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 + CH Cl2O2 --> CHCl2OH + CCl2O + O-2 (2b), CCl3O2 + HO2 --> products (3), a nd 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 measur ements and continuous photolysis/FTIR spectroscopy for end-product ana lyses. The UV absorption spectra of CHCl2O2 and CCl3O2 were determined between 220 and 280 nm; they have shapes similar to those of other al kyl 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 1 0(-13) exp[(745 +/- 58)K/T] cm(3) molecule(-1) s(-1), where quoted (1 sigma) errors represent only statistical uncertainties, Reaction 2 pro ceeds predominately (greater than or equal to 90%) by channel 2a, Whil e k(2) was not measured directly, satisfactory simulations in the CHCl 2O2 + HO2 experiments could only be achieved with k(2) values comparab le to those of the self-reactions of CCl3O2 and CH2ClO2 radicals. By a veraging the kinetic data for the CH2ClO2 and CCl3O2 radical self-reac tions, 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 reactio n 2 always reacts by Cl atom elimination so that CHClO is the major at mospheric oxidation product of CH2Cl2. The rate constants of reactions 3 and 4 were measured over the temperature range 286-440 K by generat ing 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 wit h HO2 at 296 K in 700 Torr of air: CHClO (71%) and CCl2O (29%). One pr oduct was observed following the reaction of CCl3O2 radicals with HO2: CCl2O in a yield indistinguishable from 100%, In contrast to all othe r 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(29 8)(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 implicati ons and the trends in the reactivity of chloromethyl peroxy radicals a re discussed. As part of this work, the following reaction rate consta nts were measured (units of cm(3) molecule(-1) s(-1)) at room temperat ure: 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 1 0(-12).