Study of solute-solvent interactions with vibrational CD spectroscopy and DFT calculations.

摘要

My PhD thesis work is centered on developing a suitable approach to account for solvent effects in solution spectroscopic measurements and on providing significant insights into the intermolecular interactions between chiral solute and solvent, in particular water, molecules. Vibrational absorption (VA) and vibrational circular dichroism (VCD) spectroscopic techniques have been used as the main experimental tools to study conformational distributions and most importantly solute-solvent interactions of a number of prototype chiral molecules in several common solvents. The spectral window from 800 cm-1 to 1800 m-1 was used for data acquisition. Aqueous solutions, both normal and deuterated water, and organic solvents such as methanol, dimethyl sulfoxide, and chloroform were used. Complementary optical rotatory dispersion measurements have also been carried out. Density functional theory has been employed to perform all calculations for conformational searches, geometry optimizations, VA, VCD, UV, and ECD intensities, and spectral simulations. To account for effects of water solvent, a clusters-in-a-liquid approach has been proposed. Molecular dynamics simulations and radial distribution function calculations have also been carried out to identify the representative hydration clusters, i.e. chiral solute-(water)N. Initial conformational analyses have been done using small basis set like 6-31G(d), which is a compromise between accuracy and computational cost. For final calculations, several larger basis set like 6-31++G(d,p), 6- 311++G(d,p), cc-pVTZ, and aug-cc-pVTZ, have been used and the specific choices depend on the size and complexity of systems under the investigation.;Geometries of the molecular systems of interest have been evaluated in the gas phase and in bulk solvent using the implicit solvation polarization continuum model, while the related geometries of the explicit hydration clusters of the targeted chiral molecules have been similarly evaluated. My thesis work shows that inclusion of both explicit and implicit solvent effects simultaneously is essential to interpret the experimental VA and VCD spectra whenever strong hydrogen-bonding interactions are expected between chiral solute and solvent molecules. When no strong solvent--solute hydrogen-bonding interactions are expected, it was found that the gas phase monomer model is adequate but not optimal for spectral interpretations and the inclusion of the implicit bulk water environment is highly recommended.