Raman studies of solvent effects on molecular vibration and isomerization
Abstract
Raman Scattering studies at both ambient pressures and high-pressure, in a diamond-anvil cell, are used to measure gas-liquid vibrational frequency shifts of normal modes of solute molecules dissolved in various solvents and the change in thermodynamic properties of an isomerization process. The results are used to test and refine a perturbed hard liquid model, which contains both attractive and repulsive contributions. For the study of vibrational frequency shifts, repulsive solvation forces are modeled using recently developed analytical "hard-fluid" expressions for heteronuclear two-cavity distribution functions in hard-sphere fluids, while attractive forces are assumed to contribute a van der Waals (linearly density-dependent) mean field. For the isomerization study, the solute molecule is modeled as a sphere. The Gibb's free energy for the isomerization process is again expressed in terms of the cavity distribution function and van der Waals (linearly density-dependent) mean field approximation. Raman experimental results for the vibrational frequency shift study compare favorably with theoretical predictions for all of the vibrational modes studied except for CH stretches, which appear to experience a nonlinearly density-dependent attractive frequency shift at high densities. Empirical attractive frequency-shift parameters, derived from gas-to-liquid shifts at 1 atm, agree reasonably well with those predicted using a simple dispersive and dipolar solvation force expression. Correlations of attractive force parameter with solvent polarization and dipole moment show a greater contribution to the attractive force from solvent polarizability than dipole moment in most of cases except for extreme cases such as acetone dissolved in acetonitrile. Isomerization studies are hampered by difficulties associated with measuring small changes in the intensities of Raman peaks. The theoretically predicted thermodynamic quantities nevertheless agree qualitatively with the experimental results. The theoretical model, furthermore, is used to predict thermodynamics of the isomerization at 500$\sp\circ$C.
Degree
Ph.D.
Advisors
Ben-Amotz, Purdue University.
Subject Area
Chemistry
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