Kinetics and Mechanisms of the Self-Reactions of CCl3O2 and CHCl2O2 Radicals and Their Reactions with HO2

The kinetics and mechanism of the reactions CCl 3 O 2 + CCl 3 O 2 → 2CCl 3 O + O 2 (1), CHCl 2 O 2 + CHCl 2 O 2 → 2CHCl 2 O + O 2 (2a), CHCl 2 O 2 + CHCl 2 O 2 → CHCl 2 OH + CCl 2 O + O 2 (2b), CCl 3 O 2 + HO 2 → products (3), and CHCl 2 O 2 + HO 2 → 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 CHCl 2 O 2 and CCl 3 O 2 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 CCl 2 O in the Cl atom initiated oxidation chain of chloroform, where reaction 1 was the rate-limiting step; k 1 = (3.3 ± 0.6) × 10 -13 exp[(745 ± 58)K/ T ] cm 3 molecule -1 s -1, where quoted (1σ) errors represent only statistical uncertainties. Reaction 2 proceeds predominately (≥90%) by channel 2a. While k 2 was not measured directly, satisfactory simulations in the CHCl 2 O 2 + HO 2 experiments could only be achieved with k 2 values comparable to those of the self-reactions of CCl 3 O 2 and CH 2 ClO 2 radicals. By averaging the kinetic data for the CH 2 ClO 2 and CCl 3 O 2 radical self-reactions, we derived k 2 = (2.6 ± 0.5) × 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 CHCl 2 O radical produced in reaction 2 always reacts by Cl atom elimination so that CHClO is the major atmospheric oxidation product of CH 2 Cl 2 . The rate constants of reactions 3 and 4 were measured over the temperature range 286−440 K by generating simultaneously CCl 3 O 2 (or CHCl 2 O 2 ) and HO 2; k 3 = (4.8 ± 0.5) × 10 -13 exp[(706 ± 31) K/ T ], and k 4 = (5.6 ± 1.2) × 10 -13 exp[(700 ± 64 K/ T ] cm 3 molecule -1 s -1 (errors = 1σ). Two products were observed following the reaction of CHCl 2 O 2 radicals with HO 2 at 296 K in 700 Torr of air: CHClO (71%) and CCl 2 O (29%). One product was observed following the reaction of CCl 3 O 2 radicals with HO 2: CCl 2 O in a yield indistinguishable from 100%. In contrast to all other studies of peroxy radical reactions with HO 2, there was no evidence of hydroperoxide formation. Ab initio quantum mechanical calculations (MP2/6-31G(d,p)) were used to derive Δ f H ° 298 (CHCl 2 OOH) = −46.3, Δ f H ° 298 (CCl 3 OOH) = −48.4, Δ f H ° 298 (CHCl 2 O 2 ) = −6.3, Δ f H ° 298 (CCl 3 O 2 ) = −8.0, and Δ f H ° 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 (CCl 3 O 2 + CH 3 O 2 ) = (6.6 ± 1.7) × 10 -12, k (Cl + CHCl 3 ) = (1.1 ± 0.1) × 10 -13, k (Cl + CH 2 Cl 2 ) = (3.5 ± 0.4) × 10 -13, k (Cl + CHClO) = (7.0 ± 1.0) × 10 -13, and k (F + CHCl 3 ) = (5.4 ± 1.5) × 10 -12 .