The influence of molecular shape on chemical reaction thermodynamics

Hard body fluid theoretical and computer simulation results are combined to predict the influence of both solute and solvent shape on the excess free energy, entropy, and enthalpy of model chemical reactions. The reactions studied include model dissociation, isomerization and association processes carried out in hard body fluids composed of either spherical atoms or diatomic (homonuclear dumbbell) molecules. The effects of molecular shape on the solvent excess chemical reaction thermodynamic functions are compared with both bonded-hard-sphere (BHS) predictions and predictions obtained by approximating the solvent and solute molecules as spheres of appropriately defined effective hard sphere diameters. The results suggest that solvent composed of nonspherical hard body molecules may be accurately represented by a hard sphere fluid of the same pressure, and a nonspherical solute may be represented as a sphere whose effective hard sphere diameter depends on the magnitude and surface-area-to-volume ratio of the corresponding solute-solvent excluded volume, as prescribed by the excluded volume anisotropy (EVA) model. Furthermore, general hard body fluid thermodynamic expressions are combined with simulation results to quantify local (solvation shell) and nonlocal (macroscopic) contributions to excess reaction thermodynamic functions, and the results are compared with estimates of cohesive (and internal partition function) contributions to chemical reactions. (C) 2001 American Institute of Physics.