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Chiral Recognition in Surface Explosion

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journal contribution
posted on 2004-08-04, 00:00 authored by Bahar Behzadi, Sara Romer, Roman Fasel, Karl-Heinz Ernst
The vast majority of chiral compounds crystallize into racemic crystals. It has been predicted and was experimentally established as a rule that chiral molecules on surfaces are more easily separated into homochiral domains due to confinement into a plane and lower entropic contributions. We investigated the formation and stability of two-dimensional tartrate crystals on a Cu(110) surface for the racemic mixture for the first time by means of temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). At low coverage, a bitartrate species becomes separated into homochiral domains, while at high coverage a monotartrate species forms a racemic mixture. At the same coverage and lateral arrangement, the thermally induced autocatalytic decomposition reaction occurs for the monotartrate racemate at a lower temperature than for the pure enantiomers. The stereochemistry in this so-called “surface explosion” reaction is explained by a higher stability of the enantiopure lattice due to lateral hydrogen-bond formation. The higher stability of the enantiopure two-dimensional lattice is in contrast to the higher stability of racemic three-dimensional tartaric acid crystals but is consistent with the observation that homochirality is preferred in hydrogen-bonded self-assembled biomolecular structures.

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