Adsorption of pyridine on the gold(111) surface: Implications for "alligator clips" for molecular wires

The adsorption of pyridine on metal surfaces such as gold(111) is an important and much studied process in solution, but little is known about the adsorption of isolated pyridine molecules. We determine the structure, binding energy, and dipole moment for this system using the PW91 density functional, periodic imaging, a plane-wave basis set, and ultrasoft pseudopotentials. Significant binding is found only for atop configurations in which pyridine sits vertically above a single gold atom with its nitrogen bound to the gold. For this configuration. the calculated binding strength. after extrapolation to zero coverage is 8.4 kcal mol(-1), a value that is qualitatively realistic but quantitatively may be too small in order to explain known free energies of formation from solution. The binding is insensitive to tilting of the pyridine by up to 30degrees from vertical, but a sizable energy of order 5 kcal mol(-1) is predicted to be required in order to drive pyridine flat above the surface. Qualitative arguments based on analysis of the PW91 results combined with approximate estimates of the dispersion energy suggest that the binding is essentially dispersive in nature. While the application of density functional theory to problems of this type is problematic, earlier results obtained using PW91 for the analogous binding of ammonia to gold indicate that the computational method used is appropriate, at least for vertical atop adsorption.