HȮ2 + HȮ2: High Level Theory and the Role of Singlet Channels

At low temperatures and high pressures, the H O center dot 2 + H O center dot 2 reaction is an important reaction in combus-tion. There are significant unresolved discrepancies between prior theoretical and experimental stud-ies of this reaction. It has generally been presumed to occur as an abstraction on the triplet surface to produce H2O2 + 3 O 2 . We employ a combination of high-level electronic structure theory (the ANL0 composite method or multi-reference methods as appropriate), sophisticated transition state theory (vi-brationally adiabatic torsions, variational, and variable reaction coordinate), and master equation anal-yses to predict the thermal kinetics on the H2O4 surface. Notably, this analysis suggests a significant branching to H O center dot 3 + center dot OH near 10 0 0 K via reaction on the singlet surface. This channel does not ap-pear to have been considered in prior combustion models. H O center dot 3 itself is a metastable complex (bound by only 3 kcal mol-1) that rapidly dissociates to center dot OH + 3 O 2 . Thus, the net reaction for this channel, H O center dot 2 + H O center dot 2 -> center dot OH + center dot OH + O 2 , converts two low reactivity H O center dot 2 radicals into two highly reactive center dot OH radicals. The ramifications of these newly derived rate expressions are highlighted through kinetic mod-eling studies of H 2 , CH3OH, n-heptane, and isooctane oxidation; all at the high pressures of relevance to combustion devices.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.