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 + 3O_2. We employ a combination of high-level electronic structure theory (the ANL0 composite method or multireference 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) that rapidly dissociates to ȮH + 3O₂. Thus, the net reaction for this channel, HȮ₂ + HȮ₂ ® ȮH + ȮH + O₂, converts two low reactivity radicals into two highly reactive radicals. The ramifications of these newly derived rate expressions are explored through modeling studies of H₂/O₂, CH₃OH, nheptane, and iso-octane, all at high pressures.