Molecular Distortions and Chemical Bonding of a Largeπ-Conjugated Molecule on a Metal Surface

Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are $0.18\pm 0.03\AA$ closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled—through charge transfer into the former lowest unoccupied molecular orbital—to the primary, extended chemisorption bond via the perylene skeleton.