A theoretical study of the Ḣ- and HOȮ-assisted propen-2-ol tautomerizations: Reactive systems to evaluate collision efficiency definitions on chemically activated reactions using SS-QRRK theory

Abstract

In combustion, enols can undergo keto-enol tautomerizations, which are intermediate steps in the formation of pollutant species. In this work, we performed a theoretical kinetic study of the step-wise propen-2-ol tautomerization catalyzed by hydrogen and hydroperoxyl radicals. Ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level were run, and rate constants were calculated using the multistructural torsional variational transition state theory with small-curvature tunneling corrections. Hydrogen and hydroperoxyl radicals can induce a step-wise mechanism toward keto formation with a lower barrier than that of unimolecular tautomerization. The potential energy surface comprising these reactions is complex, involving different intermediates that are connected by different types of pathways. The hydrogen-assisted tautomerization consists of two steps where the formation of an intermediate radical takes place as a result of the addition of the hydrogen atom to the double bond of propen-2-ol. The high-pressure limit rate constants of the reactions of this intermediate radical toward propen-2-ol and acetone exhibit an Arrhenius behavior, in agreement with previous works. In the hydroperoxyl-assisted tautomerization, the acetone formation has two routes involving an overall of four steps. The route with the highest energy barrier becomes prominent above 800 K due to multistructural anharmonicity effects, which must be included for an accurate kinetic description of the titled reactions. Calculations of pressure-dependent rate constants showed that the original system-specific quantum Rice-Ramsperger-Kassel theory, together with the modified strong collision model (SS-QRRK/MSC), significantly underpredict the bimolecular stabilization rate constants for the hydrogen-assisted tautomerization above 1200 K by factors of up to three orders of magnitude when compared with the benchmark Rice-Ramsperger-Kassel-Markus/master equation method. To solve this problem, we tested two alternative definitions of the collision efficiency parameter by using an improved implementation of the SS-QRRK/MSC approach developed by us for chemically activated reactions. One of these definitions, provided by Gilbert et al. (1983), corrected the bimolecular stabilization rate constant behavior and yielded a maximum deviation factor of only 4.5 at 2000 K and 100 atm. For the hydroperoxyl-assisted tautomerization, pressure effects are negligible because the stabilization of the energized adduct cannot compete with the reaction leading to the final product for most of the physical conditions studied. Our calculated rate constants can be used to perform more accurate kinetic modeling of alcohols. Besides, the implementation of the SS-QRRK theory with the collision efficiency of Gilbert et al. (1983) proposed in this work is useful for computing pressure-dependent rate constants of chemically activated reactions, including all possible refinements (multi-dimensional tunneling, multistructural anharmonicity, etc.) considered in high-pressure limit calculations.