Molecular modeling of isomer effects in naphthenic and aromatic hydrocarbons

The vapor pressure and liquid density of 12 aromatic and 11 naphthenic hydrocarbons are studied to characterize transferable step potential models. Discontinuous molecular dynamics simulations are combined with thermodynamic perturbation theory to provide the basis for molecular modeling. Since the attractive forces appear as perturbations in this approach, their characterization can be varied continuously, without re-simulating, to obtain optimal results in minimal time. A generalized grid search is used to optimize the attractive steps for multiple compounds and multiple site-types simultaneously. In general, the vapor pressure is correlated to 10% average absolute deviation (%AAD) and the liquid density to 4%AAD. These are slightly larger than the errors encountered in characterizing straight chain hydrocarbons, ethers, and alcohols. One complicating factor that may explain the larger errors in this instance is the subtle variability in molecular structure caused by the consideration of isomers with branching in different positions around the ring. The transferable potentials account for this variability in a configurational sense, but not in a quantum mechanical sense. A detailed study reveals the extent to which the vapor pressure of several isomers can be correlated qualitatively and quantitatively. A preliminary study suggests one manner in which quantum mechanical corrections can be implemented.