AVS 62nd International Symposium & Exhibition
    Surface Science Monday Sessions
       Session SS-MoA

Paper SS-MoA9
Mapping of Enantioselective Reaction Kinetics across Surface Structure Space: Tartaric and Aspartic Acid on Cu(111) Structure Spread Single Crystals

Monday, October 19, 2015, 5:00 pm, Room 113

Session: Organics and Ionic Liquids: Surfaces, Layers, Interfaces and Chirality
Presenter: Andrew Gellman, Carnegie Mellon University
Authors: A.J. Gellman, Carnegie Mellon University
A. Reinicker, Carnegie Mellon University
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On single crystal metal surfaces, enantioselectivity is, perhaps, the most subtle form of structure sensitive surface chemistry. Enantioselectivity can be observed on naturally chiral metal surfaces with structures that are traditionally described as having flat low Miller index terraces separated by kinked step edges, thereby lacking mirror symmetry. This work has mapped the enantiospecific decomposition kinetics of tartaric acid (TA) and aspartic acid (Asp) on 225 different single crystal planes exposed by the surface of a Cu(111) Surface Structure Spread Single Crystal (S4C). The Cu(111) S4C is a single crystal polished into a spherical dome shape that exposes a continuous distribution of surface orientations vicinal to the Cu(111) plane. During isothermal decomposition, XPS has been used to map the temporal evolution of the coverages of TA and Asp at points across the Cu(111) S4C. On Cu surfaces, both TA and Asp decompose by an explosive, vacancy-mediated decomposition mechanism consisting of an initiation step and a vacancy-mediated explosion step. The rate law for this process is parameterized by an initiation rate constant, ki, and an explosion rate constant, ke. Under isothermal conditions, the measured extent of reaction versus time at each point on the S4C has been used to fit the decomposition rate law and estimate the values of ki, and ke as functions of local surface structure across the Cu(111) S4C. These maps reveal that ki is maximum on surfaces with high densities of the close packed (100) steps and minimum on surfaces with high densities of the close packed (110) steps. As the angle between the surface and the (111) plane increases, ki increases linearly with step density. Along the direction containing the surfaces with (100) steps, the initiation step dominates and the kinetics look first-order. Along the directions exposing (110) steps the kinetics are dominated by the explosion step. In the case of Asp decomposition on the Cu(111) S4C, the decomposition kinetics are also sensitive to the chirality of the local surface orientation. Collectively, these data provide the deepest insight yet obtained in to the structure sensitivity of surface explosion reactions and the structural origins of enantioselectivity on naturally chiral surfaces.