Cooperativity in ionic liquids

Abstract

Cooperativity in ionic liquids is investigated by means of static quantum chemical calculations. Larger clusters of the dimethylimidazolium cation paired with a chloride anion are calculated within density functional theory combined with gradient corrected functionals. Tests of the monomer unit show that density functional theory performs reasonably well. Linear chain and ring aggregates have been considered and geometries are found to be comparable with liquid phase structures. Cooperative effects occur when the total energy of the oligomer differs from a simple sum of monomer energies. Cooperative effects have been found in the structural motifs examined. A systematic study of linear chains of increasing length ͑up to nine monomer units͒ has shown that cooperativity plays a more important role than expected and is stronger than in water. The Cl¯H distance of the chloride to the most acidic proton increases with an increasing number of monomer units. The average bond distance approaches 218.9 pm asymptotically. The dipole moment grows almost linearly and the dipole moment per monomer unit reaches the asymptotic value of 16.3 D. The charge on the chloride atoms decreases with an increasing chain length. In order to detect local hydrogen bonding in the clusters a new parametrization of the shared-electron number method is introduced. We find decreasing hydrogen bond energies with an increasing cluster size for both the first hydrogen bond to the most acidic proton and the average hydrogen bond.

Choline Saccharinate and Choline Acesulfamate: Ionic Liquids with Low Toxicities

“…Many theoretical studies on ionic liquids consider ion pairs only to understand the basic intermolecular interactions. [82][83][84][85][86] Calculation of the behavior of ionic liquids in the condensed phase is still a challenge, [87][88][89][90][91][92] not only because calculations on many particle systems are expensive in computer time, but also because the intermolecular forces that govern the dynamics are not easy to describe.…”

Spectroscopic Evidence for an Enhanced Anion–Cation Interaction from Hydrogen Bonding in Pure Imidazolium Ionic Liquids

“…[8][9][10][11][12][13][14][15][16][17][18] Indications of hydrogen bonding is also provided by theoretical studies. [19][20][21][22] Recently however, some authors have strongly challenged the presence of hydrogen bonds in ionic liquids, and claimed that hydrogen bonding need not be invoked for explaining IL properties. [23][24][25] For this reason, we initiated a program of direct spectroscopic observation of hydrogen bonds by successively increasing hydrogen bond abilities in a set of well-chosen imidazolium-based ionic liquids.…”

Improved United-Atom Force Field for 1-Alkyl-3-methylimidazolium Chloride

“…The conformers are in plane except for top, consistent with previous findings. 58,70 The most stable conformer is 5m, followed by top, which has a slightly higher energy (see Table 2). In fact, the difference in the relative energies of 5m and top is very small, which will be reversed (see Table 2) when a different ab initio method is employed.…”

“…The second drawback of fitting using isolated cation-anion pairs is that the charge transfer is exaggerated in such ion pairs because every ion is actually coordinated by 5-6 counterions in a liquid, as was found by both classical MD 40,71 and ab initio MD. 57,58 Static quantum chemical calculations of linear chains up to nine ion pairs showed 70 that the average charge (absolute value) on chloride increased from 0.735 on the isolated ion pair to 0.814 on the linear nine ion pair chain, with an extrapolated value of 0.823 for an infinitely long chain. The absolute value of the charges on chloride obtained in this work are 0.723, 0.751, 0.824, 0.662, and 0.705 for the 5m, 4m, 44, top, and dimer conformers, respectively.…”

“…In fact, the difference in the relative energies of 5m and top is very small, which will be reversed (see Table 2) when a different ab initio method is employed. 58,70 The fitted charges are highly dependent on the geometries of the conformers, that is, the position of chloride relative to the cation. It is difficult to get an averaged charge distribution from these conformers, even though the multiconformer RESP fitting can be performed.…”