Intramolecular Hydrogen Shift Chemistry of Hydroperoxy-Substituted Peroxy Radicals.

Abstract

Gas-phase autoxidation - the sequential regeneration of peroxy radicals (RO2) via intramolecular hydrogen shifts (H-shifts) followed by oxygen addition - leads to the formation of organic hydroperoxides. The atmospheric fate of these peroxides remains unclear, including the potential for further H-shift chemistry. Here, we report H-shift rate coefficients for a system of RO2 with hydroperoxide functionality produced in the OH-initiated oxidation of 2-hydroperoxy-2-methylpentane. The initial RO2 formed in this chemistry are unable to undergo α-OOH H-shift (HOOC-H) reactions. However, these RO2 rapidly isomerize (>100 s-1 at 296 K) by H-shift of the hydroperoxy hydrogen (ROO-H) to produce a hydroperoxy-substituted RO2 with an accessible α-OOH hydrogen. First order rate coefficients for the 1,5 H-shift of the α-OOH hydrogen are measured to be ∼0.04 s-1 (296 K) and ∼0.1 s-1 (318 K), within 50% of the rate coefficients calculated using multiconformer transition state theory. Reaction of the RO2 with NO produces alkoxy radicals which also undergo rapid isomerization via 1,6 and 1,5 H-shift of the hydroperoxy hydrogen (ROO-H) to produce RO2 with alcohol functionality. One of these hydroxy-substituted RO2 exhibits a 1,5 α-OH (HOC-H) H-shift, measured to be ∼0.2 s-1 (296 K) and ∼0.6 s-1 (318 K), again in agreement with the calculated rates. Thus, the rapid shift of hydroperoxy hydrogens in alkoxy and peroxy radicals enables intramolecular reactions that would otherwise be inaccessible.