The paramagnetic ionic liquids 1-ethyl-3-methylimidazolium tetrachloroferrate ([Emim][FeCl4]) and 1-butyl-3-methylimidazolium tetrachloroferrate ([Bmim][FeCl4]), exhibiting Heisenberg spin exchange, were studied using Electron Paramagnetic Resonance (EPR) from 150 to 300 K. In [Emim][FeCl4], an exchange-narrowed EPR Lorentzian line was observed in the liquid phase and in the solid phase between the solid-solid and melting/crystallization temperatures. In comparison, below the solid-solid transition, an additional EPR line appeared, resulting in two exchange-narrowed EPR Lorentzian lines. The two EPR lines likely originate from the two polymorphic forms of [Emim][FeCl4], whose crystal structures have been presented in the literature. We calculated the EPR linewidth for both lines resulting from the Heisenberg spin exchange using the observed EPR linewidths and the distances between the nearest-neighbor Fe3+ ions, following the Anderson-Weiss theory. The value of the exchange integral estimated from the additional broader EPR line, named the "low-temperature EPR line," is 0.06 K. In contrast, the exchange integral estimated from the other narrower EPR line, named the "high-temperature EPR line," was 0.235 K, implying that the crystal structure of [Emim][FeCl4] associated with this line has a stronger superexchange coupling than that associated with the low-temperature EPR line. The solid-solid transition in [Emim][FeCl4] and the melting/crystallization transition exhibited hysteretic behavior, which was also observed in differential scanning calorimetry measurements. The EPR spectrum of [Bmim][FeCl4] shows only one Lorentzian EPR line throughout the measured temperature region, regardless of temperature treatment. The width change of this EPR line indicates the glass transition in [Bmim][FeCl4] at 185 K.

We have studied the rotational diffusion of two prolate nitroxide probes, the doubly negatively charged peroxylamine disulfonate (Frémy's salt − FS) and neutral di-tert-butyl nitroxide (DTBN), in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons using electron paramagnetic resonance (EPR) spectroscopy. Though the size and shape of the probes are reasonably similar, they behave differently due to the charge difference. The rotation of FS is anisotropic, and the rotational anisotropy increases with the alkyl chain length of the cation, while the rotation of DTBN is isotropic. The hyperfine coupling constant of DTBN decreases as a function of the alkyl chain length and is proportional to the relative permittivity of ionic liquids. On the other hand, the hyperfine coupling constant of FS increases with increasing chain length. These behaviors indicate the location of each probe in RTILs. FS is likely located in the polar region near the network of charged imidazolium ions. DTBN molecules are predominately distributed in the nonpolar domains.

We applied electron spin resonance (ESR) to study tracer diffusivities of a nitroxide radical at various temper-atures in the normal alkanes (octane to tridecane) and alcohols (methanol to 1-octanol). We studied and compared radical diffusivities in these liquids because their molecules are similar, but alcohols exhibit hetero-geneous structures due to the hydrogen bonding of hydroxyl groups, which is absent in alkanes. The crossover temperature behavior of radical diffusivities was found in all liquids by relating radical diffusivities and solvent self-diffusivities. This finding evidences the transformation from a single-molecule diffusion process into a col-lective process upon temperature lowering. However, the crossover behavior strongly differs in alcohols and alkanes, indicating that the heterogeneous structure of alcohols affects the radical diffusion crossover.

We studied the diffusivities of a nitroxide radical at various temperatures in six glass-forming molecular liquids by electron spin resonance. By comparing the radical diffusivities and solvent self-diffusivities, we found that the radical diffusivities are lower than the self-diffusivities at high temperatures and approach them at low temperatures in all liquids. This crossover behavior was considered as evidence that a single-molecule diffusion process transforms into a collective process with temperature lowering. The crossover phenomenon was analyzed by a novel, simple diffusion model, combining collective and single-molecule diffusion processes, and it was compared to the Arrhenius crossover phenomenon. The obtained results suggest that future studies of tracer diffusion could contribute to a better understanding of diffusion mechanisms in glass-forming liquids. The proposed diffusion model could be used to study the crossover phenomena of tracer diffusion measured by other techniques, and it could serve as a base for developing more advanced models.

