Investigation of Vinyl Phosphonic Acid / Hydroxylated { Al 2 O 3 ( 0001 ) Reaction

Using rst-principles total energy calculations, we investigated the reaction enthalpies for bonding of a single vinyl phosphonic acid (VPA) molecule to selected three-fold sites on hydroxylated {Al2O3(0001). We examined tridentate, bidentate and unidentate coordinations for the bound VPA fragment following a condensation reaction with the surface. Based upon our calculations we found that: (a) the tridentate coordination had the most negative reaction enthalpy, and the unidentate coordination had the least negative reaction enthalpy; (b) VPA molecule binding to the oxide surface was site sensitive and depended upon the geometry of the surface and the distance of the underlying Al ion directly beneath the centroid of the site; (c) for the two VPA geometries that we examined, the angular orientation of the hydrocarbon tail relative to the bound tripodal base was found to have little e ect on the tridentate reaction enthalpies; (d) the bonding character, which was analyzed with the electron localization function, revealed a small reduction in the covalent character between the P{O bonds that anchor the VPA fragment to the oxide surface. No signi cant changes occured in the bonding character of the vinyl group. Finally, some comments on the entropic contributions of the VPA and H2O molecules to the binding energetics are o ered, along with a discussion of the e ects of H2O placement on the oxide surface and aluminum alloying agents. Typeset using REVTEX