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

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 k_3obs(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 k_3obs 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 NO_x.