United atom forcefield for vapor-liquid equilibrium (VLE) properties of cyclic and polycyclic compounds from Monte Carlo simulations

Gibbs ensemble Monte Carlo and NPT simulations are applied to the prediction of VLE properties for cyclic and polycyclic molecules with internal flexibility. An extension of the TraPPE-UA forcefield is defined, considering the molecules fully flexible with appropriate intramolecular potential terms (bond stretching, bending, torsion, Van der Waals) in cycles as well as in side chains. New United Atom types are proposed to handle the ternary and quaternary carbons involved in naphthenes and aromatics bearing alkyl groups or naphthenoaromatics, and published TraPPE-UA parameters are kept unchanged for other groups. The extended forcefield is tested for accuracy and internal consistency on a set of 34 compounds comprising between 5 and 28 carbon atoms including naphthenic hydrocarbons, aromatic hydrocarbons, naphthenoaromatics, and thiophenic compounds. This set of compounds contains several multi-functional polycyclic molecules which can now be described with the forcefield extensions provided by this work (e.g. 5α cholestane, 1-hexyl tetralin) and that are representative of a much wider range of compounds for which predictions through theoretical methods are difficult to be achieved. Comparison with available experimental data on normal boiling temperature, vapor pressure, and liquid density, reveals average absolute deviations of 6.4 K on normal boiling temperature and 0.91% on standard liquid density, without noticeable increase of deviations with molecular weight. Computational efficiency allows simulations for compounds containing up to 30 carbon atoms.