Cinchonidine Adsorption on Gold and Gold-Containing Bimetallic Platinum Metal Surfaces: An Attenuated Total Reflection Infrared and Density Functional Theory Study

Abstract

Adsorption of cinchonidine on monometallic Au and bimetallic Pt-Au and Pd-Au thin model films prepared by physical vapor deposition has been investigated with attenuated total reflection infrared (ATR-IR) spectroscopy. On Au the alkaloid forms an adsorbed layer that shows higher stability against desorption than the corresponding adsorption on Pt. In this adsorption layer the intermolecular interactions dominate over metal-adsorbate interactions as indicated by the absence of the spectroscopic features attributed to strongly flat adsorbed species. This behavior is further supported by Density Functional Theory (DFT) calculations indicating that flat and tilted orientations of the quinoline ring have comparable adsorption energy on Au but lower (7-10 kcal/mol) compared to adsorption on Pt (ca. 40 kcal/mol). As a consequence, the creation of a metal surface with isolated chiral sites is prevented by formation of an adsorbed structure formed by intermolecularly bound cinchonidine molecules on Au. While the binding to Pt is due to the formation of sigma-bonds to surface atoms, such aggregates are bound to Au mainly by van der Waals forces. Given this different nature of bonding of cinchonidine to Au and Pt, addition of Au to Pt and Pd films could be used to probe the changes of fractional coverage of the different adsorbed species of cinchonidine on the platinum metals. It is demonstrated that the lowering of the domain size of the platinum group metal by Au can simulate the effect of particle size on the distribution of the surface conformations of the alkaloid on a metal surface.