Crystal Structure Prediction for Aprotic Ionic Liquids – Searching for the Unknown

Ionic liquids (ILs) represent an extensively studied class of materials. Nevertheless, their solid state has often been overlooked, leading to frequent knowledge gaps about their phase behavior or crystal structures that such materials may form. This work focuses on the development of a crystal structure prediction (CSP) scheme suitable for aprotic ILs, relying on quasi-random crystal structure generation, dispersion-corrected density functional theory (DFT-D)-based energy reranking, and quasi-harmonic phonon treatment. The interpretation of peculiar differences in the crystallizability of very similar ILs upon cooling of their melts is presented. The versatility of the computational protocol is validated for [emIm]-[MeSO₃], an IL known to be polymorphic. The current CSP identifies the [emIm]-[MeSO₃] polymorph that is thermodynamically stable in reality at the top of the stability ranking, both in terms of DFT-D refined lattice energies and quasi-harmonic Gibbs free energies. Several low-energy, high-entropy crystal structures are also proposed for [emIm]-[MeSO₃] as candidates for the remaining known polymorphs with yet unresolved crystal structures. Our CSP modeling explains the extraordinary reluctance of [emIm]-[EtSO₄] to crystallize due to its glassy shape of the polymorph landscape with no distinct global energy minimum crystal structure.