Evaluation of molecular mechanics calculated binding energies for isolated and monolayer organic molecules on graphite

The calculated molecule–surface binding energy, Ecal∗, for physical adsorption was determined using molecular mechanics MM2 parameters for a model graphite surface and various organic molecules. The results for Ecal∗ were compared to published experimental binding energy values, E∗E∗, from gas chromatography (GC) or thermal desorption (TD). The binding energies from GC were for isolated molecules in the Henry's law region of adsorption, and the binding energies from TD were for molecules in monolayer coverage on a highly oriented pyrolytic graphite (HOPG). A simple desorption model was used to allow the calculation of monolayer coverage to include both molecule–surface and molecule–molecule interactions and then the results were compared to experimental values. For the 14 TD organic adsorbates (polyaromatic hydrocarbons, alcohols, benzene, substituted benzenes, methane, chloroalkanes, N,NN,N-dimethylformamide, and C60 Buckyball), the experimental versus calculated binding energies were E∗=1.1193Ecal∗ and r2=0.967r2=0.967. The GC E∗E∗ values were also well correlated by calculated Ecal∗ values for a set of 11 benzene and methyl substituted benzenes and for another set of 10 alkanes and haloalkanes. The TD Ecal∗ mechanics computation provides a useful comparison to the one for GC data since adsorbate–adsorbate interactions as well as adsorbate–surface must be considered.

Molecule–graphite binding energies calculated from molecular mechanics, based on a monolayer model, EcalML∗, predict experiment binding energies obtained from thermal desorption, Eavg∗.Figure optionsDownload as PowerPoint slide