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.