Low-Temperature Rate Coefficients for Reactions of the Ethynyl Radical (C2H) with C3H4 Isomers Methylacetylene and Allene

Abstract

Low-temperature rate coefficients for the reactions C2H + C3H4 → products [C3H4 = methylacetylene (propyne) CH3CCH and allene (propadiene) CH2CCH2] are measured over the temperature range of 155−298 K. Absolute rate constants are determined using laser photolysis and transient infrared absorption spectroscopy. Both reactions are fast, approaching the kinetic limit and have either no temperature dependence or a slight negative temperature dependence. Over the experimental temperature range, the rate constants are fit to the following Arrhenius expressions: kCH3CCH = (1.6 ± 0.4) × 10-10 exp[(71 ± 50)/T] cm3 molecule-1 s-1 and kCH2CCH2 = (1.3 ± 0.6) × 10-10 exp[(103 ± 136)/T] cm3 molecule-1 s-1. The corresponding rate constants at 298 K are kCH3CCH = (1.9 ± 0.3) × 10-10 cm3 molecule-1 s-1 and kCH2CCH2 = (1.7 ± 0.3) × 10-10 cm3 molecule-1 s-1. These measurements provide new reaction rate coefficients for the ethynyl radical, C2H, with C3H4 unsaturated hydrocarbons. These are necessary to extend current photo...