Capturing the Capture: Branching Ratios of O2 Addition in Isoprene Peroxy Radicals via Quasiclassical Trajectories

Abstract

Abstract A central unresolved question in isoprene chemistry concerns the branching ratio between the α- and γ-addition channels of O2 capture by the Z,Z’-OH-allyl hydroperoxy radical forming second-generation peroxy radicals, which ultimately determines the respective importance of the formation of hydroperoxy enals (HPALD, from α addition of O2) and dihydroxyperoxycarbonyl peroxy radicals (di-HPCARP, from γ addition). To address this, we performed on-the-fly quasiclassical trajectory (QCT) calculations to determine the capture rate coefficient, resolve the real-time dynamics of the O2 addition process, and derive reliable estimates of the branching ratios. Calculations were carried out for both allylic adducts formed after OH addition to isoprene, Case I (1-OH addition) and Case II (4-OH addition). We obtain capture rate coefficients of (2.4 ± 0.2) × 10–11 and (1.3 ± 0.1) × 10–11 cm3 molecule–1 s–1 for Cases I and II, respectively. The α/γ branching leans toward the α channel in Case I (85:15 α:γ) but is nearly statistical in Case II (50:50 α:γ). These results provide the first dynamical determination of the O2 capture step and place quantitative constraints on the major product yields in low-NOx isoprene oxidation.