Study of solvent effects on thermodynamic properties of molecules
Abstract
The thermodynamic parameters which characterize chemical processes may change significantly when the species involved are placed in solution. In this work, theoretical models for these changes are proposed and evaluated by comparing their predictions with available experimental measurements and computer simulations. Initially, two basic models are proposed, a spherical and a non-spherical model. The spherical model assumes that both the solvent-solvent and the solute-solvent interactions are spherically symmetric. The non-spherical model assumes that the solute-solvent interaction is the sum of the interaction potentials between each solute atom and the solvent, and the solvent-solvent interaction is spherically symmetric. The solute excess internal energy and the solvent induced vibrational frequency shifts are used to test the models, assuming that the interactions in the system are Lennard-Jones (LT). Analytical expressions for frequency shifts in the low density limit are derived and used to obtain information about the changes in the solute-solvent interaction potential with solute bond length. At liquid densities, an analytical "hard fluid" model for the cavity distribution function of a mixed hard sphere fluid is used as an alternative to Monte Carlo simulations to approximate the solvent distribution around the solute. Both the spherical and the non-spherical models are reasonably accurate, but require adjustable parameters for the solute-solvent interaction in order to fit experimental data of monatomic fluids or homonuclear diatomics in non-polar fluids. The non-spherical model is also applied to the prediction of frequency shifts in clusters. A version of the non-spherical model, which divides the solute-solvent interaction into a repulsive hard sphere term and an attractive term, is applied to the study of dissociation and isomerization reactions. Attraction is modeled using an attractive LJ potential or using a mean field treatment. This latter approach is found to accurately predict vibrational frequency shifts and changes in thermodynamic properties associated with dissociation and isomerization reactions.
Degree
Ph.D.
Advisors
Ben-Amotz, Purdue University.
Subject Area
Analytical chemistry|Chemistry
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