Kinetics of reactions of primary, secondary and tertiary β-hydroxy peroxyl radicals

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Abstract
The UV absorption spectra and kinetics of reactions of the primary, secondary and tertiary β-hydroxy peroxyl radicals HOCH2CH2O2, CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2 at or near 298 K have been studied using both the molecular modulation (MM) and the laser flash photolysis (LFP) techniques. The radicals were produced by the UV photolysis (254 or 248 nm) of mixtures containing H2O2 and one of the symmetric monoalkenes ethene, (E)-but-2-ene and 2,3-dimethylbut-2-ene, diluted in air. The spectra of CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2 were found to be very similar to that measured previously for HOCH2CH2O2, with maximum cross-sections of ca. 4 × 10–18 cm2 molecule–1 at 245 nm. At high monoalkene concentrations, the observed second-order removal of the peroxyl radicals (RO2) provided values of k3obs(in units of 10–12 cm3 molecule–1 s–1) of 3.1 ± 0.4, 0.84 ± 0.06 and 0.0114 ± 0.0017 for HOCH2CH2O2, CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2, respectively, the quoted values being the mean of the MM and LFP determinations: RO2+ RO2→ products (3), The values of k3obs are greater than the values for the elementary rate coefficients, k3, owing to secondary removal of the RO2 radicals. The details of the secondary chemistry and the probable magnitude of the additional removal are discussed. At low monoalkene concentrations, simultaneous production of HO2 in the systems allowed the reactions of RO2 with HO2 to be investigated: RO2+ HO2→ ROOH + O2(2), Values of k2(in units of 10–11 cm3 molecule–1 s–1) of 1.5 ± 0.3, 1.5 ± 0.4 and ca. 2 were determined for HOCH2CH2O2, CH3CH(OH)CH(O2)CH3 and (CH3)2C(OH)C(O2)(CH3)2, respectively, using the LFP technique. The kinetic data obtained in the present work, and those obtained previously for the allyl peroxyl radical, are used to infer rate coefficients for some of the reactions of the complex peroxyl radicals formed from the OH-initiated oxidation of isoprene. A mechanism describing the degradation of isoprene to first generation products is constructed using the derived coefficients, and key parameters obtained or inferred from other sources, which gives a reasonable description of product yields measured in the laboratory in both the presence and absence of NOx.
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