Use of ab Initio Calculations toward the Rational Design of Room Temperature Ionic Liquids

Abstract

Ionic liquids are gaining substantial interest as alternative reaction media. Despite the overwhelming amount of evidence suggesting a relationship between their structure and melting point, there still remains the problem of selectively choosing a particular ionic pair that will produce a liquid at room temperature. Ionic liquids based on 1-alkyl-3-methylimidazolium halides have been investigated using ab initio calculations utilizing Gaussian 98 and the 6-31G* and 6-31+G* basis sets. The calculated interaction energy was found to increase in magnitude with decreasing alkyl chain length at the Hartree−Fock level, although no trend was found to exist with increasing anionic radius. Correlations between melting point and interaction energy were investigated. Linear trends were found to exist in the 1-n-butyl-3-methylimidazolium (Bmim) halide series as well as the 1-alkyl-3-methylimidazolium iodide series.

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…According to the calculated minimum energy E e of the conformers, the GA was more stable than the AA conformer, but at 298.15 K the Gibbs energy of the AA conformer was 0.168 kJ mol À1 less than that of the GA conformer, indicating 52% of anti-anti vs. 48% of gauche-anti or almost equal amounts of the two conformers at equilibrium at room temperature [103]. A higher difference between the AA and GA energy was found in other calculations [131]. These results are consistent with the observation of both conformers being simultaneously present in the spectra of the [C 4 mim] þ cation liquids are given in Fig.…”

“…The calculated interaction energy was found to increase in magnitude with decreasing alkyl chain length at the Hartree-Fock level, but no trend was found with increasing halide anionic radius. Linear trends between melting point and interaction energy were found to exist for the 1-butyl-3-methylimidazolium halide series as well as for the 1-alkyl-3-methylimidazolium iodide series [131]. …”

“…Ionic liquids based on 1-alkyl-3-methylimidazolium halides have also been studied, e.g., by Turner et al [131] who performed more systematic ab-initio calculations utilizing Gaussian98 and 6-31G Ã and 6-31þG Ã basis sets but reported no spectral data. The calculated interaction energy was found to increase in magnitude with decreasing alkyl chain length at the Hartree-Fock level, but no trend was found with increasing halide anionic radius.…”

Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy

“…As an example, the observed melting point change for ionic liquids with different anions was explained in terms of the change in the interaction energy and the relative position depending on the anion. 14,40 Many calculation studies found that the relative position of the anion with respect to the cation could change with the anion type 9,[14][15][16]20,21 and, in case of imidazolium-based ionic liquid, proposed that the interaction between the anion and H-C(2) is crucial in determining this structure. 15,18,19,40,41 The recent simulated IR spectrum in the 2800-3200 cm -1 range was shown to change with anion position.…”

“…12, 13 Calculations that followed suggested that the above cation conformation change would alter the position of the anion and change the interaction between the anion and the cation in 1-alkyl-3-methylimidazolium halide ionic liquids. [14][15][16] An important issue in the structure of an ionic liquid is the relative position between the anion and cation comprising the bulk liquid. In the study of a mixture of methyl-ethyl imidazolium chloride ([MEI][Cl]) and AlCl 3 molten salt, Dieter et al first proposed that Cl -interacts preferentially with the hydrogen attached to C(2), C(4), and C(5) of the imidazolium ring.…”