ADSORPTION OF SULFUR-CONTAINING MOLECULES ON GOLD - THE EFFECT OF OXIDATION ON MONOLAYER FORMATION AND STABILITY CHARACTERIZED BY EXPERIMENTS AND THEORY

The formation and stability of self-assembled monolayers (SAMs) of ary l sulfinates and, for the first time, aryl sulfonates ire described. T he ways in which the molecules interact with the surface and the stabi lity of the resulting SAMs were characterized by surface enhanced Rama n (SER) spectroscopy. Aryl sulfinate monolayers can be reversibly oxid ized to sulfonate monolayers, but the sulfonate is readily displaced b y sulfinate in solution. The relative adsorptivities of aryl sulfur sp ecies were found to be ArSO3- much less than ArSO2- < ArS-. Through a novel application of perturbation theory, in which the adsorbate-surfa ce Coulombic and charge transfer interactions and the change in the so lvation free energy of the adsorbate are taken into account, we have b een able to explain this trend. The differences in the adsorptivities of the anions studied here are primarily attributable to differences i n the adsorbate-surface charge transfer interactions. These were evalu ated by calculating the adsorbate HOMO energies at the ab initio HF/3- 21G(d) level. Higher adsorbate HOMO energies are correlated with highe r adsorptivities, consistent with established trends in the adsorptivi ties of soft, basic anions on metal electrodes. Statistical perturbati on theory was used to calculate the relative free energies of hydratio n of the three anions. The sulfonate is most strongly solvated, follow ed by the sulfinate and thiolate. Thus, the trend in solvation energie s is consistent with and probably reinforces the trend in the adsorbat e-surface charge transfer interactions. The combination of computation al methods used here may prove generally useful for predicting the rel ative adsorptivities of molecules and ions on metal surfaces.