THEORETICAL-STUDIES OF LIQUIDS BY COMPUTER-SIMULATIONS - THE METHANOL-PYRIDINE MIXTURE

The Monte Carlo method was used to calculate thermodynamic properties and radial distribution functions for methanol-pyridine mixtures as a function of composition. All calculations were performed in the isothe rmal and isobaric ensemble at T = 298.15 K and p = 1.0 atm. The optimi zed potentials for liquid simulations (OPLS) force held was used for p yridine and methanol molecules. In the OPLS force field the methanol m olecule is represented by a three-site model, as the methyl group is c onsidered as a united atom site. Two potential models were used for py ridine: a six-site one with carbon and hydrogen atoms represented by u nited atom sites, and an eleven-site model with all atoms explicitly i ncluded. The partial charges used in the eleven-site model for pyridin e are derived from ab initio 6-31G wavefunctions. The results obtaine d for the average cofigurational energy of the methanol-pyridine syste m as a function of the mole fraction are in good agreement with experi mental data. The partitioning of the total configurational energy show s comparatively weak interaction between methanol and pyridine molecul es. The values obtained for the methanol-pyridine interaction energy a re slightly more negative when the hydrogen atoms of pyridine are cons idered explicitly. The radial distribution functions calculated for me thanol-pyridine correlation show characteristic features indicating th e formation of hydrogen-bonded dimers. The position and amplitude of t he peaks observed on these distribution functions depend on the partic ular model used to represent the pyridine molecules. The coordination numbers obtained for methanol-methanol interaction in the pure liquid and in the equimolar methanol-pyridine mixture are very similar, indic ating a small influence of pyridine in the association of methanol mol ecules. Molecular graphics representation confirms the association of methanol molecules through hydrogen bonding in the bulk of the methano l-pyridine mixture.