AVS 49th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Invited Paper SS1-MoM7
Enantioselectivity on Naturally Chiral Surfaces

Monday, November 4, 2002, 10:20 am, Room C-108

Session: Adsorption and Chirality
Presenter: A.J. Gellman, Carnegie Mellon University
Authors: A.J. Gellman, Carnegie Mellon University
J. Horvath, Carnegie Mellon University
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Chirality is an omnipresent feature of the biochemical and biophysical world. The handedness of the molecules that form the basis of life creates the need for enantiomeric purity in the chemicals used for pharmaceutical and other bio-active purposes. Many of the processes used for synthesis and preparation of enantiomerically pure compounds rely on the use of chiral surfaces. The high Miller index surfaces of metals have chiral structures and can, in principle, be used for to control enantioselectivity in chemical processes. The kinked step structures of such surface are chiral and thus serve as chiral binding sites for a number of enantiospecific adsorption, desorption and surface reactions. As an example, the orientation of chiral molecules on chiral surfaces can be shown to depend on the relative handedness of adsorbate and substrate. This has been shown by study of the infrared reflection absorption spectra of 2-butanoxy groups on the Ag(643) surface. The intensities of the absorptions by R- and S-2-butanoxy groups are dependent on the handedness of the Ag(643) substrate. Similarly, the heats of adsorption of small chiral molecules such as R- and S-propylene oxide (CH3CH(O)CH2) and R-3-methylcyclohexanone are sensitive to the handedness of surfaces such as Cu(643). This has been observed using thermally programmed desorption measurements which reveal that the desorption kinetics of these chiral molecules are enantiospecific on chiral surfaces. They do not exhibit enantiospecificity on achiral surfaces such as Cu(111). In the course of this work we have been able to identify chiral adsorption sites on high Miller index metal surfaces. Most recently we have been able to demonstrate an enantioselective separation using such surfaces to purify a racemic mixture of 3-methylcyclohexanone. Understanding and controlling these enantiospecific properties poses some extremely interesting challenges for surface chemistry and surface physics.