X-band electron paramagnetic resonance spectroscopy has been used to investigate the rotational diffusion of a stable, positively charged nitroxide 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (Cat-1) in a series of 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) having alkyl chain lengths from two to eight carbons. The rotation of Cat-1 is anisotropic with the preferential axis of rotation along the NO• moiety. The Stokes-Einstein-Debye law describes the mean rotational correlation time of Cat-1, assuming that the hydrodynamic radius is smaller than the van der Waals radius of the probe. This implies that the probe rotates freely, experiencing slip boundary condition, which is solvent-dependent. The rotational correlation time of Cat-1 in RTILs can very well be fitted to a power-law functionality with a singular temperature, which suggests that the apparent activation energy of rotation exhibits non-Arrhenius behavior. Compared to the rotation of perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO), which is neutral, the rotation of Cat-1 is several times slower. The rotational anisotropy, the ratio of the rotational times of pDTO and Cat-1, and the apparent activation energy indicate the transition from a homogeneously globular structure to a spongelike structure when the alkyl chain has four carbons, which is also observed in molecular dynamics computational studies. For the first time, we have been able to show that the rotational correlation time of a solute molecule can be analyzed in terms of the Cohen-Turnbull free volume theory. The Cohen-Turnbull theory fully describes the rotation of Cat-1 in all ionic liquids in the measured temperature range.

The Heisenberg spin exchange-dipole-dipole separation method was used to measure the translational diffusion coefficients of the 14N-labeled perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (14N-pDTEMPONE) nitroxide spin probe as a function of temperature in two methylimidazolium ionic liquid series, one based on the tetrafluoroborate (BF4) anion and another one on the bis(trifluoromethane)sulfonimide (TFSI, Tf2N) anion. The obtained translational diffusion coefficients of 14N-pDTEMPONE were analyzed in terms of the Cohen-Turnbull free volume theory. It was found that the Cohen-Turnbull theory describes, exceptionally well, the translational diffusion of 14N-pDTEMPONE in all the ionic liquids in the measured temperature range. In addition, the Cohen-Turnbull theory was applied to the viscosity and self-diffusion coefficients of the cation and anion-taken from literature-in the same ionic liquids. The critical free volume for the self-diffusion of the cation and anion in a given ionic liquid is the same, which suggests that the diffusion of each ionic pair is coordinated. The critical free volumes for the 14N-pDTEMPONE diffusion, self-diffusion, and viscosity for a given cation were about 20% greater in the TFSI based ionic liquids than in the BF4 based ionic liquids. It appears that the ratio of the critical free volumes for a given cation between the two series correlates with the ratio of their densities.

Relative diffusion of free radicals in solution modulates the Heisenberg spin exchange and dipole-dipole interactions among them, which affects their electron paramagnetic resonance (EPR) spectra. The radical concentration dependence of EPR parameters can, in turn, give information about radical diffusivity in a liquid. We studied the diffusivities of the 14N- and 15N-labeled perdeuterated TEMPONE radicals at various temperatures in three viscous liquids: 1‑ethyl‑3‑methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid, propylene carbonate, and ethylene glycol. By fitting EPR spectra at various radical concentrations, we obtained the concentration coefficients of EPR parameters. The concentration coefficients were related to the radical diffusivity by solving the kinetic equations for the spin evolution of a radical pair, considering the radicals as continuously diffusing spherical objects in the hard-core pair potential. We tested the method by comparing the calculated radical diffusivities of isotopically substituted TEMPONE radicals. Temperature dependences of radical diffusivities were discussed in terms of the Stokes-Einstein relation. Additionally, the radical diffusivities were compared to the self-diffusivities of the studied liquids. At lower temperatures, the radical diffusivities follow the self-diffusivities, while at the higher temperatures, the radical diffusivities start deviating from the self-diffusivities.

