Transferable step potentials for amines, amides, acetates, and ketones

Abstract

The vapor pressure and liquid density of 11 amines, 5 primary amides, 9 acetates, and 15 ketones are studied to characterize transferable step potential models. Discontinuous molecular dynamics simulations are performed for reference potentials with details of the molecular structure and bond lengths. The simulation results are interpreted with thermodynamic perturbation theory to provide an efficient basis for molecular modeling and characterization of the induction forces. The attractive potential is represented as four steps with only the first and last steps varied independently. The two middle steps are interpolated such that each site type is characterized by three parameters: the diameter, the depth of the inner well, and the depth of the outer well. The depths of the attractive wells are optimized to fit experimental data for vapor pressure and liquid density. In general, the vapor pressure is correlated to an overall 12% average absolute deviation (%AAD) and the liquid density to 4% AAD. These deviations are larger than the errors encountered in characterizing straight chain hydrocarbons but consistent with recent results for naphthenic and aromatic compounds. The subtle variability in molecular structure caused by the consideration of isomers with heteroatoms and branching in varied positions puts a strain on the assumption of transferability. An additional challenge is encountered as the coverage of site types is extended to broader classes of families; the availability of reliable experimental data becomes much more limited. This lack of experimental data is especially a problem for the primary amides in this study and a likely problem for most future extensions.