). Rate coefficients of (2.7 _+ 0.5) x 10-•0 and Since there are relatively few measurements of rate coefficients at temperatures relevant for the outer planets, the models of the planetary atmospheres require extrapolations from higher-temperature rate data. The rate coefficient values presently used in the models of the planetary atmospheres are often in error by an order of magnitude, even before extrapolation to lower temperatures, and at the lowest temperatures of 70 K on Titan, the actual rate coefficients may differ even more.Currently, the Laval nozzle expansion type apparatus is the only method capable of measuring low-temperature rate constants without condensation of the reactants. The Laval nozzle produces a gas flow that is uniform in velocity, density, and temperature and endures for several tens of centimeters and some hundreds of microseconds downstream of the nozzle exit.In this paper we report the first rate coefficient data obtained with a new pulsed Laval nozzle apparatus designed to study neutral radical reaction kinetics. The apparatus incorporates vacuum ultraviolet laser ionization, time-of-flight mass spectrometer detection. The measurements presented here are the first direct kinetic measurements that monitor the reaction products. Therefore we can not only measure rate constants but also identify the reaction products of the low-temperature chemistry.In the present study, we report measurements of the rate coefficients for (R1) at 90 and 120 K using the pulsed Laval nozzle apparatus, and we probe neutral radical products directly using single-photon ionization at 118 nm (10.5 eV). The results show a marked increase in the reaction rate coefficient 15,085

Dissociative Photoionization of Polycyclic Aromatic Hydrocarbon Molecules Carrying an Ethynyl Group

“…Note that C 2 H reacts also with acetylene (C 2 H 2 ), giving diacetylene (C 4 H 2 ) and a H atom. This process is also free of energy barrier at the entrance channel as shown by Lee et al (2000) with another experiment in the gas phase. Thus C 2 H may react with ethynyl-substituted PAH molecules under interstellar conditions, at an aromatic cycle or at the side chain, as suggested by the different paths described by Mebel et al (2008) for the reaction of C 2 H with ethynylbenzene.…”

The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan

“…C 2 H 3 CN + H reaction is generally taken from Monks et al (1993). However the data of Monks et al (1993), at room temperature only, are very imprecise and their rate constant for the C 2 H 3 + HCN reaction is notably higher than the rate constant of reactions of C 2 H 3 with unsaturated hydrocarbons (Wang and Frenklach, 1994;Knyazev et al, 1996;Callear and Smith, 1986;Ismail et al, 2007) which is a surprising result as several other atoms and radicals (C 2 , C 2 H, CN, OH, F, Cl) are significantly less reactive with HCN (Frost et al, 1986;Hoobler and Leone, 1997;Fukuzawa and Osamura, 1997;Sander et al, 2011) than with unsaturated hydrocarbons (Nesbitt et al, 1994;Li et al, 2006a,b;Paramo et al, 2008;Daugey et al, 2008;Canosa et al, 2007;Lee et al, 2000;Vakhtin et al, 2001;Sims et al, 1993;Gannon et al, 2007;Atkinson et al, 2004;McKee et al, 2007;Nesbitt et al, 1999;Gu et al, 2006;Mebel et al, 2006;Bouwman et al, 2012;Golden, 2012). We performed theoretical calculations for this reaction finding a barrier in the entrance valley equal to 18.0 kJ/mol at the DFT level (M06-2X/cc-pVTZ), in good agreement with Petrie (2002), and classical transition state theory leads to a rate constant equal to k(C 2 H 3 +HCN) = 1.0 Â 10 À12 exp (À2300/T) cm 3 molecule À1 s À1 , a much lower value than Monks et al (1993).…”

Reaction of the C2H Radical with 1-Butyne (C4H6): Low-Temperature Kinetics and Isomer-Specific Product Detection

“…11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26] In order to analyze and account for the presence of hydrocarbon species, modeling studies are of central importance in understanding the complex nature of Titan's atmosphere.…”