Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

Abstract

Summary.A review of the recent developments in the study and understanding of room temperature ionic liquids are given. An intimate picture of how and why these liquids are not crystals at ambient conditions is attempted, based on evidence from crystallographical results combined with vibrational spectroscopy and ab-initio molecular orbital calculations. A discussion is given, based mainly on some recent FT-Raman spectroscopic results on the model ionic liquid system of 1-butyl-3-methylimidazolium ([C 4 mim][X]) salts. The rotational isomerism of the [C 4 mim] þ cation is described: the presence of anti and gauche conformers that has been elucidated in remarkable papers by Hamaguchi et al. Such presence of a conformational equilibrium seems to be a general feature of the room temperature liquids. Then the ''localized structure features'' that apparently exist in ionic liquids are described. It is hoped that the structural resolving power of Raman spectroscopy will be appreciated by the reader. It is of remarkable use on crystals of known different conformations and on the corresponding liquids, especially in combination with modern quantum mechanics calculations. It is hoped that these interdisciplinary methods will be applied to many more systems in the future. A few examples will be discussed.

Investigation ofL(+)-Ascorbic acid with Raman spectroscopy in visible and UV light

“…The reason is that differences in the conformation geometry are substantially reflected in the couplings among the vibrational modes, leading to noticeable differencies in frequencies and intensities. This is similar to what has been found for other molecules such as amphetamine (82,83) or for ionic liquids (84,85).…”

Raman spectroscopy of ionic liquids derived from 1‐n‐butyl‐3‐methylimidazolium chloride and niobium chloride or zinc chloride mixtures

“…24,25 To our knowledge the ionic species formed in mixtures of alkyl imidazolium chloride and niobium or zinc halides have been scarcely studied so far. Nb 6 26 Electrochemical and electronic absorption spectroscopy of the melts have identified anionic complexes of the type [ Nb 6 Cl 12 Cl 6 ] 6 z .…”

Vibrational Spectroscopy for Studying Hydrogen Bonding in Imidazolium Ionic Liquids and their Mixtures with Cosolvents

“…The basics of such quantum chemical calculations can be found, for example, in Refs [17] and [18]. Moreover, the use of Raman spectroscopy in combination with ab initio calculations has recently been summarized in an excellent review by Berg [19]. In the following, an overview of vibrational spectroscopic investigations of hydrogen bonding in pure ionic liquids and their mixtures with cosolvents is given.…”