Peculiarities of Intramolecular Motions in Ionic Liquids

Abstract

Room temperature ionic liquids (RTILs) have been studied intensely during last two decades, because they can often be used as green and efficient solvents for many chemical reactions in organic chemistry, for electrochemical applications, and for industrial catalytic and extraction processes (Mamantov & Popov, 1994; Wasserscheid & Welton, 2002, 2008; Rogers & Seddon, 2003; Dyson & Geldbach, 2005; Anastas et al., 2010). RTILs have been investigated with various physical-chemical methods because they can easily interact with different surfaces and chemical environments, that is sure important for practice. For better practical applications, characterization of the transport and solvating properties, of the molecular mobility in RTILs as in a new class of reaction media. Recent studies with the use of different spectroscopic methods form now a new research area, although to date there are rather few reports on fundamental investigations of such systems. Among various unique properties distinguish ionic liquids from common molecular solvents, such as: high viscosity, a broad liquid temperature range, high thermal stability, negligible vapor pressure, a wide electrochemical window, recyclability, and high solvation ability, the mechanism of molecular motions in RTILs media is of great importance. There are two, from the first view independent, types of reorientation movements: mobility of the whole particle (any molecule or ion, including both ionic parts of RTIL itself and dissolved molecules) and the intramolecular reorganization. Also, historically, all movements are usually divided on rotational and translational with different physical formalism for describing them. Relatively recently it was experimentally shown by Wasserman & Kovarski, 1986; Kovarski, 1997 that there is a certain quantitative correlation between rotational, Drot, and translational, Dtr, diffusion coefficients. Easy to see, that knowledge of such dynamic characteristics for various ionic liquids is very valuable for applications, i.e., in catalysis, organic synthesis, absorption and separation, for electro-chemical processes in RTILs. Traditional physical and physico-chemical methods present information about various macroscopic (bulk) parameters such as viscosity, η, dielectric constants e, dipole moments μ, refractive indices nD, 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