HȮ2 + HȮ2: High level theory and the role of singlet channels

At low temperatures and high pressures, the HȮ₂ + HȮ₂ reaction is an important reaction in combustion. There are significant unresolved discrepancies between prior theoretical and experimental studies of this reaction. It has generally been presumed to occur as an abstraction on the triplet surface to produce H₂O₂ + ³O₂. We employ a combination of high-level electronic structure theory (the ANL0 composite method or multi-reference methods as appropriate), sophisticated transition state theory (vibrationally adiabatic torsions, variational, and variable reaction coordinate), and master equation analyses to predict the thermal kinetics on the H₂O₄ surface. Notably, this analysis suggests a significant branching to HȮ₃ + ȮH near 1000 K via reaction on the singlet surface. This channel does not appear to have been considered in prior combustion models. HȮ₃ itself is a metastable complex (bound by only 3 kcal mol⁻¹) that rapidly dissociates to ȮH + ³O₂. Thus, the net reaction for this channel, HȮ₂ + HȮ₂ → ȮH + ȮH + O₂, converts two low reactivity HȮ₂ radicals into two highly reactive ȮH radicals. The ramifications of these newly derived rate expressions are highlighted through kinetic modeling studies of H2, CH₃OH, n-heptane, and isooctane oxidation; all at the high pressures of relevance to combustion devices.