Tunnelling and the kinetic isotope effect in CH3+CH4→CH4+CH3: An application of semiclassical transition state theory

Activation volume and quantum tunneling in the hydrogen transfer reaction between methyl radical and methane: A first computational study

We present a theory of the effect of quantum tunneling on the basic parameter that characterizes the effect of pressure on the rate constant of chemical reactions in a dense phase, the activation volume. This theory results in combining, on the one hand, the extreme pressure polarizable continuum model, a quantum chemical method to describe the effect of pressure on the reaction energy profile in a dense medium, and, on the other hand, the semiclassical version of the transition state theory, which includes the effect of quantum tunneling through a transmission coefficient. The theory has been applied to the study of the activation volume of the model reaction of hydrogen transfer between methyl radical and methane, including the primary isotope substitution of hydrogen with deuterium (H/D). The analysis of the numerical results offers, for the first time, a clear insight into the effect of quantum tunneling on the activation volume for this hydrogen transfer reaction: this effect results from the different influences that pressure has on the competing thermal and tunneling reaction mechanisms. Furthermore, the computed kinetic isotope effect (H/D) on the activation volume for this model hydrogen transfer correlates well with the experimental data for more complex hydrogen transfer reactions.

Perceptions on the Treatment of Apparent Isotope Effects during the Analyses of Reaction Rate and Mechanism

Isotope substitution, a compelling tool of physical chemistry, has been broadly applied in the research field of heterogeneous catalysis. In general, upon the differences in mass-related atomic vibrational frequencies and...

Calculations of quantum tunnelling rates for muonium reactions with methane, ethane and propane

Thermal rate constants for Mu + CH₄, Mu + C₂H₆ and Mu + C₃H₈ and their equivalent reactions with H were evaluated with ab initio instanton rate theory. The potential-energy surfaces are fitted using Gaussian process regression to high-level electronic-structure calculations evaluated around the tunnelling pathway. This method was able to successfully reproduce various experimental measurements for the rate constant of these reactions. However, it was not able to reproduce the faster-than-expected rate of Mu + C₃H₈ at 300 K reported by Fleming et al. [Phys. Chem. Chem. Phys., 2015, 17, 19901 and Phys. Chem. Chem. Phys., 2020, 22, 6326]. Analysis of our results indicates that the kinetic isotope effect at this temperature is not significantly influenced by quantum tunnelling. We consider many possible factors for the discrepancy between theory and experiment but conclude that in each case, the instanton approximation is unlikely to be the cause of the error. This is in part based on the good agreement we find between the instanton predictions and new multiconfigurational time-dependent Hartree (MCTDH) calculations for Mu + CH₄ using the same potential-energy surface. Further experiments will therefore be needed to resolve this issue.

Calculations on the unimolecular decomposition of the nerve agent VX

It is very difficult to perform experiments on the physical parameters for the thermal decomposition of chemical nerve agents such as VX and computations, therefore, are useful. The reaction dynamics of the gas-phase pericyclic hydrogen transfer of the nerve agent VX is studied computationally. The geometries of the stationary structures are calculated at M06-2X/jul-cc-pVTZ level of theory. Single point energy calculations are carried out at the CBS/QB3 level to correct the energy barriers. Canonical reaction rate constants are calculated as a function of temperature. The one-dimensional semiclassical transition state theory is used to analyse the quantum tunneling effects. A reduced-dimensional hindered rotor model is proposed, tested, and applied to calculate the vibrational partition functions. It is found that the ester (O-side) and thioester (S-side) side chains of VX undergo pericyclic H-transfer reactions that result in decomposition of the molecule. The S-side reaction is favoured both kinetically and thermodynamically and dominates the pyrolysis over the temperature range from 600 K to 1000 K. It is predicted that VX completely decomposes in 2 s at temperatures above 750 K.

Hydrogen tunnelling in the rearrangements of carbenes: the role of dynamical calculations

A tunnelling controlled reaction is studied with semiclassical transition state theory, rationalising the results of experiment.

Understanding H2 Formation on Hydroxylated Pyroxene Nanoclusters: Ab Initio Study of the Reaction Energetics and Kinetics

The rate constants of H₂ formation on five models of silicate nanoclusters with varying degrees of hydroxylation, (Mg₄Si₄O₁₂)(H₂O)_N, were computed over a wide temperature range [180-2000 K]. We tested nine combinations of density functional methods and basis sets for their suitability for calculating reaction energies and barrier heights, and we computed the minimum energy H + H → H₂ reaction paths on each nanocluster. Subsequently, we computed the rate constants employing three semiclassical approaches that take into account tunneling and nonclassical reflection effects by means of the zero curvature tunneling (ZCT), the small curvature tunneling (SCT), and the one-dimensional semiclassical transition state theory (SCTST) methods, which all provided comparable results. Our investigations show that the H₂ formation process following the Langmuir-Hinshelwood (LH) mechanism is more efficient on the hydroxylated (N = 1-4) nanoclusters than on the bare (N = 0) one due to relatively higher reaction barrier height on the latter. H₂ formation is found to have the smallest barrier and the most exothermic reaction for the moderately hydroxylated (Mg₄Si₄O₁₂)(H₂O)₂ nanocluster for all nine considered methods. Overall, we conclude that all the considered nanoclusters are very efficient catalyzing grains for H₂ formation in the physical conditions of the interstellar medium (ISM) with pyroxene nanosilicates having moderate to high hydroxylation being more efficient than bare nanograins.

Theoretical Study of Gas-Phase Unimolecular Decomposition of Simulants of the Nerve Agent VX

In order to further understand and support approaches for the degradation and destruction of toxic chemicals, the thermal decomposition of the nerve agent VX through possible pericyclic hydrogen transfer reactions is investigated using simulant molecules. A total of four simulant molecules are studied. Three of them have only one possible H-transfer site, while the other has two. They are chosen to bring physical insights into individual steps of the pericyclic reaction mechanism as well as the possible existence of competing mechanisms. The unimolecular reaction rate constants at the high-pressure limit are calculated. Geometries of stationary structures on the potential energy surfaces are calculated with the MP2 method as well as the B3LYP and M06-2X functionals and 6-311++G(d,p), jul-cc-pVTZ, and aug-cc-pVTZ basis sets. The barrier heights are corrected using energy values obtained at the CBS/QB3 level of theory. The contribution of the quantum tunneling effect to the reaction rate constants is included using one-dimensional semiclassical transition state theory. Adiabatic barrier heights, reaction rate constants, and branching ratio of the competing mechanisms are reported.

Spiers Memorial Lecture : Introductory lecture: quantum dynamics of chemical reactions

This Spiers Memorial Lecture discusses quantum effects that can be calculated and observed in the chemical reactions of small molecules.

Catalysis and tunnelling in the unimolecular decay of Criegee intermediates

Semi-classical Transition State theory can be applied to catalysed atmospheric reactions, but reaction mode anharmonicity must be treated carefully.