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 (k4), methyl cyanide (k 5 ), ethyl cyanide (k 6 ), and propyl cyanide (k 7 ) are studied at low temperature using a pulsed Laval nozzleapparatus. 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 Δ) chemiluminescence from the C 2 H + O 2 reaction. The rate constants at low and high temperatures are k 1 = (1.3 ′ 0.3) x 10 - 1 0 and (1.0 ′ 0.2) x 10 - 1 0 for isobutane, k 2 = (2.1 ′ 0.4) x 10 - 1 0 and (2.2 ′ 0.4) × 10 - 1 0 for 1-butene, k 3 = (1.4 ′ 0.3) x 10 - 1 0 and (1.2 ′ 0.2) x 10 - 1 0 for isobutylene, and k4 = (2.9 ′ 0.6) x 10 - 1 0 and (3.3 ′ 0.6) x 10 - 1 0 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) x 10 - 1 1 exp(-766 ′ 38/T), k 6 = (1.5 ′ 0.3) × 10 - 1 1 exp(-145 ′ 10/T), and k 7 = (2.1 ′ 0.4) × 10 - 1 1 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.