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

Low-temperature rate coefficients for the reactions C 2 H + C 3 H 4 → products [C 3 H 4 = methylacetylene (propyne) CH 3 CCH and allene (propadiene) CH 2 CCH 2 ] 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: k CH 3 CCH = (1.6 ± 0.4) × 10 -10 exp[(71 ± 50)/ T ] cm 3 molecule -1 s -1 and k CH 2 CCH 2 = (1.3 ± 0.6) × 10 -10 exp[(103 ± 136)/ T ] cm 3 molecule -1 s -1 . The corresponding rate constants at 298 K are k CH 3 CCH = (1.9 ± 0.3) × 10 -10 cm 3 molecule -1 s -1 and k CH 2 CCH 2 = (1.7 ± 0.3) × 10 -10 cm 3 molecule -1 s -1 . These measurements provide new reaction rate coefficients for the ethynyl radical, C 2 H, with C 3 H 4 unsaturated hydrocarbons. These are necessary to extend current photochemical models of Jupiter, Saturn, and its satellite Titan.