An Electrochemical and ESR Spectroscopic Study on the Molecular Dynamics of TEMPO in Room Temperature Ionic Liquid Solvents

Abstract

The field of room temperature ionic liquid (RTIL) research is currently very active, with the potential of fashioning "greener" alternatives to traditional molecular solvents driving intensive studies into their applications across many chemical disciplines, which include the synthetic and electrochemical industries. [1,2] Characterisation of the transport properties and solvating ability of this new class of reaction media will presumably form an important part in realising this endeavour to its full potential, although to date, reports on more fundamental investigations of this nature have been less common. To aid in this effort, we recently explored the temperature dependence of the translational diffusion of probe molecules N,N,N',N'-tetramethyl-para-phenylenediamine, its radical cation and dication in several RTIL solvents, by means of a chronoamperometric technique. [3] Amongst these species, the variation in translational diffusion coefficient, D T , with temperature in each ionic liquid was found to obey an Arrhenius relation of general form [Eq. (1)]:where E a represents the activation energy associated with the temperature-dependent process, f 0 is a constant of proportionality and, in the current context, f = 1/D T . Furthermore, the diffusional activation energies calculated by plotting ln f versus 1/T and measuring the slope were, for a given RTIL, largely independent of solute charge or size. Subjecting the measured viscosity, h, of the ionic liquids at each temperature to the same Arrhenius-type analysis [with f = h in Equation (1)] systematically yielded an activation energy of viscous flow that also closely resembled this diffusional activation energy. This implies an inverse linear relationship between translational diffusion coefficient and solvent viscosity, akin to that commonly observed in conventional molecular solvents and generally formulated in terms of the Stokes-Einstein equation [Eq. (2)]:where a is the hydrodynamic radius of the (spherical) diffusing molecule. The current report aims to build on this work by now expanding the investigation to include rotational diffusion. The equivalent hydrodynamic description of molecular rotation in a liquid is given by the Debye-Stokes-Einstein equation [Eq. (3)]:where D R and t R are termed the rotational diffusion coefficient and rotational correlation coefficient, respectively. Note that an inverse linear relationship between rotational diffusion coefficient and solvent viscosity is again predicted. For sufficiently viscous solvents, electron spin resonance (ESR) can be utilised to measure the rotational correlation coefficient of paramagnetic species via analysis of the spectral line-shapes, a technique which exploits the fact that restricted molecular tumbling of the rotational probe leads to an incomplete averaging of the ESR signal on the experimental timescale, ultimately manifesting as asymmetry in the resulting spectrum. While ESR has seldom been employed to study ionic liquids, the reports that have appeared describe the successful app...

The electrochemistry of TEMPO-mediated oxidation of alcohols in ionic liquid

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Electroreduction of Sulfur Dioxide in Some Room-Temperature Ionic Liquids

“…No trends in solubility or diffusion coefficient are observed with respect to viscosity in these systems, though other systems sometimes exhibit an inverse linear relationship between diffusion coefficient and solvent viscosity as described by the Stokes-Einstein equation. 45 SO 2 appears to be the most soluble in [C 6 mim][Cl], a fact seemingly at odds with the comparatively low current response observed for that RTIL in Figure 2. However, a particularly low diffusion coefficient in that RTIL prevents the diffusion layer from being replenished with SO 2 quickly enough to produce a large response even when the solution has a high concentration of SO 2 overall.…”

Electrochemical reduction of nitrobenzene and 4-nitrophenol in the room temperature ionic liquid [C4dmim][N(Tf)2]

“…Recently, room temperature ionic liquids (RTILs) have become increasingly popular for use as solvents in electrochemical experiments [1][2][3][4][5][6][7][8][9][10][11][12]. Their wide electrochemical windows allow the extension of study to more negative (and positive) potentials than in conventional organic solvents [5].…”