Computational Study of Molecular Interactions in ZnCl2(urea)2 Crystals as Precursors for Deep Eutectic Solvents

Abstract

Deep eutectic solvents (DESs) are now enjoying an increased scientific interest due to their interesting properties and growing range of possible applications. Computational methods are at the forefront of deciphering their structure and dynamics. Type IV DESs, composed of metal chloride and a hydrogen bond donor, are among the less studied systems when it comes to their understanding at a molecular level. An important example of such systems is the zinc chloride–urea DES, already used in chemical synthesis, among others. In this paper, the ZnCl2(urea)2 crystal is studied from the point of view of its structure, infrared spectrum, and intermolecular interactions using periodic density functional theory and non-covalent interactions analysis. The two main structural motifs found in the crystal are a strongly hydrogen-bonded urea dimer assisted by chloride anions and a tetrahedral Zn(II) coordination complex. The crystal is composed of two interlocking parallel planes connected via the zinc cations. The infrared spectrum and bond lengths suggest a partially covalent character of the Zn−Cl bonds. The present analysis has far-reaching implications for the liquid ZnCl2–urea DES, explaining its fluidity, expected microstructure, and low conductivity, among others.