Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Materials Science

Background:

• Ionic liquids, such as 1,3-dimethylimidazolium chloride ([C1C1im]Cl), are crucial in various chemical applications.
• Understanding the interactions within ion pairs (IPs) and their dimers is essential for predicting their behavior in condensed phases.

Purpose of the Study:

• To explore the structural and energetic landscape of gas-phase ion pair conformers and dimers of [C1C1im]Cl.
• To investigate the roles of π(+)-π(+) stacking, hydrogen bonding, and anion-π(+) interactions.
• To characterize novel structural motifs and their relation to liquid-phase properties.

Main Methods:

• Computational modeling to explore potential energy surfaces.
• Identification and analysis of stable ion pair conformers and dimers.
• Inclusion of dispersion effects in calculations.

Main Results:

• Classification of cation-cation ring stacking as electron-deficient π(+)-π(+) interaction.
• Identification of anion-donor π(+)-acceptor interactions in anion-on-top motifs.
• Obtained 21 stable IP-dimers within 0-126 kJ mol⁻¹ energy range.
• Demonstrated that low-energy IP-dimers do not necessarily originate from low-energy IP conformers.
• Observed that IP-dimers reveal structural features not apparent in individual IPs, reflecting local liquid structure.

Conclusions:

• The structural and energetic landscape of [C1C1im]Cl ion pairs and dimers is complex.
• IP-dimers provide a more comprehensive understanding of local liquid structures than individual ion pairs.
• Dispersion effects subtly influence relative energies and geometries, with functional dependence.