H-Bonding Enables Molecular Dancing

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.