Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures

The kinetics of the reactions of CH2Br and CH2I radicals with O-2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O-2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O-2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O-2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O-2 reaction is found to depend on temperature as k(CH2I + O-2) (1.39 +/- 0.01) x 10(-12) (T/300 K)(-1.55) +/- (0.06) cm(3) s(-1) (220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O-2 reaction in the third-body region is k(CH2Br + O-2 + He) (1.2 +/- 0.2) x 10(-30) (T/300 K)(-4.8) (+/-) (0.3) cm(6) s(-1) (241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F-cent value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/- 25%.