Extension of a charged anisotropic united atoms model to polycyclic aromatic compounds.

A new potential model for polycyclic aromatic hydrocarbons has been developed on the basis of a charged anisotropic united atoms (AUA) potential with six AUA force centers and three electrostatic point charges per aromatic ring. Using quantum mechanical calculations, quadrupolar moments of several aromatic molecules were computed and a correlation has been observed that links the magnitude of the point charges with respect to the number of aromatic rings. The Lennard-Jones parameters of quaternary carbon atoms bridging two aromatic rings have been optimized with the minimization of a dimensionless error criterion incorporating various thermodynamic data of naphthalene. The new potential model, called ch-AUA, was then evaluated on its abilities to predict thermodynamic and transport properties for a series of polycyclic aromatic compounds in a wide range of temperatures. Although the relative errors with respect to the experimental density, vaporization enthalpy, and vapor pressure data are similar to those computed with the noncharged AUA potential, the new ch-AUA potential noticeably improves the prediction of the shear viscosities of polycyclic aromatic compounds. Comparisons between experimental viscosities of 1-methylnaphthalene at different pressures and those computed using the new ch-AUA and the noncharged AUA potentials show that the new potential improves the prediction of viscosities at high pressures.