Theoretical Investigation of Formamide Adsorption on Ag(111) Surfaces

摘要

The cooperativity of intermolecular hydrogen bonds and adsorbate-surface interaction was studied theoretically for the prototype system formamide-silver. A dispersion-corrected density-functional method was employed for the study of the adsorption of hydrogen-bonded formamide networks on the Ag(111) surface, in order to account for weak van der Waals interaction. The dispersion correction considerably improved the calculated adsorption energy of formaldehyde with respect to experiment. We observed antieooperativity of formamide hydrogen bonds and molecule-surface interaction: at low coverage the formamide monomers adsorb vertically at the Ag(111) surface forming a weak O-Ag chemical bond. In contrast, formamide dimers, and also higher polymers formed by linear dimer chains, preferentially adsorb parallel to the surface. The surface-adsorbate interaction is rather weak, as demonstrated by small interaction energies, large vertical distances, and small perturbation of the molecular electronic structure. Consequently the adsorbate structures are very close to the corresponding gas-phase polymers. In this way, the reduction of adsorption energy per molecule in the parallel arrangement compared to the vertical adsorption is overcompensated.