Force Fields for Deep Eutectic Mixtures: Application\nto Structure, Thermodynamics and 2D-Infrared Spectroscopy

Parametrizing\nenergy functions for ionic systems can be challenging.\nHere, the total energy function for an eutectic system consisting\nof water, SCN<sup>–</sup>, K<sup>+</sup> and acetamide is improved\nvis-a-vis experimentally measured properties. Given the importance\nof electrostatic interactions, two different types of models are considered:\nthe first (model <b>M0</b>) uses atom-centered multipole whereas\nthe other two (models <b>M1</b> and <b>M2</b>) are based\non fluctuating minimal distributed charges (fMDCM) that respond to\ngeometrical changes of SCN<sup>–</sup>. The Lennard-Jones parameters\nof the anion are adjusted to best reproduce experimentally known hydration\nfree energies and densities, which are matched to within a few percent\nfor the final models irrespective of the electrostatic model. Molecular\ndynamics simulations of the eutectic mixtures with varying water content\n(between 0 and 100%) yield radial distribution functions and frequency\ncorrelation functions for the CN-stretch vibration. Comparison with\nexperiments indicates that models based on fMDCM are considerably\nmore consistent than those using multipoles. Computed viscosities\nfrom models <b>M1</b> and <b>M2</b> are within 30% of\nmeasured values and their change with increasing water content is\nconsistent with experiments. This is not the case for model <b>M0</b>.