We measured electron paramagnetic resonance (EPR) spectra of 14N- and 15N-labeled perdeuterated TEMPONE radicals in normal and supercooled water at various radical concentrations. By fitting the EPR spectra to spectral shape functions based on the modified Bloch equations, we obtained concentration dependences of EPR parameters of radicals at each measured temperature. From concentration dependences of the EPR parameters quantifying spin dephasing, coherence transfer, and hyperfine splitting, we determined linear concentration coefficients, whose values depend on the relative motion of radicals due to modulation of the Heisenberg spin exchange (HSE) and dipole-dipole (DD) interactions between them. We applied the continuous diffusion model for relative motion of radicals and we evaluated the diffusion coefficients of radicals from the concentration coefficients using the standard relations and the relations derived from kinetic equations for the spin evolution of interacting radical pair. It was found that the latter equations lead to the better agreement between the diffusion coefficients calculated from different concentration coefficients. The calculated diffusion coefficients of 14N- and 15N-labeled radicals show similar values, which is an expected result that supports the presented method. Upon lowering the temperature into the supercooled state, the calculated diffusion coefficients decrease slower than is predicted by the Stokes-Einstein relation and slower than the rotational diffusion coefficient. Similar effects were detected in NMR studies of the rotational and translational motion of water molecules in supercooled water.

The X-band electron paramagnetic resonance spectroscopy (EPR) of a stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO) has been used to study the nanostructural organization of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids (ILs) with alkyl chain lengths from two to eight carbons. By employing nonlinear least-squares fitting of the EPR spectra, we have obtained values of the rotational correlation time and hyperfine coupling splitting of pDTO to high precision. The rotational correlation time of pDTO in ILs and squalane, a viscous alkane, can be fit very well to a power law functionality with a singular temperature, which often describes a number of physical quantities measured in supercooled liquids. The viscosity of the ILs and squalane, taken from the literature, can also be fit to the same power law expression, which means that the rotational correlation times and the ionic liquid viscosities have similar functional dependence on temperature. The apparent activation energy of both the rotational correlation time of pDTO and the viscous flow of ILs and squalane increases with decreasing temperature; in other words, they exhibit strong non-Arrhenius behavior. The rotational correlation time of pDTO as a function of η/T, where η is the shear viscosity and T is the temperature, is well described by the Stokes-Einstein-Debye (SED) law, while the hydrodynamic probe radii are solvent dependent and are smaller than the geometric radius of the probe. The temperature dependence of hyperfine coupling splitting is the same in all four ionic liquids. The value of the hyperfine coupling splitting starts decreasing with increasing alkyl chain length in the ionic liquids in which the number of carbons in the alkyl chain is greater than four. This decrease together with the decrease in the hydrodynamic radius of the probe indicates a possible existence of nonpolar nanodomains.

Bimolecular collision rate constants of a model solute are measured in water at T = 259-303 K, a range encompassing both normal and supercooled water. A stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl, is studied using electron paramagnetic resonance spectroscopy (EPR), taking advantage of the fact that the rotational correlation time, τ(R), the mean time between successive spin exchanges within a cage, τ(RE), and the long-time-averaged spin exchange rate constants, K(ex), of the same solute molecule may be measured independently. Thus, long- and short-time translational diffusion behavior may be inferred from K(ex) and τ(RE), respectively. In order to measure K(ex), the effects of dipole-dipole interactions (DD) on the EPR spectra must be separated, yielding as a bonus the DD broadening rate constants that are related to the dephasing rate constant due to DD, W(dd). We find that both K(ex) and W(dd) behave hydrodynamically; that is to say they vary monotonically with T/η or η/T, respectively, where η is the shear viscosity, as predicted by the Stokes-Einstein equation. The same is true of the self-diffusion of water. In contrast, τ(RE) does not follow hydrodynamic behavior, varying rather as a linear function of the density reaching a maximum at 276 ± 2 K near where water displays a maximum density.

