Pasteur's separation of the optical antipodes of ammonium sodium tartrate tetrahydrate in 1848 and the concept of the asymmetrically substituted carbon atom, introduced by van't Hoff and le Bel, marked the birth of stereochemistry. Although the Fischer–Rosanoff convention based on d-(+)glyceraldehyde got general acceptance, it was not possible to link the optical rotary power to the absolute configuration of chiral molecules. In 1951, however, Bijvoet and co-workers determined the absolute configuration of (+)-sodium rubidium tartrate through anomalous X-ray scattering and showed that the arbitrary d-/l-convention matches reality. Increasing computational power and improved quantum mechanical codes nowadays allow to connect chiroptical spectra with absolute configuration, but this approach is still limited to relatively small or rigid molecules. Other methods for determining the absolute configuration, such as wetting or enantioselective adsorption on polar crystals and measurements of the electric polarization in ferroelectric liquid crystals, have been reported. The conformation of chiral molecules adsorbed on a surface plays an important role in biomineralization and in stereoselective heterogeneous catalysis. For example, tartaric acid serves as chiral modifier for the enantioselective hydrogenation of b-ketoesters over supported nickel catalysts. To gain deeper insight into the mechanisms of enantioselective surface chemistry and biomineralization, a detailed analysis of the local adsorbate structure is necessary. The absolute chirality of molecules adsorbed on surfaces has been inferred from scanning tunneling microscopy (STM) images. However, in most cases of molecular adsorption it is very difficult to correlate the observed electron densities with the absolute atomic positions, and modeling of STM images requires extensive theoretical and computational effort. Herein we demonstrate that the absolute configuration of chiral molecules adsorbed on single-crystal surfaces can be determined in a straightforward fashion by means of anglescanned X-ray photoelectron diffraction (XPD), a method delivering direct real-space structural information. For our study we chose the classic tartaric acid molecule adsorbed on the Cu(110) surface. This adsorbate system has been studied in great detail. At low coverages and after activation at 405 K, tartaric acid becomes doubly deprotonated, with the resulting bitartrate species forming long-range ordered chiral structures on the surface. Based on IR spectroscopy results, a local C2 symmetry for the bitartrate adsorbate complex has been proposed. Figure 1 illustrates the basic principles of the XPD experiment for the case of the adsorbed bitartrate species.
Read full abstract
Dec 6, 2023
Zhengnan Yuan + 5
Open Access