TEMPERATURE-DEPENDENCE OF CH(CD)+D2 REACTION-KINETICS

The temperature dependence of the rate coefficients for the reactions of ground-state CH and CD + D2 has been measured over the temperature range 298-1260 and 560-1120 K, respectively. The methylidyne radical is generated by laser photolysis at 248 nm of CHBr2Cl for Ch and CDBr3 for CD and detected via laser-induced fluorescence. No pressure dependence over the range 10-100 Torr at room temperature is observed for the CH + D2 reaction. The CH + D2 rate coefficients are fit to the Arrhenius expression k(T) = A exp[-E(a)/RT] and yield k(T) = (4.29 +/- 0.27) X 10(-11) cm3 s-1 exp[(450 +/- 50 cal mol-1)/RT]. The data are fit equally well by k(T) = (1.13 +/- 0.36) X 10(-9) T-(0.45 +/- 0.06). For the reaction CD + D2, a rate expression for the endothermic addition-elimination channel is derived from the measured temperature dependence. The fit expression yields k(abs)(T) = (5.56 +/- 2.82) X 10(-11) cm3 s-1 exp[(-2775 +/- 900 cal mol-1)/RT]. Error limits of the fitted parameters represent 1-sigma-values. The observed negative temperature dependence of the CH + D2 reaction is consistent with recent calculations for CH + H-2 which predict no barrier to CH3 formation along a non-least-motion path. The results of the CD + D2 study indicate the activation energy for addition-elimination is approximately equal to the reaction enthalpy. Above 2100 K the dominant reaction channel is expected to be addition-elimination.