KINETICS OF ELEMENTARY REACTIONS AT LOW-TEMPERATURES - RATE CONSTANTS FOR THE REACTIONS OF OH WITH HCL (298 GREATER-THAN-OR-EQUAL-TO T/K GREATER-THAN-OR-EQUAL-TO 138), CH4 (298 GREATER-THAN-OR-EQUAL-TO T/K GREATER-THAN-OR-EQUAL-TO 178) AND C2H6) (298 GREATER-THAN-OR-EQUAL-TO T/K GREATER-THAN-OR-EQUAL-TO 138)

The pulsed laser photolysis (PLP)-laser-induced fluorescence (LIF) method has been used to study the kinetics at low temperatures of the reactions of the OH free radical with HCl (138-298 K), CH4 (178-298 K) and C2H6 (138-298 K). In deriving rate constants for the reactions of OH with CH4 and C2H6, small but significant corrections were made to allow for the last secondary reactions between OH and the alkyl radicals produced in the primary reactions. The results are compared with predictions based on extrapolations of kinetic data obtained at higher temperatures, and limitations of the experimental method at low temperatures are considered. The rate constants for OH + CH4 and OH + C2H6 are in reasonable agreement with values calculated using modified Arrhenius expressions which have been fitted to data obtained at higher temperatures, although our rate constant for OH + CH4 at 216 K is significantly higher than a recent determination at 223 K by Vaghjiani and Ravishankara [Nature (London), 1991, 350, 406]. However, the rate constants for OH + HCl at lower temperatures deviate significantly from such predictions.