Surface X-ray diffraction on large organic molecules: thiouracil on Ag(111)

The geometric structure of an ordered monolayer of large organic molecules adsorbed on a metallic surface [thiouracil on Ag(lll)] was analyzed using s urface;X-ray diffraction. As in the bulk crystal structure, the surface ads orbed molecules are arranged in parallel chains. However, owing to the inte raction of the S atoms with the substrate as well as to the reduced dimensi onality and lower lateral packing density, all but one of the hydrogen bond s characteristic of the bulk structure are broken. This causes a doubling o f the chain repeat periodicity. The intact pyrimidine rings are tilted by 3 1 and 36 degrees with respect to the Ag surface. In addition, an elongation of the C=S bond distance by 0.17 Angstrom and an increase of the intra-pyr imidine-ring distances by about 0.10 Angstrom relative to the bulk are obse rved. (C) 1999 Elsevier Science B.V. All rights reserved.