Kinetics of C₂H Reactions with Hydrocarbons and Nitriles in the 104−296 K Temperature Range

Abstract

Reactions of C 2 H with isobutane (k 1 ), 1-butene (k 2 ), isobutylene (k 3 ), 1,3-butadiene (k 4 ), methyl cyanide (k 5 ), ethyl cyanide (k 6 ), and propyl cyanide (k 7 ) are studied at low temperature using a pulsed Laval nozzle apparatus. The C 2 H radical is prepared by 193-nm photolysis of acetylene, and the C 2 H concentration is monitored using CH(A 2 ∆) chemiluminescence from the C 2 H + O 2 reaction. The rate constants at low and high temperatures are k 1 ) (1.3 ( 0.3) × 10 -10 and (1.0 ( 0.2) × 10 -10 for isobutane, k 2 ) (2.1 ( 0.4) × 10 -10 and (2.2 ( 0.4) × 10 -10 for 1-butene, k 3 ) (1.4 ( 0.3) × 10 -10 and (1.2 ( 0.2) × 10 -10 for isobutylene, and k 4 ) (2.9 ( 0.6) × 10 -10 and (3.3 ( 0.6) × 10 -10 for 1,3-butadiene at T ) 104 and 296 K, respectively (in units of cm 3 molecule -1 s -1 ). Comparison with existing data shows a trend of a decrease in activation energy with increasing size of the hydrocarbon chain. For these reactions of hydrocarbons containing four carbon atoms, the activation energy is zero within experimental uncertainty, and the rate constants do not depend on temperature in the 104-296 K temperature range. The rate constants for C 2 H reactions with methyl cyanide, ethyl cyanide, and propyl cyanide are measured at three temperatures, 104, 165, and 298 K. Measured rate constants are fit to an Arrhenius expression and are k 5 ) (1.8 ( 0.35) × 10 -11 exp(-766 ( 38/T), k 6 ) (1.5 ( 0.3) × 10 -11 exp(-145 ( 10/T), and k 7 ) (2.1 ( 0.4) × 10 -11 exp(-51 ( 4/T) (in units of cm 3 molecule -1 s -1 , T is in Kelvin). At T ) 296 K, k 3 , k 5 , k 6 , and k 7 are measured as a function of total pressure and show no pressure dependence in the 0.6-8 Torr (0.08-1.07 kPa) pressure range. Results from this work are compared with the results of previous investigations of C 2 H reactions at low temperature and are discussed in relation to the atmospheres of Saturn and Titan.

Formation of benzene in the interstellar medium

“…We would like to stress that, in our computations, the barrierless addition was verified by a careful examination of the potential energy surface in the entrance channel (intrinsic reaction coordinate calculations), which indicates that the potential energy of the system steadily and monotonically decreases as the D1-ethynyl radical approaches 1,3-butadiene. The barrierless nature of this reaction is also supported by Nizamov and Leone's low-temperature kinetics studies in the range from 104 to 296 K, which suggest rate coefficients within gas kinetics limits of a few 10 −10 cm 3 s −1 (14); however, these experiments did not determine the nature of the reaction products. In the bimolecular crossed beam reaction, the resulting doublet radical intermediate [i1] was found to either decompose forming the 1,3-hexadien-5-yne isomer or to undergo ring closure followed by hydrogen shift yielding ultimately the singly hydrogenated benzene molecule [i4], which then loses a hydrogen atom forming D1-benzene.…”

“…In our network, we implemented a rate constant of 3.0 AE 0.9 × 10 −10 cm 3 s −1 and accounted for the branching fractions of benzene versus the 1,3-hexadien-5-yne isomer as elucidated in our present study. We recognize that Nizamov and Leone's data were recorded at temperatures between 104 and 296 K (14). However, an analysis of ethynyl-radical reactions with unsaturated hydrocarbons shows that their rate constants are almost invariant with temperature (16).…”

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

“…Our data is also consistent with the measured rate coefficients of the C 2 H with 1,3-butadiene reaction. 39 Indeed, our rate constant is slightly lower Before considering the possible mechanism for the reaction, it is useful to compare the energetics of propargyl recombination, 61 which has been identified as a benzene formation pathway in fuelrich flames. 44 Figure 9 shows the heats of reaction (∆H r° in kcal/mol) in which the three most stable C 6 H 6 isomers, i.e.…”

H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

“…Diacetylene is believed to play a key role in the formation of polyynes and polycyclic aromatic hydrocarbons (PAHs) that partially comprise the haze layer in Titan's upper atmosphere (2)(3)(4). It is well established that the formation of diacetylene is initiated by photodissociation of acetylene below 217 nm (2,(5)(6)(7)(8) according to the following reaction mechanism: C 2 H 2 ϩ hv ¡ C 2 H ϩ H͑ Ͻ 217 nm͒ C 2 H ϩ C 2 H 2 ¡ C 4 H 2 ϩ H The importance ascribed to diacetylene arises in part because it absorbs light at longer wavelengths, where the solar flux is higher, than any other major constituents of Titan's atmosphere; moreover, experimental results suggest it is still photochemically reactive even well below the threshold for dissociation (9)(10)(11)(12). Understanding the dynamics of diacetylene photoexcitation is thus key to revealing the factors driving the chemistry of Titan's atmosphere.…”