Theoretical Studies of Chemical Systems and Reactions at High Pressures

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Pressure is an important physical parameter that determines selectivity, direction, and rates of chemical reactions. The effects of pressure on chemical kinetics are well studied and a vast amount of experimental data has been collected over the last sixty years. Reaction mechanisms are usually discussed in terms of properties of transient states (transition states), through which reactants transform into products. Traditionally, the experimentally measured effects of pressure on reaction rates are expressed in terms of so-called volumes of activation, which represent the difference in volumes of transition states and reactants. Since the volumes of reactants are readily available experimentally, the volumes of activation provide a direct measure of the transition state volumes and thus a unique glimpse at these transient species. This requires a reliable technique relating the volume of a microscopic system to its geometrical parameters. Until recently, such a technique did not exist. Over the past few years we developed a precise method based on computer simulations known as molecular dynamics. Within our approach, the volume of a microscopic reaction system is determined as an Archimedean displacement volume of the system in a solvent and is found as a difference between the volumes of the pure solvent (N particles) and the solvent containing the system (N+1 particles). The calculated volumes are sensitive to molecular geometry, strength and type of solute-solvent interactions, and physical parameters such as temperature and pressure, and closely reproduce the experimental volumes obtained at given conditions. We are now developing various applications of this method to problems of high pressure chemistry, biochemistry, and spectroscopy. Our ability to accurately calculate molecular volumes is at the heart of each of these applications.