Computational Studies of the Effects of Pressure on Reaction Kinetics and Reaction Mechanisms

Both experiment and first principles calculations unequivocally indicate that properties of elements and their compounds undergo a tremendous transformation at ultra-high pressures due to the fact that the difference between intra- and intermolecular interactions disappears under such conditions. Yet, even at much milder pressures, when molecules still retain their individual identity, their chemical properties and reactivity change dramatically. We propose a set of techniques, based on molecular dynamics simulations and quantum mechanical calculations, which can aid in the understanding and prediction of the behavior of chemical systems over a wide range of high pressures.Experimentally, the effects of pressure on reaction rates and equilibrium constants are described by their pressure derivatives, known as volumes of activation and reaction volumes respectively. These quantities are directly linked to partial molar volumes of reactants, transition states, and products. We formulate a molecular dynamics method for the accurate calculation of molecular volumes. This method can be applied to both stable and transient species, which makes it suitable for quantitative analysis of experimental volumes of activation and reaction volumes. The calculated partial molar volumes, as well as reaction and activation volumes obtained from them, agree well with experimental data. To assess the reliability of the experimental activation and reaction volumes, we also present an analysis of the most common empirical analytical functions used to obtain them from pressure dependences of the rate and equilibrium constants. Since mechanisms of chemical reactions are often described in terms of properties of their potential energy surfaces (PES) or Gibbs energy surfaces (GES), we present an analysis of pressure-induced deformations of GES of solvated reaction systems and use quantum mechanical and molecular dynamics simulations to construct energy surfaces and reaction profiles of compressed species, and to analyze how their shapes and topography change in response to compression. We also discuss the important role of volume profiles in assessing pressure-induced deformations of GES.