Natalia A. Chumakova

Rotational and Translational Diffusion of Spin Probes in Room-Temperature Ionic Liquids

We have studied the rotational and translational diffusion of the spin probe 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL) in five imidazolium-based room-temperature ionic liquids (RTILs) and glycerol by means of X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times and rate constants of intermolecular spin exchange have been determined by analysis of the EPR line shape at various temperatures and spin probe concentrations. The model of isotropic rotational diffusion cannot account for all spectral features of TEMPOL in all RTILs. In highly viscous RTILs, the rotational mobility of TEMPOL differs for different molecular axes. The translational diffusion coefficients have been calculated from spin exchange rate constants. To this end, line shape contributions stemming from Heisenberg exchange and from the electron-electron dipolar interaction have been separated based on their distinct temperature dependences. While the Debye-Stokes-Einstein law is found to apply for the rotational correlation times in all solvents studied, the dependence of the translational diffusion coefficients on the Stokes parameter T/η is nonlinear; i.e., deviations from the Stokes-Einstein law are observed. The effective activation energies of rotational diffusion are significantly larger than the corresponding values for translational motion. Effects of the identity of the RTIL cations and anions on the activation energies are discussed.

Determination of structural characteristics of all-organic radical liquid crystals based on analysis of the dipole-dipole broadened EPR spectra.

The angular dependences of g-value and line width of EPR spectra of paramagnetic all-organic liquid crystalline (LC) materials were measured for the quantitative characterization of the nematic, cholesteric, and smectic C phases. The detailed molecular alignment in mesophases was determined by means of numerical spectra simulation focusing on spin exchange and dipole-dipole magnetic interactions of neighboring molecules. The obtained structural data indicate that the spin polarization mechanism between neighboring molecules rather than the direct through-space interactions between paramagnetic centers is responsible for the specific magnetic properties of the studied LC materials.

Orientational order of guest molecules in aligned liquid crystal as measured by EPR and UV–vis techniques

Orientational order of guest molecules in aligned liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) is studied via optical dichroism and electron paramagnetic resonance (EPR) spectra measurements. The guest molecules used are bifunctional molecules bearing paramagnetic nitroxide group and photochromic azobenzene moiety. The bifunctional probe with rigidly bonded nitroxide and azobenzene moieties was found to align as a whole, while flexible long spacer between the moieties provides independent alignment for the nitroxide and azobenzene parts. Intermolecular interactions responsible for the alignment of azobenzene and nitroxide moieties of the probe molecules are discussed. The molecules with cis-configuration of azobenzene moiety are able to align in the liquid-crystalline medium, but to a lesser extent than the molecules with trans-configuration. Directions of orientational axes and characteristics of rotational mobility of spin probes are determined. Second, fourth and, in some cases, sixth rank order parameter values are found.

Copper(II) organic compounds as intermediates of photochemical transformations of quaternary ammonium tetrachlorocuprates

171 Stable organometallic compounds with a Cu(II)–C σ bond have not been described [1]. Pulse radiolysis and photolysis of divalent copper salts in aqueous solutions [2, 3] and in n -donor solvents [4] gave short-lived copper complexes with organic radicals in which the copper oxidation state was presumably two. Dissociation [2] CuR + Cu + + R • and bimolecular processes [3] 2CuR + 2Cu + + R 2 , CuR + + H 3 O + Cu 2+ + RH + H 2 O, were considered as the key routes for the destruction of these complexes. Copper(II) Organic Compounds as Intermediates of Photochemical Transformations of Quaternary Ammonium Tetrachlorocuprates E. N. Golubeva a, A. V. Lobanov a , V. I. Pergushov a , N. A. Chumakova a, and A. I. Kokorin b Presented by Academician A.L. Buchachenko February 5, 2008

Simulation of Rigid-Limit and Slow-Motion EPR Spectra for Extraction of Quantitative Dynamic and Orientational Information

© 2012 Vorobiev and Chumakova, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Simulation of Rigid-Limit and Slow-Motion EPR Spectra for Extraction of Quantitative Dynamic and Orientational Information

The Novel Stable Nitroxide Radicals as Perspective Spin Probes for Study of Orientation Order of Liquid Crystals and Polymers

Stable nitroxide radicals with rigid core were used as spin probes for investigation of orientational order of nematic liquid crystals MBBA and 5CB, uniaxially stretched polyethylene and porous polyethylene. The orientation distribution functions of probe molecules were determined. The order parameters of the true orientation axes were estimated. It was demonstrated that the novel nitroxide radicals show orientational alignment of both liquid crystals and polymers more effectively than ordinary spin probes.

Molecular Orientational Order of Nitroxide Radicals in Liquid Crystalline Media

The orientational distribution of a set of stable nitroxide radicals in aligned liquid crystals 5CB (nematic) and 8CB (smectic A) was studied in detail by numerical simulation of EPR spectra. The order parameters up to the 10th rank were measured. The directions of the principal orientation axes of the radicals were determined. It was shown that the ordering of the probe molecules is controlled by their interaction with the matrix molecules more than the inherent geometry of the probes themselves. The rigid fused phenanthrene-based (A5) and 2-azaphenalene (A4) nitroxides as well as the rigid core elongated C11 and 5α-cholestane (CLS) nitroxides were found to be most sensitive to the orientation of the liquid crystal matrixes.

Impregnation of polymers with 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl (TEMPONE) paramagnetic probe in sub- and supercritical CO2

The spin probe method is used to study the impregnation of polycarbonate (PC) based on bisphenol A, polyethylene oxide (PEO), and crosslinked acrylamide–acrylic acid copolymer (PAA) with organic molecules in sub- and supercritical CO2 media. Electron spin resonance (EPR) data show that, at 196 bar and 307 K, 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl (TEMPONE) paramagnetic spin probe molecules penetrate into the PC and PEO matrices, which are, respectively, in the glassy and elastic states under normal conditions. The degree of impregnation of PAA under these conditions is negligibly small. Estimates of the local concentration of probe molecules show that, in the PEO matrix, TEMPONE is distributed much more uniformly than in the PC matrix. Analysis of the effect of temperature on the shape of the EPR spectra of the radical in the polymer matrix shows that, under the same conditions, the mobility of TEMPONE molecules in the PEO matrix is much higher than in the PC matrix. The results suggest that the spin probe method is promising for studying the characteristics of macro- and micro-processes in polymer–supercritical fluid solvent–organic molecule ternary systems.

Rotational mobility of Aerosil surface grafted ≡ SiNNO• radicals with and without adsorbate at 77 K

The rotational mobility of Aerosil surface grafted ≡ SiNNO• radicals without adsorbate and in adsorbed carbon dioxide or sulfur hexafluoride layers was estimated at 77 K as (4–5) × 107 s−1.