Collaborative Research: Structure Sensitive Surface Chemistry - Enantioselectivity on Chiral Surfaces

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Enantiomers are two molecules that have the same arrangement of chemical bonds, but have three-dimensional structures that are mirror images of each other, like left and right hands. These 'chiral' chemicals play a central role in living-organisms, where only the left- or right-handed forms are present; but not both. As a result, most biologically active molecules, such as pharmaceuticals, must also be chiral molecules, and they must match the handedness of the biochemicals found in life. However, producing enantiomerically pure forms of molecules is a challenging problem. In this collaborative research project funded by the Chemical Structure Dynamics and Mechanism A (CSDM-A) program of the National Science Foundation Division of Chemistry, Professors Andrew Gellman (Carnegie Mellon University) and Charles Sykes (Tufts University) are studying the chemistry of chiral molecules adsorbed on metal surfaces where the atomic structure also has a specific handedness. Working with their students, Professors Gellman and Sykes are developing curved metal surfaces that present a systematic variation in chiral structures. The designed surfaces enable the study of many atomic configurations with a single sample. Sophisticated spectroscopies map the chemical reactivity across the surface, while isolated chemical reactions are observed using a high-resolution microscope that can map the positions of individual atoms. In addition to providing training opportunities for future scientists, insights gained from the project could guide the design of surfaces for enantiospecific production of chiral chemicals in the pharmaceutical industry. The research targets the surface chemistry of two chiral compounds, tartaric acid (TA) and aspartic acid (Asp), on chiral Cu(hkl) surfaces. Single crystal Cu surfaces are polished into spherical shapes such that each point on the surface of the sphere exposes a plane with a different atomic structure. This makes it very efficient to study the adsorption and surface reactions of TA and Asp on all possible structures of a Cu surface; i.e. all possible Miller indices (hkl). The atomic structures of these surfaces are imaged with and without adsorbed TA and Asp using scanning tunneling microscopy. Independent measurements of TA and Asp decomposition kinetics are made on hundreds of different Cu(hkl) surfaces, all on a single sample. These data elucidate surface reaction rate constants as a function of surface orientation, ks(hkl), and hence enable an understanding of the effect of surface structure on chemical and enantiospecific reactivity. Combined, these data are used to identify those surface orientations with the highest enantioselectivities towards decomposition of the two enantiomers of TA and Asp. Imaging with the scanning tunneling microscope is then used to determine the atomic structure of the surfaces having greatest enantioselectivity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.