Molecular dynamics calculation of molecular volumes and volumes of activation.

Experimentally, the effects of pressure on reaction rates are described by their pressure derivatives, known as volumes of activation. Transition state theory directly links activation volumes to partial molar volumes of reactants and transition states. We discuss a molecular dynamics method for the accurate calculation of molecular volumes, within which the volumes of molecular species are obtained as a difference between the volumes of pure solvent and solvent with a single molecule inserted. The volumes thus obtained depend on the molecular geometry, the strength and type of the solute–solvent interactions, as well as temperature and pressure. The partial molar volumes calculated using this approach agree well with experimental data. Since this method can also be applied to transition state species, it allows for quantitative analysis of experimental volumes of activation in terms of structural parameters of the corresponding transition states. The efficiency of the approach is illustrated by calculation of volumes of activation for three nonpolar reactions in nonpolar solvents. The results agree well with the experimental data.