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

Abstract

The kinetics of the reactions of CH₂Br and CH₂I radicals with O₂ 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 O₂ being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH₂I + O₂ reaction in the range 0.2–15 × 10¹⁷ cm⁻³ of He at 298 K. In this same density range the CH₂Br + O₂ reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH₂I + O₂ reaction is found to depend on temperature as k(CH₂I + O₂) = (1.39 ± 0.01) × 10⁻¹² (T/300 K)^{−1.55 ± 0.06} cm³ 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 + O₂ reaction in the third-body region is k(CH₂Br + O₂ + He) = (1.2 ± 0.2) × 10⁻³⁰ (T/300 K)^{−4.8 ± 0.3} cm⁶ 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%.