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 st udied include model dissociation, isomerization and association processes c arried out in hard body fluids composed of either spherical atoms or diatom ic (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 appro ximating the solvent and solute molecules as spheres of appropriately defin ed effective hard sphere diameters. The results suggest that solvent compos ed of nonspherical hard body molecules may be accurately represented by a h ard sphere fluid of the same pressure, and a nonspherical solute may be rep resented as a sphere whose effective hard sphere diameter depends on the ma gnitude and surface-area-to-volume ratio of the corresponding solute-solven t excluded volume, as prescribed by the excluded volume anisotropy (EVA) mo del. Furthermore, general hard body fluid thermodynamic expressions are com bined with simulation results to quantify local (solvation shell) and nonlo cal (macroscopic) contributions to excess reaction thermodynamic functions, and the results are compared with estimates of cohesive (and internal part ition function) contributions to chemical reactions. (C) 2001 American Inst itute of Physics.