Chemical transformations, molecular transport, and kinetic barriers in creating the chiral phase of (R,R)-tartaric acid on Cu(110)

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. B 103 (48), 10,661 (1999). Of these, only the low-coverage (4 0, 2 3) and (9 0, 12) 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, 12) 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, 12) 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 shown molecule occurs in which intramolecular bonds break to release H-2, 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. (C) 2002 Elsevier Science.