Raman spectroscopy, ab-initio model calculations, and conformational, equilibria in ionic liquids

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-methyl-imidazolium ([C4C1Im]+X-) salts. The rotational isomerism of the [C4C1Im]+ cation is described: the presence of anti and gauche conformational forms 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. The “localised 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, when used on crystals of known conformation and on the corresponding liquids, especially in combination with modern quantum mechanics calculations.

It is hoped that these inetrdisciplinary methods will be applied to many more systems in the future. A few examples will be discussed. Contents 12.1 Introduction...........307 12.2 Brief introduction to Raman spectroscopy ..............309 12.2.1 Basics .....................309 12.2.2 Experimental, fluorescence and fouriertransform- Raman spectroscopy instrumentation ...... 311 12.3 Brief introduction to ab-initio model calculations .... 312 12.4 Case study on Raman spectroscopy and structure of imidazolium-based ionic liquids ..... 312 12.5 Raman spectra and structure of [C4C1Im]+ liquids ..... 315 12.6 Normal mode analysis and rotational isomerism of the [C4C1Im]+ cation .........318 12.7 Other studies on [CnC1Im]+ liquids ... 329 12.8 Conformations equilibria in liquids versus temperature .... 332 12.9 Local structures in ionic liquids .....335 12.10 Other systems .................338 12.11 Other applications of Raman spectroscopy .................346 12.12 Conclusions .............346 Acknowledgments .............347 References ..................347 12.1 Introduction