Changes in the Relative Acidities of Organic Compounds upon Surface Interaction: A Computational Study on the Rutile TiO2(110) Surface

Unlike acidity scales for organic compounds in the gas or aqueous phases, there are no defined scales when these molecules are adsorbed on solids. This is due to the complexity of surface-adsorbate interactions and the absence of information about the associated thermodynamics. In this work, we have computed the adsorption energies and changes in charge distribution of a large number of nonsubstituted and F-substituted alcohols on the surface of rutile TiO2(110), probably the most understood metal oxide surface, used as a prototype, in order to compile such a scale. The dissociative adsorption that follows an acid-base type of reaction is accompanied by charge redistribution which could reasonably be modeled by the degree of charge retention of the alkoxy that linearly scales with the net charge to surface. Equally important, the computed dissociative adsorption energies were consistent with experimental results for acid-base titrations on the surface in demonstrating that free energies of titration reactions were compressed relative to gas phase acidities because of the partial compensation of the cost of breaking O-H bonds by forming O-Ti bonds.