DFT and ATR-IR insight into the conformational flexibility of cinchonidine adsorbed on platinum: Proton exchange with metal

The adsorption mode of cinchonidine on platinum is discussed in the light of new computational studies in which the alkaloid is adsorbed on a large metal cluster. Previous computations focused on the role of 1(S)-(4-quinolinyl)ethanol as anchoring group, but in the present study the role of the quinuclidine ring is analyzed, leading to a consistent view of the roles of both moieties of the alkaloid in the adsorption process. The tertiary nitrogen of the quinuclidine moiety can participate in the anchoring of the alkaloid and can be protonated by surface hydrogen. The conformational flexibility of the quinuclidine moiety was investigated by attenuated total reflection infrared experiments under nearly in situ conditions, by comparing the adsorption behavior of cinchonidine and O-phenyl-cinchonidine on platinum. Close agreement was found between experimental observations and theoretical calculations. A mechanism is proposed whereby the tertiary nitrogen can promote charge polarization of hydrogen and its transfer to the substrate.