Chemical Transformations, Molecular Transport, and Kinetic Barriers in Creating the Chiral Phase of (R, R)-Tartaric Acid on Cu(110)

Abstract

Metal surfaces modified by chiral molecules have been shown to be effective heterogeneous catalysts for enantioselective reactions; however, their performance is found to be critically affected by modification conditions. Recently, model chirally modified surfaces created by the adsorption of the well-known chiral modifier, (R, R)-tartaric acid on a Cu(110) single crystal surface, have been shown to exhibit a variety of surface phases (Ortega Lorenzo, M., Haq, S., Bertrams, T., Murray, P., Raval, R., and Baddeley, C. J., J. Phys. Chem. B103 (48), 10,661 (1999). Of these, only the low-coverage (4 0, 2 3) and (9 0, 1 2) phases are thought to be important for the enantioselective reaction. In this paper we report a detailed study of these two phases using the surface spectroscopic techniques of RAIRS, LEED, STM, and TPRS, and show that a remarkable dynamic interplay exists between them depending on adsorption temperature, coverage, and holding time. At low exposures, the conversion from the initially formed (4 0, 2 3) phase to the thermodynamically preferred (9 0, 1 2) phase is associated with a local chemical transformation from the monotartrate to the bitartrate form, accompanied by a change in the two-dimensional organization of the adsorbed modifier molecules which involves significant molecular mass transport and expansion in adsorption area. Time-dependent RAIRS data following this process show that it conforms to first-order kinetics and possesses a significant kinetic barrier of 73±2 kJ mol−1. Interestingly, increasing coverage of modifiers at the surface reverses the phase stabilities and causes reverse transformation of the (9 0, 1 2) bitartrate phase into the more densely packed monotartarte (4 0, 2 3) phase. Thermal evolution of the surface phases shows they are very robust and stable up to temperatures of >430 K, after which explosive decomposition of the molecule occurs in which intramolecular bonds break to release H2, CO2, and CO products into the gas phase. This work provides a fundamental insight into the delicate balances responsible for the creation or destruction of chiral phases at modified metal surfaces.