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

The calculated molecule-surface binding energy, E(cal)( *), for physical adsorption was determined using molecular mechanics MM2 parameters for a model graphite surface and various organic molecules. The results for E(cal)( *) were compared to published experimental binding energy values, 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,N-dimethylformamide, and C(60) Buckyball), the experimental versus calculated binding energies were E( *)=1.1193E(cal)( *) and r(2)=0.967. The GC E( *) values were also well correlated by calculated E(cal)( *) values for a set of 11 benzene and methyl substituted benzenes and for another set of 10 alkanes and haloalkanes. The TD E(cal)( *) mechanics computation provides a useful comparison to the one for GC data since adsorbate-adsorbate interactions as well as adsorbate-surface must be considered.

Adsorption energies for a nanoporous carbon from gas-solid chromatography and molecular mechanics.

Gas-solid chromatography was used to obtain second gas-solid virial coefficients, B2s, in the temperature range 342-613 K for methane, ethane, propane, butane, 2-methylpropane, chloromethane, chlorodifluoromethane, dichloromethane, and dichlorodifluoromethane. The adsorbent used was Carbosieve S-III (Supelco), a carbon powder with fairly uniform, predominately 0.55 nm slit width pores and a N2 BET surface area of 995 m2/g. The temperature dependence of B2s was used to determine experimental values of the gas-solid interaction energy, E*, for each of these molecular adsorbates. MM2 and MM3 molecular mechanics calculations were used to determine the gas-solid interaction energy, E*(cal), for each of the molecules on various flat and nanoporous model surfaces. The flat model consisted of three parallel graphene layers with each graphene layer containing 127 interconnected benzene rings. The nanoporous model consisted of two sets of three parallel graphene layers adjacent to one another but separated to represent the pore diameter. A variety of calculated adsorption energies, E*(cal), were compared and correlated to the experimental E* values. It was determined that simple molecular mechanics could be used to calculate an attraction energy parameter between an adsorbed molecule and the carbon surface. The best correlation between the E*(cal) and E* values was provided by a 0.50 nm nanoporous model using MM2 parameters.