(77e) Novel Reaction Pathways in Low-Temperature Oxidation: Applications in Liquid-Phase and Atmospheric Systems

Low temperature oxidation in the gas and condensed phases has been the subject of experimental investigations for many decades owing to applications in many areas of practical significance like thermal stability, combustion, atmospheric chemistry and industrial syntheses. However, dues to their intrinsic complexity, several fundamental aspects of the underlying kinetics remain a mystery. For example, 30 years ago Korcek and co-workers showed that the major oxidation products of alkanes are carboxylic acids and methyl ketones, but the known reactions producing these products are much too slow to explain the observed yields.

In this work, we use electronic structure methods and reaction rate theory are used elucidate mechanistic details of new pathways in liquid-phase and atmospheric oxidation. The first of these studies focuses on pathways that establish γ-ketohydroperoxides (KHP), well-known products in low-temperature alkane oxidation, as precursors to acids through a two-step process. Ab initio calculations are used to identify pathways leading from KHP to a cyclic peroxide isomer which decomposes through novel concerted reactions into carbonyl and carboxylic acid products. The predicted rate coefficients found to be in excellent agreement with experiment lending theoretical support to the 30-year old Korcek hypothesis.

Next, insights from the Korcek reaction are extended to atmospheric chemistry where similar cyclic peroxides are formed by reactions of the Criegee Intermediate (•CH2OO•) with double bonds. The role of chemical activation in reactions between •CH2OO• and aldehydes/ketones species is explored revealing chemically activated formation of organic acids as the major product channel at low pressures, in agreement with recent direct measurements. The presentation also discusses ongoing efforts at discovering as yet unknown pathways in oxidation chemistry and their impact on our understanding of oxidation mechanisms.