Spin Exchange in Solutions of TEMPOL in n-Octanol and 1-Methyl-3-octylimidazolium Hexafluorophosphate in the Temperature Range from 300 to 500 K

Abstract

Comprehensive examinations of the motional properties (rotational correlation time τ(R)) and the spin exchange ω(SS) of the spin probe TEMPOL have been carried out using ESR spectroscopy in two different solvents. For the first time, the dynamic parameters τ(R) and ω(SS) have been determined simultaneously by simulation of spectra measured at three different ESR frequencies (L-, X-, and Q-band) between 293 and 500 K using a dynamic model based on a stochastic fitting program and, for comparison, two alternative models involving the shift of the hyperfine lines and considering the line broadening due to spin exchange in a wide range of conditions. Possibilities and limits of the used models are shown upon comparing the obtained results of the spin exchange. Moreover, the analysis of the ESR spectra gave evidence for the existence of cage effects that produce re-encounters of the spin probes. This has been done for the activation energies, which have been calculated from the temperature dependence of the rate constants of the spin exchange. From the ratio of the activation energies and the influence of the viscosities on the dynamics of the examined systems in n-octanol and an ionic liquid, conclusions can be drawn for the re-encounter effects in solvent cages. However, in contrast to n-octanol, the dynamics of the spin probe in the ionic liquid depends on specific and anisotropic interactions. The temperature dependence of the Q-band measurements required the development of a novel Q-band cavity.

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

“…The mobility of stable nitroxide radical provides information on local viscosity and environment, and this approach is widely used in various fields of science, including 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 studies of ILs. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] However, in most cases previous CW EPR data were obtained at temperatures close to ambient and well above melting temperatures of corresponding ILs, and only a few works addressed the low temperature regions between glass transition (T g ) and melting (T m ) temperatures.…”

“…A number of EPR studies in ILs using spin probes have been accomplished, both in neat ILs and in IL/water mixtures. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In particular, it was shown that nanostructuring of ILs in water can be monitored via polaritydependent hyperfine splitting. However, in general, the shape of the nitroxide spectrum was found to be not very sensitive to the effects of nanostructuring, therefore developing other types of probes for heterogeneities in ILs would be a topical task.…”

The microwave cavity perturbation technique for contact-free and in situ electrical conductivity measurements in catalysis and materials science

“…8-12 GHz) resonator is difficult, if the reactor setup has not to be extensively miniaturized (e.g. by using a Q-band dewar and quartz tube 29 ) and a standard gas delivery and analysis system operating with ml min À1 rather than ml min À1 gas flows has to be used. Thus, we investigated the TM 110 mode of a cylindrical silver-plated brass resonator with a diameter of 38.5 mm and a height of 19.5 mm originally designed for the investigation of lossy samples in electron paramagnetic resonance spectroscopy.…”

Three Linear Trinuclear Zinc(II) Complexes with Acenaphthenequinone Dioxime and the Ancillary Ligand 2‐Amino‐2‐methyl‐1‐propanol in Different Coordination Modes

“…EPR spectroscopy in general and the spin probing approach in particular have proven to be a powerful method for studying the structures and dynamics of neat ILs 21–31. The isotropic nitrogen hyperfine coupling constant has served as tool for assessing the micropolarity, which was found to significantly exceed that expected from the relative permittivity (for [bmim + ][BF 4 − ], ε r =12.2 at 298 K) 23.…”

“…The Stokes–Einstein–Debye law (SED) was found to describe the rotational motion at high temperatures, whereas subdiffusional behavior, free‐volume effects, and a fractional SED law characterize the low‐temperature region 21. 24, 26–28, 30, 31 Spin‐exchange rate constants have been determined with the aim of addressing the translational diffusion in ILs 29. 31…”