Molecular distortions and chemical bonding of a large pi-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 +/- 0.03 angstroms 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.