Structure sensitive enantioselectivity on surfaces: tartaric acid on all surfaces vicinal to Cu(111)

Abstract

Comprehensive mapping of enantiospecific surface reactivity versus the crystallographic orientation of Cu(hkl) surfaces vicinal to Cu(111) has been conducted using a spherically shaped single crystal on which the surface normal vectors, [hkl], span all possible orientations lying with 14° of the [111] direction. This has allowed direct measurement on 169 different Cu(hkl) surfaces of the two rate constants, k^(hkl)_i and k^(hkl)_e, that determine the kinetics of the vacancy-mediated, explosive decomposition of tartaric acid (TA). The initiation rate constant, k^(hkl)_i, quantifies the kinetics of an initiation step that creates vacancies in the adsorbed TA monolayer. The explosion rate constant, k^(hkl)_e, quantifies the kinetics of a vacancy-mediated explosion step that results in TA decomposition and product desorption. Enantiospecificity is revealed by the dependence of TA decomposition kinetics on the chirality of the local surface orientation. Diastereomerism is demonstrated by the fact that D-TA is more reactive than L-TA on S surfaces while L-TA is more reactive on R surfaces. The time to reach half coverage, t^(hkl)_1/2, during isothermal TA decomposition at 433 K allowed determination of the most enantiospecific surface orientation; Cu(754). The ideal Cu(754) surface structure consists of (111) terraces separated by monoatomic steps formed by the (100) and (110) microfacets.