Molecular dynamics simulations of cinchonidine-modified platinum in ethanol: comparisons with surface studies

The interaction of the chiral modifier cinchonidine with Pt(1 1 1) both in ultrahigh vacuum (UHV) and in ethanol solvent has been studied using molecular dynamics (MD) simulation. In UHV at low coverage (0.0125 molecules/Pt atom) and 298.15 K the cinchonidine was found to adsorb with the quinoline ring oriented largely parallel (alpha = 6degrees) to the surface. Cinchonidine surface attachment was found to be through both pi bonding of the aromatic group and adsorption of the C=C double bond of the vinyl group. The dihedral angles T-1 and T-2 are reduced about 30degrees and 20degrees, respectively, with respect to those corresponding to the vacuum free molecule, revealing the strong molecule-surface interaction. A previous UHV experimental study in the literature performed at similar coverage (theta approximate to 0.03 molecules/Pt atom) of the related aromatic molecule quinoline shows an aromatic tilt of approximate to15degrees, which is in reasonable agreement with the present MD simulations. The interactions between ethanol solutions of cinchonidine (0.129 and 1.035 M) and the platinum surface were also simulated at 298.15 K. The cinchonidine coverage was found to be 0.0125 molecules/Pt atom for a 0.129 M solution and approximately 0.0375 molecules/Pt for 1.035 M. The results indicate that either the entire quinotine group or at least part of the quinoline group is attached to the platinum surface (via the pi system) in all cases. For the less concentrated solution (0.129 M) we found two different equilibrium conformations, one in which only part of the quinoline is attached to the surface, and a slightly more stable conformation that involves the quinoline group being adsorbed parallel to the platinum surface. We found that cinchonidine conformation at the surface was not only affected by the ethanol solvent, but also by the cinchonidine-cinchonidine steric interactions and their competition for surface sites. These MD simulations are compared with recent in situ Raman and infrared studies of this important adsorption system, and show overall good agreement. (C) 2004 Elsevier B.V. All rights reserved.