Crystal Structure Prediction for Aprotic Ionic Liquids - Searching for the Unknown.

Area of Science:

• Materials Science
• Computational Chemistry
• Crystallography

Background:

• Ionic liquids (ILs) are widely studied, but their solid-state properties, including crystal structures and phase behavior, remain poorly understood.
• Existing knowledge gaps hinder the complete characterization and application of ILs, particularly concerning their solid forms.
• Understanding IL crystallization is crucial for controlling their solid-state properties and predicting their behavior.

Purpose of the Study:

• To develop and validate a computational crystal structure prediction (CSP) scheme specifically for aprotic ionic liquids.
• To investigate the factors influencing the crystallizability of ILs, especially the differences between similar compounds.
• To explain the observed crystallization behaviors, including polymorphism and glass formation, in ILs.

Main Methods:

• Developed a CSP protocol combining quasi-random crystal structure generation, dispersion-corrected density functional theory (DFT-D) for energy reranking, and quasi-harmonic phonon calculations.
• Applied the protocol to [emIm]-[MeSO4], a known polymorphic ionic liquid, to validate its accuracy.
• Analyzed the energy landscape and thermodynamic stability of predicted crystal structures using lattice energies and Gibbs free energies.

Main Results:

• The developed CSP scheme successfully identified the thermodynamically stable polymorph of [emIm]-[MeSO4] at the top of the stability ranking.
• Several low-energy, high-entropy crystal structures were proposed as candidates for the unresolved polymorphs of [emIm]-[MeSO4].
• The CSP modeling explained the reluctance of [emIm]-[EtSO4] to crystallize by revealing a glassy polymorph landscape with no distinct global energy minimum.

Conclusions:

• The new computational protocol is effective for predicting ionic liquid crystal structures and understanding their phase behavior.
• The study provides insights into the crystallizability differences between similar ILs, linking it to their solid-state energy landscapes.
• This work advances the understanding of IL polymorphism and glass formation, crucial for designing and utilizing ILs in various applications.