Low-temperature rate coefficients for reactions of the ethynyl radical (C2H) with C3H4 isomers methylacetylene and allene

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: k(CH3CCH) = (1.6 +/- 0.4) X 10(-10) exp[(71 +/- 50)/T] cm(3) molecule(-1) s(-1) and k(CH2CCH2) = (1.3 +/- 0.6) x 10(-10) exp[(103 +/- 136)/T] cm(3) molecule(-1) s(-1). The corresponding rate constants at 298 K are k(CH3CCH) = (1.9 +/- 0.3) X 10(-10) cm(3) molecule(-1) s(-1) and k(CH2CCH2) = (1.7 +/- 0.3) X 10(-10) cm(3) 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 photochemical models of Jupiter, Saturn, and its satellite Titan.