KINETIC-STUDY OF THE REACTIONS OF CH2CLO2 WITH ITSELF AND WITH HO2, AND THEORETICAL-STUDY OF THE REACTIONS OF CH2CLO, BETWEEN 251-K AND 600-K

The UV absorption spectrum, self-reaction (2CH2ClO2 --> 2CH2ClO + O2 (1)) kinetics and the kinetics of the reaction with HO2 (CH2ClO2 + HO2 --> products (3)) of the chloromethylperoxy radical, CH2ClO2, have been studied between 251 and 600 K and at 760 Torr total pressure. Chloromethylperoxy radicals were produced by the flash photolysis of Cl2/CH3Cl/O2/N2 mixtures; for the study of reaction 3, chloromethylperoxy and hydroperoxy radicals were produced simultaneously by the flash photolysis Of Cl2/CH3Cl/CH3OH/O2/N2 mixtures. Radical concentrations were monitored by UV absorption spectrometry from 205 to 290 nm. The chloromethoxy radicals formed in reaction 1 produce hydroperoxy radicals via reaction with molecular oxygen (CH2ClO + O2 --> HO2 + CHClO (2)). These hydroperoxy radicals are removed via reaction 3 and by their self-reaction (HO2 + HO2 --> H2O2 + O2 (4)). Unlike the majority of systems studied to date, the rate constant for the RO2 + HO2 reaction is not significantly faster than that for the RO2 + RO2 reaction and significant transient concentrations of HO2 are produced. Consequently, the observed second-order rate constant for radical disappearance depends strongly on the monitoring wavelength, and UV absorption cross-section and values of k1 and k3 could only be obtained by an iterative process. Sensitivity analyses with respect to all model parameters on fitted rate constants were performed. Arrhenius fits to the data on k1 and k3 gave k1 = (1.95 +/- 0.16) X 10(-13) exp[(874 +/- 26)K/7]cm3 molecule-1 s-1 and k3 = (3.26 +/- 0.61) X 10(-13) exp[(822 +/- 63)K/T] cm3 molecule-1 s-1. Average absolute uncertainties in k1 and k3 over the temperature range studied, including experimental scatter and uncertainties in the analysis parameters, were estimated to be 31 and 33%, respectively. The UV absorption spectrum of the chloromethylperoxy radical is typical of the spectra of alkylperoxy radicals, peaking at 230 nm, where sigma = (4.06 +/- 0.53) X 10(-18) cm2 molecule-1. In sharp contrast to the behavior of fully halogenated methylperoxy radicals, there was no evidence for the elimination of a chlorine atom from the chloromethoxy radical, CH2ClO, even at the highest temperature employed in this study. Semiempirical MNDO calculations and RRKM calculations were performed which suggest that unimolecular elimination of HCl from CH2ClO is important at high temperatures.