Spin probing ionic probes

Carper, W. R., Pflug, J. L., and Wilkes, J. S., Dual spin probe NMR relaxation studies of ionic structure in l-ethyl-3-methylimidazolium chloride-AlClg molten-salts, Inorg. Chim. Acta, 202,89,1992. 

General regularities of molecular dynamics and local organization of micellar phase of polyelectrolytes complexes with ionic SAS [16-22, 26] were formulated for the solution of this problem spin probes were used. Formulas of some of the last ones are presented in Scheme 3. 

Transition metal ions are less widely applicable as spin probes but may be convenient for addressing ionic clusters or ligand moieties or for substituting diamagnetic ions. 

Fig. 12. Ionic clusters in telechelic ionomers based on diblock copolsmiers (see also Fig. 11). (a) Spin probes attached to ionic clusters located at the interface between the polystsrrene (PS) and polyisoprene (PI) microphases. The inset shows the CW ESR spectrum for PS-Q-PI, where Q is a quaternary ammonium group. The fast component (arrows) was assigned to probes in the PI microphase, (b) The distribution of ionic clusters in monoionic diblock copolymers suggested by DEER experiments, (c) The distribution of ionic clusters in zwit-terionic diblock copolymers suggested by DEER experiments.

Fig. 5. Distribution of local concentrations c/cq and corresponding background functions B(t) for monovalent charged ionic spin probes in a dispersion of charged planar platelets in water (simulation based on the Poisson-Boltzmann equation). A concentration of 1.67 mM of a monovalent salt corresponding to a Debye screening length of 7.5 nm was assumed, (a) Local concentration as a function of distance r from the platelet surface for counterions with unlike charge (enrichment near the surface), (b) Dipolar evolution function for counterions with unlike charge (solid line) and fit of the data between t = 1 and 2 ps by an exponential decay (dashed line), (c) Local concentration as a function of distance r from the platelet surface for counterions with like charge (depletion near the surface), (d) Dipolar evolution function for counterions with like charge (solid line) and fit of the data between t = 1 and 2 ps by an exponential decay (dashed line).

Fig. 12. The CW ESR spectra (=9.6 GHz) of surfactant spin probes in aqueous polymer lat-ices. (a) Surfactant labeled near the ionic head group (5-DOXYL-stearate). (b) Surfactant...

Fig. 15. Pulsed ELDOR (DEER) distance measurements on the ionic spin-probe TEMPO-4-carboxylate attached to ionic clusters in ionically modified diblock copolymers, (a) Schematic structure of a monoionic polystyiene-polyisoprene diblock copolymer modified by sulfonate end groups on the polyisoprene bloek (sample series S). (b) Schematic structure of an a,0)-zwitterionic polystyrene- ly-isoprene diblock copolymer modified by a quaternary ammonium end group on the polystyrene block and a sulfonate end group on the polyisoprene block (sample series Z). (c) Schematic structures of the polymer chains. Tlie solid line corresponds to the harder block polystyrene, the dotted line to the softer block polyisoprene. (d) Dependence of ionic cluster size (ri) and intercluster distance (r2> on molecular weight. Squares correspond to sample series Z, circles to sample series S, and diamonds to monionic homopolymers (polystyrene modified with quaternary ammonium end groups). The dotted and dashed lines are fits of a constant function. The solid line is the best-fit scaling law r2 = 2.09...

Another example is a recent study of confinement effects in ionomers carried out at W-band. Electronic Tx and T2 relaxation as a function of temperature was measured at W-band for spin probes localized at the interface between the ionic clusters and the polymer. Excellent angular selection W-band EPR which resolves x-, y-, and z-orientations of the nitroxides in the magnetic field allowed authors to probe electronic relaxation along those orientations while the sample remained macroscopically disordered. Based on these measurements of the electronic relaxation as a function of the nitroxide orientation, it was concluded that reorientation of these spin probes has clear uniaxial features. Moreover, evidence was presented that the dynamic constraints on the poly(isoprene) chains in the diblock copolymer propagate over the whole chain consisting of approximately 170 monomer units. 

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 effects of halides, carbon dioxide and water on the physical properties of Emim+[(CF3SQ3)2N] have been extensively studied by Barrosse-Antle et al., 2009. The system was studied using cyclic voltammetry, chronoamperometiy, and EPR spectroscopy. Diffusion coefficients in the pure and CC>2-saturated ionic liquid revealed a decrease in activation energy E ir of translational diffusion from 29.0 to 14.7 kj/ mol, suggesting a reduction in the viscosity of the RTIL with addition of C02. EPR spectroscopy was used to calculate Tc coefficients of a spin probe R H. Arrhenius plots of Tc in the pntre and C02-saturated RTIL resulted in a similar drop in E ot from 28.7 to 18.2 kj/mol. It was concluded that the voltammetric respense of the electroactive species in RTILs is not independent of other solutes. 

We would like to stress that in all these publications the authors investigated peculiarities of the rotational and translational diffusion of spin-probe molecules in various room temperature ionic liquids, comp)ared them with molecular dynamics in common organic solvents. Correlations with Stokes-Debye-Einstein or Stokes-Enstein laws were foimd. Areas in RTILs (polar, non-polar), in which spin probes (hydropElic, charged, hydrophobic) are localized were determined. Just recently, attention of the scientists was attracted to another type of molecular motions in the ionic liquids (Tran et al., 2007a, 2009). Such processes as well as solvent effects on them can be examined in detail by EPR sp>ectroscopy with the use of stable nitroxide biradicals (Parmon et al., 1977a, 1980). 

Chumakova, N. A. Pergushov, V. L Vorobiev, A. Kh. Kokorin, A I. (2010). Rotational and translational mobility of nitroxide spin probes in ionic liquids and molecular solvents. Appl. Magn. Reson., ISSN 0937-9347 (in press)... 

Stoesser, R. Herrmann, W. Zehl, A. Laschewsky, A. Strehmel, V. (2006). Microviscosity and micropolarity effects of imidazolium based ionic liquids investigated by spin probes. Their diffusion and spin exchange. Z. Phys. Chem., Vol. 220, No. 10, 1309-1342, ISSN 0942-9352... 

This report presents an overview of the literature on the use of EPR spectroscopy to study ionic liquids as solvents. After a short in overview on the history and the outstanding properties of ionic liquids, the report focusses on EPR investigations of rotational correlation times obtained with various spin probes, on biradicals, on electron selfexchange reactions as well as on synthetic and mechanistic aspects of ionic liquids. 

EPR spectroscopic investigations in ionic liquids using spin probes... 

Based on the unique properties of ILs regarding the wide range of viscosities available and the ionic character of the solvents, several spin probes have been used to investigate their rotational and translational mobilities. As spin probes, mainly derivatives of TEMPO and Fremy s salt are employed. 

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