A structural investigation of ionic liquid mixtures
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Accepted version
Published version
Author(s)
Type
Journal Article
Abstract
The structures of mixtures of ionic liquids (ILs) featuring a common 1-butyl-3-methylimidazolium ([C4C1im]+
) cation but
different anions have been investigated both experimentally and computationally. 1
H and 13C NMR of the ILs and their
mixtures has been performed both on the undiluted liquids and these diluted by CD2Cl2. These experiments have been
complemented by quantum chemical density functional theory calculations and molecular dynamics simulations. These
techniques have identified the formation of preferential interactions between H2
of the imidazolium cation and the most
strongly hydrogen bond (H-bond) accepting anion. In addition, a preference for the more weakly H-bond accepting anion
to interact above the imidazolium ring through anion-π
+
interactions has been identified. The modelling of these data has
identified that the magnitude of these preferences are small, of the order of only a few kJ mol−1, for all IL mixtures. No
clustering of the anions around a specific cation could be observed, indicating that these interactions arise from the
reorientation of the cation within a randomly assigned network of anions. π
+
-π
+
stacking of the imidazolium cations was
also studied and found to be promoted by ILs with a strong H-bond accepting anion. Stacking interactions are easily
disrupted by the introduction of small proportions (< 50 mol%) of a weakly coordinating anion due to their propensity to
form anion-π
+
interactions. These results suggest that the formation of IL mixtures with different anions leads to subtle
structural changes of much lower energy than the Coulombic ordering of ions, accounting for why most IL mixtures exhibit
ideal, or nearly ideal, behaviour.
) cation but
different anions have been investigated both experimentally and computationally. 1
H and 13C NMR of the ILs and their
mixtures has been performed both on the undiluted liquids and these diluted by CD2Cl2. These experiments have been
complemented by quantum chemical density functional theory calculations and molecular dynamics simulations. These
techniques have identified the formation of preferential interactions between H2
of the imidazolium cation and the most
strongly hydrogen bond (H-bond) accepting anion. In addition, a preference for the more weakly H-bond accepting anion
to interact above the imidazolium ring through anion-π
+
interactions has been identified. The modelling of these data has
identified that the magnitude of these preferences are small, of the order of only a few kJ mol−1, for all IL mixtures. No
clustering of the anions around a specific cation could be observed, indicating that these interactions arise from the
reorientation of the cation within a randomly assigned network of anions. π
+
-π
+
stacking of the imidazolium cations was
also studied and found to be promoted by ILs with a strong H-bond accepting anion. Stacking interactions are easily
disrupted by the introduction of small proportions (< 50 mol%) of a weakly coordinating anion due to their propensity to
form anion-π
+
interactions. These results suggest that the formation of IL mixtures with different anions leads to subtle
structural changes of much lower energy than the Coulombic ordering of ions, accounting for why most IL mixtures exhibit
ideal, or nearly ideal, behaviour.
Date Issued
2016-02-26
Date Acceptance
2016-02-23
Citation
Physical Chemistry Chemical Physics, 2016, 18, pp.8608-8624
ISSN
1463-9084
Publisher
Royal Society of Chemistry
Start Page
8608
End Page
8624
Journal / Book Title
Physical Chemistry Chemical Physics
Volume
18
Copyright Statement
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Sponsor
Commission of the European Communities
Engineering & Physical Science Research Council (EPSRC)
Grant Number
226438
EP/K039946/1
Subjects
Chemical Physics
02 Physical Sciences
03 Chemical Sciences
Publication Status
Published