Role of Microsolvation and Quantum Effects in the Accurate Prediction of Kinetic Isotope Effects: The Case of Hydrogen Atom Abstraction in Ethanol by Atomic Hydrogen in Aqueous Solution.

Suraj Kannath,Paweł Adamczyk,Agnieszka Dybala-Defratyka,Antonio Fernández-Ramos,Dan Thomas Major,David Ferro-Costas

doi:10.1021/acs.jctc.9b00774

Abstract

Hydrogen abstraction from ethanol by atomic hydrogen in aqueous solution is studied using two theoretical approaches: the multipath variational transition state theory (MP-VTST) and a path-integral formalism in combination with free-energy perturbation and umbrella sampling (PI-FEP/UM). The performance of the models is compared to experimental values of H kinetic isotope effects (KIE). Solvation models used in this study ranged from purely implicit, via mixed–microsolvation treated quantum mechanically via the density functional theory (DFT) to fully explicit representation of the solvent, which was incorporated using a combined quantum mechanical-molecular mechanical (QM/MM) potential. The effects of the transition state conformation and the position of microsolvating water molecules interacting with the solute on the KIE are discussed. The KIEs are in good agreement with experiment when MP-VTST is used together with a model that includes microsolvation of the polar part of ethanol by five or six water molecules, emphasizing the importance of explicit solvation in KIE calculations. Both, MP-VTST and PI-FEP/UM enable detailed characterization of nuclear quantum effects accompanying the hydrogen atom transfer reaction in aqueous solution.

Highlights

Hydrogen abstraction from ethanol by atomic hydrogen is a well-known reaction which is one of the most important steps in ethanol decomposition.[1−5] This type of hydrogen abstraction reaction is used as competition kinetic standards to determine relative reaction rate constants for different solutes where hydrogen is not released during the reactions.[2−5] Depending on temperature, this reaction can proceed via three different channels arising from the hydrogen atom abstracted from different positions within the ethanol molecule.[6,7]
In two of the structures the methyl group is in the gauche configuration (g+ and g−), whereas in the other one it is in the alternate or trans (t) configuration
Using MP-variational transition state theory (VTST) we investigate the number of specific microsolvating water molecules required, within a continuum environment, to reach converged kinetic isotope effects (KIEs) values

Summary

Introduction

Hydrogen abstraction from ethanol by atomic hydrogen is a well-known reaction which is one of the most important steps in ethanol decomposition.[1−5] This type of hydrogen abstraction reaction is used as competition kinetic standards to determine relative reaction rate constants for different solutes where hydrogen is not released during the reactions.[2−5] Depending on temperature, this reaction can proceed via three different channels arising from the hydrogen atom abstracted from different positions within the ethanol molecule (methyl or methylene or the hydroxyl group).[6,7] at room temperature the reaction involving the abstraction from the closest carbon atom to the hydroxyl group (α-carbon) is much faster than the H abstraction from the hydroxyl or methyl groups Only this channel needs to be considered for the reaction occurring in aqueous solution (reaction R1). The total rate constant for the process is twice the one obtained from the one that only considers the abstraction of one hydrogen atom (Figure 1)

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Results

Conclusion