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 x 10(17) 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)x 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)x 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%.

Gas Phase Kinetics and Equilibrium of Allyl Radical Reactions with NO and NO2

“…The flow reactor with laser photolysis coupled to the resonance gas lamp photoionization mass spectrometer (LP-RPIMS) used in this work has been described previously, 15,16 and only a brief 10 The main photolysis channel leads to C3−C4 bond fission, but in addition other products are formed:…”

UV photodissociation dynamics of CHI2Cl and its role as a photolytic precursor for a chlorinated Criegee intermediate

“…2,3 Haloalkyl fragments also play a role in atmospheric chemistry as they are generally thought to form peroxy radicals after reacting with O2. 37 As with the CH2I radical, reaction of the CHICl radical with O2 appears to forms a carbonyl oxide or Criegee intermediate in comparably high yield. Pressuredependent measurements of the CH2I + O2 reaction indicate a CH2OO yield of 77% at 15 Torr (the pressure used in the measurement of the ClCHOO absorption cross section), decreasing to around 30% at atmospheric pressure.…”

“…[31][32][33][34][35] Photolysis of CH2I2 in the presence of O2 was first demonstrated by Welz et al 36 to be an efficient route to generate CH2OO in sufficient concentration for kinetics studies at low pressure, following initial work by Eskola et al who had identified I atom formation in the CH2I + O2 reaction. 37 The CH2OO yield in the CH2I + O2 reaction approaches unity at low pressure, decreasing to ~30% at atmospheric pressure. [38][39][40] The UV absorption spectrum of CH2OO has been measured using several spectroscopic techniques.…”

Perspective: Spectroscopy and kinetics of small gaseous Criegee intermediates

“…Sunlight can efficiently induce the photolytic decomposition of CH 2 I 2 to form CH 2 I + I; the reaction of CH 2 I + O 2 eventually releases a second I atom; these I atoms might destroy ozone or become transformed to aerosol. 58,59 Even though the reaction of CH 2 I + O 2 has been investigated extensively, [52][53][54][55][56][60][61][62][63][64][65][66] a detailed mechanism of this reaction was established only recently. 67 The proposed mechanisms in the early studies were incomplete because only product channels for the formation of ICH 2 OO and H 2 CO + IO, not CH 2 OO + I, were considered.…”