The most stable adsorption geometries of two chiral modifiers on Pt(111)

Abstract The molecular description of chirality transfer on chirally modified metal surfaces is unclear due to the complexity of the systems and the weak energetic biases favoring specific enantioselective pathways. In order to move to an enhanced level of molecular understanding it is critical to define the chemisorption geometries of the modifiers responsible for enantiodifferentiation. Model surface science studies of chiral molecules on single crystals provide an indirect method to probe catalytic chirality transfer molecular mechanisms. Apart from the great difference between ultrahigh vacuum (UHV) and reaction conditions, the extent to which such studies can inform the interpretation of data obtained under reaction conditions depends on the degree to which the model systems are correctly defined. The same holds true for any comparison of UHV surface spectroscopy data to spectroscopic measurements made at the catalyst-solution interface under either in-situ or operando conditions. We present detailed reflection absorption infrared spectroscopy (RAIRS) data on ( R )-1-(1-naphthyl)ethylamine, ( R )-NEA, and its simple derivative, ( R )-1-(8-methyl-1-naphthyl)ethylamine, on Pt(111). Various conflicting proposals for the structure of NEA and analogous modifiers on Pt catalyst particles and extended Pt surfaces are found in the literature. Here, we find that below high sub-monolayer coverages on Pt(111), ( R )-NEA adopts a chemisorption geometry where the entire naphthyl group is π-bonded to the surface and the amine group forms a dative bond to the surface. The same general adsorption geometry is found for the methyl-substituted derivative. The study provides reference spectra for these chiral modifiers and confirms the molecular structures described in our previous studies of diastereomeric complexes formed by the ( R )-NEA/Pt(111) system.