Peculiarities of Intramolecular Motions in Ionic Liquids

Traditional physical and physico-chemical methods paesent information about various macroscopic (bulk) parameters such as viscosity, q, dielectric constants 8, dipale moments p, refractive indices no, and some others at different temperatures. Several experimental methods are used for recording molecular dynamic properties in liquid solutions nuclear magnetic resonance spectroscopy (NMR), fluorescence, ionic conductivity, electron paramagnetic resonance (EPR) spectroscopy, and some others. However, such processes as salvation, chemical interaction between reagents, electron and energy transfer, etc., take 

The l-alkyl-3-methylimidazolium ionic liquids can be depicted in general as it is shown in Fig. 1, where A- is an anion, and Cn means a cation alkyl group CJT2n+i. All RTTLs of this line have common abbreviations Mmim+A-, or simply MmimA (n = 1, M is methyl), EmimA (n = 2, E is ethyl), PmimA (n = 3, P is propyl), BmimA (n = 4, B is butyl), HmimA (n = 6, H is hexyl), OmimA (n = 8, O is octyl), DmimA (n = 10, D is decyl), etc. 

One of the most powerful methods for investigation the structure, spatial organization and physical-chemical properties of complex and supramolecular systems on the microscopic, molecular level is EPR spectroscopy in its spin label/probe technique variant (Berliner, 1976 Buchachenko Wasserman, 1976 Likhtenstein, 1976). Usually, nitroxide radicals of different structure were used for studying of structural peculiarities of ionic liquids and the mobility of spin probes in them. We will discuss shortly the most important results obtained by different authors below. 

the first paper on the EPR study of RTIL Emim+b, was published by Noel et al., 1992. EPR spectra of 2,2,6,6-tetramethylpiperidinle-l-oxyl (RaH, see Fig. 1) and of 4-amino-2,2,6,6-tetramethylpiperidinl-e o xyl (R6NH2) were obtained at several temperatures, and the rotational correlation time values, t were plotted vs. Stokes coordinates T/q, where T is the absolute temperature, and q is a macroscopic viscosity. In the molecular solution, mineral oil, the rotational correlation time of R6NH2 was ca. 30 times longer than for R H. 

This was explaind as the result of a specific interaction of the RTIL ions with the NH2 moiety of R6NH2 contrary to ReH. 

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