The formation and stability of self-assembled monolayers (SAMs) of aryl sulfmates and, for the first time, aryl sulfonates are described. The ways in which the molecules interact with the surface and the stability of the resulting SAMs were characterized by surface enhanced Raman (SER) spectroscopy. Aryl sulfinate monolayers can be reversibly oxidized to sulfonate monolayers, but the sulfonate is readily displaced by sulfinate in solution.The relative adsorptivities of aryl sulfur species were found to be ArSC>3_ < < ArSC>2-< ArS-. Through a novel application of perturbation theory, in which the adsorbate-surface Coulombic and charge transfer interactions and the change in the solvation free energy of the adsorbate are taken into account, we have been able to explain this trend. The differences in the adsorptivities of the anions studied here are primarily attributable to differences in the adsorbate-surface charge transfer interactions. These were evaluated by calculating the adsorbate HOMO energies at the ab initio HF/3-21G(d) level. Higher adsorbate HOMO energies are correlated with higher adsorptivities, consistent with established trends in the adsorptivities of soft, basic anions on metal electrodes. Statistical perturbation theory was used to calculate the relative free energies of hydration of the three anions. The sulfonate is most strongly solvated, followed by the sulfinate and thiolate. Thus, the trend in solvation energies is consistent with and probably reinforces the trend in the adsorbate-surface charge transfer interactions. The combination of computational methods used here may prove generally useful for predicting the relative adsorptivities of molecules and ions on metal surfaces.

Fluorescence excitation and fluorescence spectra of 2,2'-dithenyl (commonly known as bithiophene) seeded into a supersonic helium expansion show complete vibronic resolution. The fluorescence excitation spectrum has two components whose relative intensity depends on expansion conditions. Measurement and analysis of these spectra for systematic variation of expansion conditions leads to their assignment to the s-trans and s-cis conformers and provides information on the potential for s-trans to s-cis interconversion in the ground and first excited singlet states.

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