Low temperature rate coefficients for reactions of the butadiynyl radical, C4H, with various hydrocarbons. Part I: reactions with alkanes (CH4, C2H6, C3H8, C4H10)

The kinetics of the reactions of the linear butadiynyl radical, C4H (CCCCH), with methane, ethane, propane and butane have been studied over the temperature range of 39-300 K using a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) apparatus combined with the pulsed laser photolysis-laser induced fluorescence technique. The rate coefficients, except for the reaction with methane, show a negative temperature dependence and can be fitted with the following expressions over the temperature range of this study: kC(2)H(6) = 0.289 x 10(-10) (T/298 K)(-1.23) exp(-24.8 K/T) cm(3) molecule(-1) s(-1); kC(3)H(8) = 1.06 x 10(-10) (T/298 K)(-1.36) exp(-56.9 K/T) cm(3) molecule(-1) s(-1); kC(4)H(10) = 2.93 x 10(-10) (T/298 K)(-1.30) exp(-90.1 K/T) cm(3) molecule(-1) s(-1). The rate coefficients for the reaction with methane were measured only at 200 K and 300 K yielding a positive temperature dependence: kCH(4) = 1.63 x 10(-11) exp(-610 K/T) cm(3) molecule(-1) s(-1). Possible reaction mechanisms and product channels are discussed in detail for each of these reactions. Potential implications of these results for models of low temperature chemical environments, in particular cold interstellar clouds and planetary atmospheres such as that of Titan, are considered.