The Structure of Imidazolium-Based Ionic Liquids: Insights From Ion-Pair Interactions

Patricia A. Hunt; Ian R. Gould; Barbara Kirchner

doi:10.1071/CH06301

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The Structure of Imidazolium-Based Ionic Liquids: Insights From Ion-Pair Interactions

Patricia A. Hunt ^A ^C , Ian R. Gould ^A and Barbara Kirchner ^B

+ Author Affiliations

- Author Affiliations

^A Chemistry Department, Imperial College London, London SW7 2AZ, United Kingdom.

^B Institut für Physikalische und Theoretische Chemie, Universität Bonn, 53115 Bonn, Germany.

^C Corresponding author. Email: p.hunt@imperial.ac.uk

Australian Journal of Chemistry 60(1) 9-14 https://doi.org/10.1071/CH06301
Submitted: 17 August 2006 Accepted: 12 October 2006 Published: 29 January 2007

Abstract

A large number of ab-initio (B3LYP/6–31++G(d,p)) computed ion-pair structures have been examined in order to determine if such calculations are capable of offering insight into the physical properties of the liquid state, particularly viscosity and melting point. Ion pairings based around the 1-butyl-3-methylimidazolium (C₄C₁im) cations and a range of anions (Cl, BF₄, and N(Tf)₂ where N(Tf)₂ is bis(trifluoromethylsulfonly)imide) were chosen because of the range of viscosities exhibited by the corresponding ionic liquids. We have used these results to build up a ‘picture’ of the ionic liquid structure which is consistent with molecular dynamics simulations and experimental evidence. However, further work is required to established if such an analysis could be predictive. Nevertheless, we establish clear relationships relating ion-pair association energy, a derived ‘connectivity index’, and the diversity of structures with viscosity and melting point. Our calculations indicate that ions in C₄C₁imCl form a strong, highly connected and regular array thus rationalizing the high viscosity and melting point. In contrast the ion-pairs of C₄C₁imN(Tf)₂ form a weakly interacting, highly disordered, and low connectivity network consistent with the low viscosity and melting point. C₄C₁imBF₄ lies midway between these two extremes.

Acknowledgments

P.A.H. gratefully acknowledges the Royal Society for a University Research Fellowship.

References

The Structure of Imidazolium-Based Ionic Liquids: Insights From Ion-Pair Interactions

Abstract

Acknowledgments

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