Time-resolved measurements of OH and HO2 product formation in pulsed-photolytic chlorine atom initiated oxidation of neopentane

Abstract

The time-resolved production of HO₂ and OH has been measured in pulsed-photolytic Cl-initiated oxidation of neopentane (2,2-dimethylpropane) between 573 and 750 K. The initial reaction in the oxidation process is the reaction of the neopentyl radical (R), formed by Cl + neopentane, with O₂. The neopentylperoxy radical (RO₂) formed can then isomerize to a hydroperoxyalkyl radical (QOOH) and dissociate. The production of HO₂ in the neopentane oxidation is attributed to secondary reaction of the RO₂ or QOOH radicals, since the neopentylperoxy radical cannot form a conjugate alkene + HO₂, and formation of 1,1-dimethylcyclopropane + HO₂ is energetically inaccessible. Significant HO₂ formation is measured above 623 K, and the formation of HO₂ increases with increasing temperature. The overall amount of HO₂ produced increases with increasing O₂ at 673 K, consistent with the proposed role for QOOH + O₂ in chain branching for this system. A simple kinetic model is constructed based on comparison with previous time-dependent master equation calculations of analogous processes in the reaction of n-propyl with O₂. The present experimental results require inclusion of formally direct pathways for several chemical activation reactions, especially direct production of OH from R + O₂. Estimation of these direct components by analogy with n-propyl + O₂ is reasonably successful. The isomerization from RO₂ to QOOH is found to be significantly larger than previously proposed. A possible course towards a generally applicable theoretically-based model for alkyl radical oxidation is suggested.