Using a precise method of least-squares nonlinear electron paramagnetic resonance (EPR) line fitting, we have obtained experimental evidence of a decoupling of the rotational motion of four nitroxide spin probes from the viscosity of bulk water at 277 K. This decoupling is about 50 K higher than another such phenomenon observed in interstitial supercooled water of polycrystalline ice by Banerjee et al. (Proc Natl Acad Sci USA 106 (2009) 11448-11453). Above 277 K the activation energies of the rotation of the probes and water viscosity are very close, while in the supercooled region the activation energies of the probes' rotation are greater than that of the viscosity of water. The rotational correlation times of the probes can be fit well to a power law functionality with a singular temperature. The temperature dependence of the hydrodynamic radii of the probes indicates two distinct dynamical regions, which cross at 277 K.

Electron spin–lattice relaxation (SLR) of TEMPO radical was measured in the crystalline and glassy states of deuterated ethanol in the temperature range 5–80K using X-band electron paramagnetic resonance (EPR). The measured SLR rates are higher in the glassy than in crystalline state and the excess SLR rate in glassy state is much lower than in ethanol. This result suggests that extra modes in glassy state, i.e. glassy modes, produce the excess SLR rate via the electron-nuclear dipolar (END) interaction between the electron spin of radical and the matrix protons or deuterons. Using the soft-potential model and assuming the END interaction between the electron spin and the matrix protons, the contributions to SLR rate of various mechanisms of glassy modes were theoretically analyzed. The evaluations of SLR rates in glassy ethanol indicate two main mechanisms of glassy modes: thermally activated relaxation of double-well systems and phonon-induced relaxation of quasi-harmonic local modes. The SLR rates induced by these mechanisms correlate well with the experimental data.

Istraživanje pripada grupi multidisciplinarnih proucavanja dinamike u materijalima karakteriziranim neuređenoscu molekula kao na primjer stakla. Pri tome se pokazuje da su molekularna stakla, poput stakla secera, alkohola i sl. prikladan eksperimentalni sustav. Trehaloza je disaharid koji se u prirodi nalazi u stanju stakla ili kristalinicnom stanju. Buduci da trehaloza ima najvisu temperaturu staklastog prijelaza i najizraženiju mrežu vodikovih veza od svih saharida, zanimljiva je za proucavanje anomalija opaženih u staklima opcenito [1]. Prednost primjene elektronske paramagnetske rezonancije (EPR) je u osjetljivosti međudjelovanja spina elektrona i resetke s obzirom na dinamiku/vibracijska svojstva promatranog sistema. Predstavit ce se rezultati EPR spektroskopije istraživanja dvaju polimorfa krute trehaloze (polikristal, staklo) [2] u kojima su paramagnetski centri inducirani ionizirajucim zracenjem [3]. Usporedit ce se spin - resetka relaksacijsko vrijeme u okruženjima razlicite neuređenosti. Pokazat ce se da je teorijski pristup u analizi EPR mjerenja zasnovan na tzv. mekom potencijalu (eng. soft potential model) pogodan za opis dinamike u stanju neuređenosti krute matrice [4]. [1] D.A. Parshin, Soft Potential Model and Universal Properties of Glasses, Phys Scripta, (1993) 180. [2] M. Kveder, I. Saric, D. Merunka, M. Jokic, S. Valic and B. Rakvin, The anhydrous solid trehalose: low-temperature EPR study of glassy and boson peak modes. Journal of Non-Crystalline Solids, 10.1016\/j.jnoncrysol.2013.05.016, (2013) [3] A. Graslund, G. Lofroth, Free-Radicals in Gamma-Irradiated Single-Crystals of Trehalose Dihydrate and Sucrose Studied by Electron-Paramagnetic Resonance, Acta Chem Scand B, (1975) 475. [4] D. Merunka, M. Kveder, B. Rakvin, Effect of thermally activated dynamics on electron spin-lattice relaxation in glasses, Chem Phys Lett, (2011) 19.