Reaction of CH with H2O: Temperature Dependence and Isotope Effect

Abstract

The reactions of CH(ν=0) and CD(ν=0) radicals with H2O and D2O were investigated in the temperature range between 291 and 723 K. The reactions of CH(ν=1) and CD(ν=1,2) with H2O and D2O were studied at 293 K. CH(D) radicals were generated by multiphoton dissociation of CH(D)Br3 using pulsed laser photolysis and the resulting CH(D) time profiles were monitored by laser induced fluorescence. The reactions of CH(D) in the ν = 0 state exhibit a negative temperature dependence and are independent of total pressure between 20 and 200 Torr at room temperature. The rate coefficients can be fitted over the experimental temperature range by the following expressions: k(T)CH+H20 = (1.56 ± 0.25) × 10-11(T/298)-1.42 ± 0.31 cm3 molecule-1 s-1; k(T)CH+D20 = (1.49 ± 0.25) × 10-11(T/298)-1.04 ± 0.29 cm3 molecule-1 s-1; k(T)CD+H20 = (1.38 ± 0.21) × 10-11(T/298)-1.16 ± 0.25 cm3 molecule-1 s-1; k(T)CD+D20 = (1.71 ± 0.28) × 10-11(T/298)-1.13 ± 0.32 cm3 molecule-1 s-1. The errors on the parameters correspond to ±1 σ standard deviation from nonlinear least-squares fitting, and the data were weighted by 1/σ2 using a σ of 20%. Deuteration of either the CH radical or H2O has no discernible effect on the kinetics of the reaction, suggesting formation of an intermediate complex prior to the insertion of CH(D) into the OH(D) bond. Rate coefficients for CH(ν=1) and CD(ν=1,2) were found to be faster than those for the radicals in the vibrational ground state. The results are compared with previous experimental and theoretical studies, and the mechanism is discussed.