Electronic Effects of Solvation: A Comparison of Gas-Phase and Condensed-Phase Photoelectron Spectroscopy

Abstract

The research presented in this dissertation focuses on the changes to the electronic structures of metal diphosphine and paddlewheel molecules due to different environments, especially intermolecular interactions present in the condensed phase. These intermolecular interactions, measured by photoelectron spectroscopy, represent important aspects of solvation, and an experimental measure of the interactions provides tests for modeling solvent effects. The condensed-phase molecular environment, in the form of thin films on a gold substrate, acts as an effective system to probe the electronic relaxation component of solvent effects by comparing the ionization energies of a molecule in the gas and condensed phase.The challenges of preparing thin films appropriate for study by condensed-phase photoelectron spectroscopy, without appreciable ionization broadening or substrate effects, and the development of a method for calibrating the resulting spectra to the vacuum level have been addressed. This method of calibration has been critical to allow for accurate comparisons of gas-phase and surface photoelectron data that had not been possible previously.The gas-phase and condensed-phase electronic structures of M(CO)4(P-P) (M=Mo, W; P-P=dmpe, dppe) and Mo2(O2CR)4 (R=Me, Ph) have been explored to probe the relative donor strength of methyl- and phenyl-substituted ligands in different environments. A reversal in the relative donor strengths of dmpe and dppe is observed, where dppe is stronger in the gas phase, and dmpe is stronger in the condensed phase; conversely, O2CPh- is a better donor than O2CMe- in both phases. The condensed-phase ionizations are destabilized relative to the gas-phase ionizations for all molecules due to the additional electron relaxation in the solvent environments which stabilizes the positive ion state in the condensed phase. The observed shift of the ionization energies is larger for the methyl-substituted molecules as compared to the phenyl-substituted molecules. Density functional calculations are in agreement with the experimental results and correctly predict the trend in the relative ionization energies of these molecules.The results of these studies indicate that intermolecular interactions, which are accounted for reasonably well by implicit and explicit solvation models, have a greater affect on the electronic structure of the smaller methyl-substituted molecules as compared to the larger phenyl-substituted molecules.