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

Abstract

The kinetics and mechanism of the reactions CCl3O2 + CCl3O2 → 2CCl3O + O2 (1), CHCl2O2 + CHCl2O2 → 2CHCl2O + O2 (2a), CHCl2O2 + CHCl2O2 → CHCl2OH + CCl2O + O2 (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 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] cm3 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 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) × 10-13 exp[(800 ± 60) K/T] cm3 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 simultaneously CCl3O2 (or CHCl2O2) and HO2; 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] cm3 molecule-1 s-1 (errors = 1σ). 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 Δf H°298(CHCl2OOH) = −46.3, Δf H°298(CCl3OOH) = −48.4, Δf H°298(CHCl2O2) = −6.3, Δf H°298(CCl3O2) = −8.0, and Δf 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 cm3 molecule-1 s-1) at room temperature: k(CCl3O2 + CH3O2) = (6.6 ± 1.7) × 10-12, k(Cl + CHCl3) = (1.1 ± 0.1) × 10-13, k(Cl + CH2Cl2) = (3.5 ± 0.4) × 10-13, k(Cl + CHClO) = (7.0 ± 1.0) × 10-13, and k(F + CHCl3) = (5.4 ± 1.5) × 10-12.

Ab initio and DFT investigation of fluorinated methyl hydroperoxides: Structures, rotational barriers, and thermochemical properties

“…The enthalpy of formation of CH 3 OOH has been previously reported [9][10][11][12][13][14]26]. For the sake of verifying the accuracy of the adopted methods of calculation, the general isodesmic reaction: 4 (1)…”

“…To our knowledge, there exists only experimental determination of some thermochemical parameters for HOOH and CH 3 OOH [7,8]. Theoretical investigations of structures, vibrational frequencies, and thermodynamic properties of methyl [9,10], a-chlorinated ethyl [11], and chlorinated methyl [12][13][14] hydroperoxides have been performed. Concerning the fluorinated methyl hydroperoxides, there is a theoretical estimate of the heat of formation and internal bond energies of only CF 3 OOH [15].…”

Structural and heat of formation studies of halogenated methyl hydro-peroxides

“…Knowledge of their structure and thermodynamics is useful for studying the mechanism of the oxidation pathways in the chemistry of combustion and flames, in atmospheric chemistry and in various biochemical oxidizing processes. Hence, several experimental and theoretical investigations have been carried out to examine and characterize the geometrical, spectroscopic and thermochemical properties of simple and halogenated alkyl hydro-peroxides [10][11][12][13][14][15][16][17][18][19][20][21][22].…”

Kinetics of the Self-Reactions of Peroxy Radicals Arising from Chlorine-Initiated Oxidation of Chloroethenes

“…For example, the CCl 3 O 2 radical presents a similar rate constant for its selfreaction as those measured for CH 2 ClO 2 and CHCl 2 O 2 . 8 Another objective of this work was to expand the kinetic database for self-reactions, with the principal aim of improving the characterization of the effect of the Cl functionality on the reactivity of peroxy radicals.…”