Kinetics and mechanisms of the self-reactions of CCl3O2 and CHCl2O2 radicals and their reactions with HO2

The kinetics and mechanism of the reactions CCl3O2 + CCl3O2 --> 2CCl(3)O + O-2 (1), CHCl2O2 + CHCl2O2 --> 2CHCl(2)O + O-2 (2a), CHCl2O2 + CHCl2O2 --> CHCl2OH + CCl2O + O-2 (2b), CCl3O2 + HO2 --> products (3), and CHCl2O2 + HO2 --> products (4) have been investigated as a function of temperature at total pressures of 700-760 Torr. Two complementary techniques were used flash photolysis/UV absorption for kinetic measurements and continuous photolysis/FTIR spectroscopy for end-product analyses. The UV absorption spectra of CHCl2O2 and CCl3O2 were determined between 220 and 280 nm; they have shapes similar to those of other alkyl peroxy radicals, but with broader and less intense bands. The rate constant k(1) was determined between 273 and 460 K from the formation rate of CCl2O in the Cl atom initialed oxidation chain of chloroform, where reaction 1 was the rate-limiting step; k(1) = (3.3 +/- 0.6) x 10(-13) exp[(745 +/- 58)K/T] cm(3) molecule(-1) s(-1), where quoted (1 sigma) errors represent only statistical uncertainties, Reaction 2 proceeds predominately (greater than or equal to 90%) by channel 2a, While k(2) was not measured directly, satisfactory simulations in the CHCl2O2 + HO2 experiments could only be achieved with k(2) values comparable to those of the self-reactions of CCl3O2 and CH2ClO2 radicals. By averaging the kinetic data for the CH2ClO2 and CCl3O2 radical self-reactions, we derived k(2) = (2.6 +/- 0.5) x 10(-13) exp[(800 +/- 60) K/T] cm(3) molecule(-1) s(-1). The observation of a chain reaction at low temperature (250 K) showed that the CHCl2O radical produced in reaction 2 always reacts by Cl atom elimination so that CHClO is the major atmospheric oxidation product of CH2Cl2. The rate constants of reactions 3 and 4 were measured over the temperature range 286-440 K by generating simultaneouslyCCl(3)O(2) (or CHCl2O2) and HO2; k(3) = (4.8 +/- 0.5) x 10(-13) exp[(706 +/- 31) K/T], and k(4) = (5.6 +/- 1.2) x 10(-13) exp[(700 +/- 64, K/T] cm(3) molecule(-1) s(-1) (errors = 1 sigma). Two products were observed following the reaction of CHCl2O2 radicals with HO2 at 296 K in 700 Torr of air: CHClO (71%) and CCl2O (29%). One product was observed following the reaction of CCl3O2 radicals with HO2: CCl2O in a yield indistinguishable from 100%, In contrast to all other studies of peroxy radical reactions with HO2, there was no evidence of hydroperoxide formation. Ab initio quantum mechanical calculations (MP2/6-31G(d,p)) were used to derive Delta(f)H degrees(298)(CHCl2OOH) = -46.3, Delta(f)H degrees(298)(CCl3OOH) = -48.4, Delta(f)H degrees(298)(CHCl2O2) = -6.3, Delta(f)H degrees(298)(CCl3O2) = -8.0, and Delta(f)H degrees(298)(CHClO) = -43.9 kcal mol(-1). The mechanistic implications and the trends in the reactivity of chloromethyl peroxy radicals are discussed. As part of this work, the following reaction rate constants were measured (units of cm(3) molecule(-1) s(-1)) at room temperature: k(CCl3O2 + CH3O2) = (6.6 +/- 1.7) x 10(-12), k(Cl + CHCl3) = (1.1 +/- 0.1) x 10(-13), k(Cl + CH2Cl2) = (3.5 +/- 0.4) x 10(-13), k(Cl + CHClO) = (7.0 +/- 1.0) x 10(-13), and k(F + CHCl3) = (5.4 +/- 1.5) x 10(-12).