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.

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, τR, the mean time between successive spin exchanges within a cage, τRE, and the long-time-averaged spin exchange rate constants, Kex, of the same solute molecule may be measured independently. Thus, long- and short-time translational diffusion behavior may be inferred from Kex and τRE, respectively. In order to measure Kex, 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, Wdd. We find that both Kex and Wdd 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, τRE 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.

Numerical simulations of dipole dynamics have been studied by employing a recently proposed modified strong dipole proton coupling ͑MSDPC͒ model of the phase transition in the hydrogen-bonded type of ferroelectrics. The obtained results in the form of the spectral density of the polarization fluctuation along the polar axis and along the lateral direction are simulated and correlated with known experimental data obtained primarily by Raman and infrared spectroscopy on KDP and DKDP lattices. A central peak ͑CP͒ appears in the calculated spectral density of the longitudinal polarization fluctuation in the ferroelectric phase of KDP and DKDP. The simulated CP indicates an order-disorder mechanism of excitation in the paraelectric phase near T c in both crystals. By increasing the temperature to 460 K, the crossover to the displacive behavior for KDP lattice is predicted. The model also predicts a low-frequency flat anomaly for KDP and a CP for DKDP in the spectral density of transversal polarization fluctuations, as was earlier detected by infrared and Raman spectroscopy. The superiority of the MSDPC model is additionally tested in the description of the experimentally obtained pressure-induced change of the CP line shape for KDP.

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.

The Heisenberg spin exchange-dipole-dipole separation method was used to measure the translational diffusion coefficients of the 14 N-labeled perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl ( 14 N-pDTEMPONE) nitroxide spin probe as a function of temperature in two methylimidazolium ionic liquid series, one based on the tetrafluoroborate (BF 4 ) anion and another one on the bis(trifluoromethane)sulfonimide (TFSI, Tf2N) anion. The obtained translational diffusion coefficients of 14 N-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 14 N-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 14 N-pDTEMPONE diffusion, self-diffusion, and viscosity for a given cation were about 20% greater in the TFSI based ionic liquids than in the BF 4 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.