A new united atom force field for alkylamines.

Abstract

We develop and validate a new united-atom (UA) model for alkylamines under the polarization consistent approach (PolCA) formalism. Making use of recent theoretical developments in the treatment of polarization effects, we apply post facto polarization corrections to improve the accuracy and transferability of the model. We parameterize the model against the experimental density, enthalpy of vaporization and dielectric constant of selected amine molecules. The model is then validated against self-solvation free energies and solvation free energies in n-hexadecane. Overall, the new PolCA model outperforms the state-of-the-art GROMOS UA model, eliminating systematic deviations in that model's predictions for methylamine. We also present further evidence that polarization energy corrections are required when transferring the model from pure liquids to mixtures, particularly when the solute and solvent have significantly different polarity. The main shortcoming of the model is the lack of accuracy in predictions of the dielectric constant of small amine molecules, which is overestimated relative to experimental data. We show that this is likely due to the effective dipole moment of practically all previous non-polarizable models (including PolCA) being higher than the real dipole moment of amines in the liquid phase, which we estimated using the recent Self-Consistent Electrostatic Embedding approach.