Enantiospecific Adsorption of ( R )-3-Methylcyclohexanone on Naturally Chiral Cu(531) R&S Surfaces
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Enantiospecific Adsorption of (R)-3-Methylcyclohexanone on Naturally Chiral Cu(531)R&S Surfaces Ye Huang Æ Andrew J. Gellman
Received: 22 May 2008 / Accepted: 22 July 2008 / Published online: 14 August 2008 Ó Springer Science+Business Media, LLC 2008
Abstract The enantiospecific adsorption and desorption of (R)-3-methylcyclohexanone on naturally chiral Cu(531)R&S surfaces was studied using temperature programmed desorption. The Cu(531)R&S surfaces are of interest because they lie at the center of the stereographic triangle and thus, have the highest density of chiral adsorption sites possible on the surface of a face centered cubic metal. Several (R)-3-methylcyclohexanone desorption features were resolved in the TPD spectra from Cu(531)R&S surfaces and were assigned to desorption of molecules from terrace, step, and kink sites. The peaks associated with (R)-3-methylcyclohexanone desorbing from the R- and S-kink sites differed in temperature by 2.2 ± 0.6 K. This corresponds to an enantiospecific difference in the desorption energies of 0.5 ± 0.2 kJ/mol, with a preference for adsorption of (R)-3-methylcyclohexanone at the S-kinks on the Cu(531)S surface. Keywords Chiral Enantioselective Enantiospecific Surface High Miller index Copper
1 Introduction Many organic molecules are chiral and thus, exist in two forms that are non-superimposable mirror images of one another. The two non-superimposable forms of chiral Y. Huang Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA A. J. Gellman (&) National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, PA 15236, USA e-mail: [email protected]
molecules, called enantiomers, have identical physical and chemical properties in achiral environments. Because life on Earth is based on proteins and enzymes that are homochiral, living organisms are chiral environments. As a result, the two enantiomers of a chiral compound behave differently when ingested by living organisms. Thus, in order to achieve the desired therapeutic effects, chiral pharmaceuticals must be produced in enantiomerically pure form. This raises great interest in the development and use of chiral media for enantiospecific production of enantiomerically pure pharmaceuticals [1]. Currently, the most common method used to produce an enantiomerically pure compound is to prepare a racemic mixture of its two enantiomers by traditional synthesis using achiral reactants, followed by separation of the racemic mixture into its enantiomerically pure components. Methods such as selective crystallization [2] and chiral chromatography [3] can be used to separate the racemic mixture. The drawback of synthesizing a racemic mixture and then separating the two enantiomers is that it wastes starting materials and the separation process is always expensive. Hence, the production of enantiomerically pure products via a direct enantioselective catalytic route is very attractive, provided that one can identify and prepare appropriate chiral catalysts. The
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