A Predictive Transferable Force Field for Primary, Secondary and Tertiary Amines

Based on the Anisotropic United Atom approach AUA, we propose a transferable force field that
includes primary, secondary and tertiary amines. A set of partial charges was calculated using Density
Functional Theory (DFT) while for the Lennard-Jones parameters an optimization procedure was
carried out fitting vaporization enthalpies, liquid densities and vapor pressures of a few reference
molecules. For the hydrocarbonated groups involved in the studied molecules, we have transferred
the parameters of the AUA4 force field for hydrocarbons. In order to include different molecular
structures in our study, a set of torsional potentials has been proposed including linear, branched and
multi-functional amines.

Monte Carlo simulations have been carried out in the Gibbs ensemble in order to calculate
thermodynamic properties at different temperatures such as phase equilibrium diagrams, vaporization
enthalpies, vapor pressures, normal boiling points, critical densities, critical temperatures and critical
pressures.

The transferability of the force field has been evaluated for different amine molecules obtaining
an excellent accuracy on the predictions of equilibrium properties as well as on critical coordinate
estimations. Our force field is also able to reproduce with high accuracy the normal boiling points of
the different studied amines with average deviations less than 2%.

We have also studied the capability of our force field to predict transport properties. Viscosity
coefficients have been calculated using molecular dynamics in the isothermal-isobaric ensemble by
means of the Einstein formalism. We have found a good agreement between the predictions of our
force field and the experimental data.