Application of empirical interatomic potentials to the calculation of the structure and dynamics of paraffin molecules adsorbed on graphite

Abstract

The adsorption of a single molecule of ethane, propane, and butane on a basal-plane surface of graphite is investigated using empirically derived carbon-hydrogen and carbon-carbon potentials. The principal motivation for this study is the calculation of a number of structural and dynamical properties which can be compared with neutron scattering experiments on monolayer paraffin films adsorbed on graphite. For each molecule the equilibrium orientation and position with respect to the surface and contours of minimum potential energy have been calculated. We find that these molecules prefer to align themselves with their carbon skeleton parallel to the graphite basal plane with the hydrogen atoms closest to the surface occupying the center of the graphite carbon hexagons. We have also found that each of the molecules tends to distort upon adsorption by small antisymmetric rotation of the C${\mathrm{H}}_3$ groups about the terminal C-C bonds. Two different force-constant models have been used to calculate the frequencies of the surface vibratory modes of the adsorbed molecules. The calculated frequencies and, with few exceptions, the force constants are in reasonable agreement with values inferred previously from inelastic-neutron-scattering spectra of monolayer butane films. Finally, the anharmonicity of the molecule-substrate potential has been briefly studied by examining the temperature dependence of the height of a butane molecule above the graphite basal plane and of the frequency of the molecular vibration normal to the surface.