Enantiospecific desorption of R- and S-propylene oxide from a chiral Cu(643) surface.

The creation or preparation of surfaces with chiral structures offers opportunities for enantioselective heterogeneous chemical processing such as catalysis or adsorption. Although such heterogeneous processes may have advantages over their homogeneous counterparts, the development of enantioselective surfaces lags far behind developments in enantioselective homogeneous chemistry. One approach to the preparation of a chiral surfaces is the irreversible adsorption of a chiral molecule on a substrate.1-3 Such templated surfaces do exhibit enantiospecific properties. This communication describes a type of naturally chiral surface formed by cutting a single crystal, face-centered cubic metal to expose a surface with periodic steps with kinks. We demonstrate enantiospecificity in the kinetics of a well-defined elementary reaction step from such naturally chiral surfaces: molecular desorption of a chiral adsorbate. Although the bulk face-centered cubic structure has high symmetry and is not chiral, it has been pointed out that that such high Miller index planes do expose chiral surfaces.4-6 One example is the naturally chiral Cu(643) surface shown in Figure 1. The Cu(643) and Cu(-6,-4,-3) surfaces are nonsuperimposable mirror images of one another and are therefore chiral. These surfaces are denoted Cu(643)S and Cu(643)R on the basis of the orientations of the microfacets that comprise the kinks on the surface.5,7,8 The chirality of this type of surface can be demonstrated using low-energy electron diffraction (LEED) and is described in more detail elsewhere.4,5 This handedness implies that the two enantiomers of a chiral compound should interact enantiospecifically with such surfaces. We have demonstrated such enantiospecificity by measuring the desorption kinetics of Rand S-propylene oxide from Cu(643)R and Cu(643)S using temperature-programmed desorption (TPD). The first attempt to observe enantiospecific effects on naturally chiral, high Miller index metal surfaces examined the desorption energies of Rand S-butan-2-ol from the Ag(643)R and Ag(643)S surfaces and the activation barriers to â-hydride elimination for Rand S-butan-2-oxide on the Ag(643)R and Ag(643)S surfaces.4,5 No enantiospecific differences in kinetics were detected for either of these reactions. If the adsorption of these molecules is influenced by the chirality of the surface, the differences in the desorption energies and the activation barriers must be less than the experimental resolution of ∼0.1 kcal/mol. Later experiments by Attard et al. investigated the electro-oxidation of the Dand L-glucose and other molecules at chiral platinum electrodes.7-9 Pt(643) and Pt(531) electrodes do exhibit enantiospecific oxidation kinetics for the two enantiomers of glucose. Glucose is a fairly complex molecule with five chiral carbon atoms, and its electrooxidation is a multistep reaction. The experiments described in this communication have made use of a simpler chiral molecule, propylene oxide, with a single chiral center and have focused on the kinetics of a simple elementary reaction step, the desorption of propylene oxides from the Cu(643) surfaces. The experiments were performed in an ultrahigh vacuum chamber in which the surfaces of the Cu single-crystal sample were cleaned and characterized. The Cu(643)R and Cu(643)S surfaces were cleaned by cycles of Ar+ sputtering followed by annealing to 1000 K. The cleanliness of the surfaces was determined by Auger electron spectroscopy and the observation of sharp low-energy electron diffraction patterns.4-6 The temperature-programmed desorption (TPD) experiment is performed in several steps. Once the surfaces were clean, the propylene oxide was adsorbed by exposure to its vapor with the surface at a temperature of 125 K. A mass spectrometer was then used to measure the rate of desorption during heating at 1 K/s. The mass spectrometer housing is designed to sample desorption from one face of the crystal only. The result of such a TPD experiment is a plot of the propylene oxide desorption rate as a function of temperature. TPD spectra were acquired for both Rand S-propylene oxide adsorbed on both Cu(643)R and Cu(643)S surfaces. These experiments revealed that the propylene oxide adsorbs and desorbs from the surface reversibly. Figure 1 shows TPD spectra for Rpropylene oxide on Cu(643)R following exposures in the range 0.4-3.0 L (1 L ) 1.0 × 10-6 Torr‚sec). As the exposures increased, desorption peaks appeared at 222, 154, and 118 K, sequentially. The lowest-temperature peak did not saturate with (1) Blaser, H. U.; Jallet, H. P.; Lottenback, W.; Studer, M. J. Am. Chem. Soc. 2000, 122, 12675. (2) Baiker, A. J. Mol. Catal. A 1997, 115, 473. (3) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376-379. (4) Horvath, J. D.; Gellman, A. J.; Sholl, D. S.; Power, T. D. In Physical Chemistry of Chirality; Hicks, J., Ed.; ACS Books; American Chemical Society: Washingtong, DC, 2001; Vol. in press. (5) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483-2487. (6) Gellman, A. J.; Horvath, J. D.; Buelow, M. T. J. Mol. Catal. A 2001, 167, 3-11. (7) Attard, G.; Ahmadi, A.; Feliu, J.; Rodes, A.; Herrero, E.; Blais, S.; Jerkiewicz, G. J. Phys. Chem. B 1999, 103, 1381-1385. (8) Ahmadi, A.; Attard, G. Langmuir 1999, 15, 2420-2424. (9) Attard, G. J. Phys. Chem. B 2001, 105(16), 3158-3167. Figure 1. Series of TPD spectra taken following increasing exposures of R-propylene oxide to the Cu(643)R surface. The two highesttemperature peaks (∼222, ∼154 K) correspond to different adsorption states for R-propylene oxide on the Cu(643)R surface, while the lowtemperature peak (118 K) corresponds to multilayer desorption. The inset shows the Cu(643)R surface. The spectra were obtained by monitoring the signal at m/q ) 27 while heating at 1 K/s. 7953 J. Am. Chem. Soc. 2001, 123, 7953-7954