A New Force Field of Formamide and the Effect of the Dielectric Constant on Miscibility

Abstract

Current force fields underestimate significantly the dielectric constant of formamide at standard conditions. We present a derivation of an accurate potential for formamide, which functional form builds on the OPLS/AA force field. Our procedure follows the approach introduced by Salas et al. (J. Chem. Theory and Comp., Just accepted, 2015), that can use information from ab initio calculations and molecular dynamics simulations. We consider several strategies to derive the atomic charges of formamide. We find that the inclusion of polarization effects in the quantum mechanical computations is essential to obtain reliable force fields. By varying the atomic charges and the LennardJones parameters describing the dispersion interactions in the OPLS/AA force field, we derive an optimum set of parameters to obtain accurate results for the dielectric constant, surface tension and bulk density of liquid formamide in a wide range of thermodynamic states. We test the transferability of our parameters to investigate liquid/liquid mixtures. We have chosen as case study an equimolar mixture of formamide and hexan-2-one. This mixture involves two fluids with very different polar characteristics, namely, large differences in their dielectric constants and their performance as polar solvents. The new potential highlights the importance of the correct parametrization of the pure liquid phases to investigate liquid mixtures. Finally, we examine the microscopic origin of the observed inmiscibility between formamide and hexa-2-one.

Miscibility and Thermodynamics of Mixing of Different Models of Formamide and Water in Computer Simulation

“…At 278 least 25% of the trial moves have been successful in every case. the OPLS/AA model 54 modified by Peŕez de la Luz et al 28 (referred to here as OPLS/AA_mod), the model belonging to the CHARMM force field, and those proposed by Cordeiro 27 and by Macchiagodena et al, 29 referred to here as MMPB. For water, we consider the three-site SPC/E 56 and four-site…”

Thermodynamic, structural and dynamic properties of ionic liquids [C4mim][CF3COO], [C4mim][Br] in the condensed phase, using molecular simulations

“…[12][13][14][15] Recent contributions have modied the atomic partial charges and Lennard-Jones parameters, in computer simulations by considering the dielectric constant as a target property in order to mimic the miscibility observed experimentally between two different liquids. 16,17 In the literature, there is a constant need for reliable and accurate IL FF 14,18,19 able to capture, among others, the local electrostatic properties, 20 microscopic dynamics, 21 and vibrational spectra; 22 as indicated by Maginn, 18 many FFs already published have not been validated yet against reliable and sensitive properties such as enthalpy of vaporization, and dynamic properties. The lack of an accurate classical FF for predicting thermodynamic, structural, and transport properties for specic substances hinders the design of new ILs solvents for condensed applications.…”

Accurate prediction of bulk properties in hydrogen bonded liquids: amides as case studies

“…We point out that use of VS is not based on additional empirical parameters, but only in an accurate fitting of QM level properties. FFs tuned towards the nanoscopic structure 34 or bulk properties 38 for HCO-NX2 (X=H and/or CH3) molecules have been proposed also quite recently but, in addition to being based on empirically derived charges, these models failed to reproduce some of the cited properties. To assess the results obtained with our FF we performed extensive MD simulations, investigating the nanoscopic liquid structure of the three organic liquids as well as the level of reproduction of key thermodynamic properties.…”