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4-Acetylbiphenyl is a small aromatic molecule that binds via dispersive forces (van der Waals interaction) with a gold surface. Experiments performed in the group of Dr. Moresco in TU Dresden show that under certain conditions of deposition these molecules formed a 4-molecule superstructure nicknamed the “windmill” because of its cross-shaped structure. Hydrogen bonding is known to be efficient in creating molecular superstructures, and the presence of oxygen in the acetyl group makes it natural to assign the formation of the supramolecular structure to H-bonding. Closer inspection of the scanning tunneling microscope images (STM) reveals that the unique seemingly H-bond should take place between a methyl group and the O atoms. This is one of the weakest H bonds if it binds at all. We undertook density functional theory calculations completed by empirical van-der-Waals corrections to study the supramolecular structure. From the simulation of STM images, total energy calculations and Gibbs free-energy estimates, we reach the startling conclusion that windmills take place only if gold adatoms join the four acetylbiphenyl molecules together.

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