Comparative Density Functional Theory Study of Cinchonidine and Hydrogen Coadsorption on Platinum Group Metals (Rh, Ir, Pd, and Pt) and Its Implications on Surface Chiral Site Formation

Coadsorption of hydrogen and cinchonidine (CD), the chiral modifier most frequently applied to bestow chiral sites onto platinum group metals, has been studied theoretically. In focus was the stability and configuration (spatial orientation) of CD adsorbed on the (111) surfaces of Rh, Ir, Pd, and Pt. On all these metals, CD is shown to be adsorbed with the quinoline moiety laying nearly parallel to the surface, likely through π-bonding. At high H-coverages, the position of the quinoline ring becomes tilted and the binding energy of CD decreases. While the configuration of adsorbed CD on Rh, Ir, and Pt is comparable, on Pd, it is significantly different, particularly as regards the position of the vinyl group of the quinuclidine moiety, which lays flatly on the surface and is involved in the adsorption bonding. On all metals, hydrogen coadsorption notably influences the stability and configuration of adsorbed CD. Generally, the stability of adsorbed CD decreases with increasing hydrogen coverage. The metals also show different propensities for H-transfer from the surface to adsorbed CD. Analyses of density of states projected on the orbitals of CD adsorbed on the (111) metal surfaces provide insights into the adsorption mechanisms depending on the H-coverage.