Structural and thermodynamic properties of liquid ethylene carbonate and propylene carbonate by Monte Carlo Simulations

Monte Carlo simulations have been used to investigate molecular association in pure liquid ethylene carbonate (EC) and propylene carbonate (PC). Standard force fields have been developed in order to reproduce accurately experimental pure liquid properties. Intermolecular interactions were described by combining standard OPLS Lennard-Jones parameters and partial atomic charges from fitting to the MP2/6-31G* electrostatic potential surface (EPS) with the CHELPG procedure. It was found that the inclusion of MP2 electron correlation on calculations of the partial atomic charges is required in order to achieve a proper description of the experimental liquid properties. The resultant force fields yielded average errors of 1-2% in computed densities and heats of vaporization. A thorough characterization of the liquid structures was performed with radial distribution functions, coordination numbers, energy distributions and dipole-dipole correlations. The electrostatic interactions in condensed phase lead the neighboring molecules to a preferential head to tail alignment of the dipoles. In contrast, the most energetically favored configuration for the EC and PC dimers in the gas phase exhibited antiparallel dipoles. The results of simulations with the partial charges set to zero revealed that the electrostatic term plays a determining role in structuring the liquids. Basis set dependence of EPS was investigated comparing the CHELPG MP2/6-31G* charges and dipole moments with the ones obtained at the MP2/{6-311G**, 6-31++G**, 6-311++G** and 6-311++G(3df,3pd)} level. Polarization effects due to intermolecular interaction were also investigate and a good reproduction of the experimental density and heat of vaporization (1.5% and 2% of deviation, respectively) was accomplished for liquid EC using CHELPG charges obtained for the most stable EC dimer configuration (antiparallel) at the MP2/6-31++G** level of theory.