Kinetic Isotope Effects in Multipath VTST: Application to a Hydrogen Abstraction Reaction

Abstract

In this work we apply multipath canonical variational transition state theory with small-tunneling corrections (MP-CVT/SCT) to the hydrogen abstraction reaction from ethanol by atomic hydrogen in aqueous solution at room temperature. This reaction presents two transition states which can interconvert by internal rotations about single bonds and another two transition states that are non-interconvertible enantiomers to the former structures. The study also includes another three reactions with isotopically substituted species for which there are experimental values of thermal rate constants and kinetic isotope effects (KIEs). The agreement between the MP-CVT/SCT thermal rate constants and the experimental data is good. The KIEs obtained by the MP-CVT/SCT methodology are factorized in terms of individual transition state contributions to facilitate the analysis. It was found that the percentage contribution of each transition state to the total KIE is independent of the isotopic substitution.

Influence of Multiple Conformations and Paths on Rate Constants and Product Branching Ratios. Thermal Decomposition of 1-Propanol Radicals

“…Multistructural VTST (MS-VTST) − uses a multifaceted dividing surface and includes the contributions from the different structures (in general, rotamers) of the reactant and the transition state (TS) in the calculation of the rate constant. , This approach takes into account only the single lowest energy path associated with the multiple reactant conformers and TS conformers. In multipath VTST (MP-VTST) ,,− a total rate constant is calculated using the contributions from the individual reaction paths that connect reactant conformers with the corresponding TS conformers. Quantum tunneling and variational effects are evaluated for each individual path separately.…”

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

“…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. These three structures interconvert easily by internal rotations about the C–O bond . One would expect also three transition state structures, but there are only two, with the methyl group in g (g+, or g–), depending on the hydrogen abstracted and in t configurations with respect to the OH group (Figure ).…”

“…Three different isotopic substitutions along with the contribution of each conformer to the total rate constant and accompanying isotopic fractionation were studied (reactions , , and ). Among the existing theories for calculating biomolecular rates, the transition state theory (TST) is one of the most widely used. ,, The main advantage of the TST over other theories is that it is easy to use and most suitable for analogizing the trends of a chemical reaction in terms of quantities which can be easily interpreted. , In the past few decades, there has been extensive research going on for exploring the parameters that control the chemical reaction rates. ,− Hydrogen abstraction reaction is a subject of intense theoretical research focused on seeking the most adequate treatment of quantum effects which constitute the origin of KIEs and are necessary to describe the behavior of light elements like hydrogen and its isotopes. One of the theoretical approaches is the variational transition state theory (VTST) ,,, and its recent developments, such as, e.g., multipath VTST (MP-VTST), ,− which allows for taking care of reactions with multiple conformers. This methodology also enables the incorporation of zero-point energy and multidimensional tunneling variational effects as well as multiple conformations in the determination of thermal rate constants.…”

“…This methodology also enables the incorporation of zero-point energy and multidimensional tunneling variational effects as well as multiple conformations in the determination of thermal rate constants. All these corrections can be used to make more accurate predictions of KIEs. , It is well-known that zero-point energy and nuclear quantum mechanical effects are essential when studying H transfer reactions. , …”

“…When dealing with solvent effects it is more common to model them using a continuum solvent methodology, so explicit solvation is typically not investigated within VTST calculations ,, (however, the so-called ensemble averaged VTST with multidimensional tunneling, EA-VTST/MT, which allows for incorporation of explicit environment, was regularly used a few years ago to study numerous enzymatic reactions − ). This implicit solvent assumption may lead to severe simplification, in particular in cases where specific solute–solvent interactions play an important role.…”