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Hydrogen bonding plays a key role in the diffusion of organic molecules across solid surfaces in a dancelike performance, according to a study published in Science (2010, 328, 882). Previous surface studies based on area-averaging techniques have established that hydrogen adsorbed on solids influences diffusion of other adsorbates. But atomic-scale details of such diffusion processes, which can control chemical reactivity and self-assembly on surfaces, have remained largely unknown. With a combination of scanning tunneling microscopy and computational methods, Tulane University’s Shao-Chun Li and Ulrike Diebold (now at Vienna University of Technology, in Austria) and coworkers have worked out a mechanism by which catechol, C6H4(OH)2, moves across a titanium dioxide crystal surface. Upon adsorption, catechol’s OH groups dissociate, leaving the molecule to bind via its oxygen atoms in a bridging configuration to two titanium sites. The liberated hydrogen atoms bind to two surface oxygen atoms, but they can easily shuttle back and forth between TiO2 and catechol. Hydrogen’s shuttling actions make it energetically feasible for catechol to “lift one of its legs” by breaking an O–Ti bond, rotate 180° on the other oxygen leg, rebind to TiO2, and thereby “dance” across the surface via repeated rotations, the team says.
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