Adsorption of Chiral Alcohols on “Chiral” Metal Surfaces

Abstract

Kink sites on high Miller index surfaces are either left- or right-handed and can be thought of as chiral, when the step lengths on either side of the kink site are unequal. A silver single crystal was oriented and cut to expose the Ag(643) surface on one side and the Ag(6̄4̄3̄) surface on the other side. A system is proposed for naming these surfaces as Ag(643) S and Ag(643) R, respectively, in analogy with the Cahn−Ingold−Prelog rules used in the nomenclature of organic stereoisomers. The left hand/right hand relationship of the two surfaces was manifested by the direction of the splitting of the low-energy electron diffraction (LEED) spots. The interaction of the enantiomers of a chiral alcohol (( R )-2-butanol and ( S )-2-butanol) with each surface was studied using temperature-programmed desorption (TPD) measurements in order to ascertain the magnitude of the effect of surface chirality on the heats of adsorption. Desorption of the alcohols following exposure to the clean surfaces was molecular and exhibited first-order kinetics. No difference was observed between ( R )- and ( S )-2-butanol in either desorption temperature (225 K) or peak shape. Upon exposure to preoxidized surfaces, the alcohols deprotonated to form ( R )- and ( S )-2-butanoxide, both of which decomposed upon heating via β-hydride elimination. The decomposition product, 2-butanone, desorbed at 282 K. Again, no difference in the reaction kinetics of the enantiomeric alkoxides was observed on the two surfaces. From these results it can be concluded that the difference in (a) the heat of adsorption of the enantiomeric alcohols and (b) the difference in the energy barrier to β-hydride elimination for the enantiomeric alkoxides is less than 0.1 kcal/mol.