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Translational diffusion of nitroxide radical in water

The X-band electron paramagnetic resonance (EPR) was applied to study translational diffusion of perdeuterated nitroxide radical 2, 2, 6, 6-tetramethyl-4-oxopiperidine-1-oxyl (pDTEMPONE) in water [1]. The measured temperature interval −18-30°C lies in both normal (T>0°C) and supercooled (T<0°C) regions. At each measured temperature, the EPR spectra were recorded for various concentrations C of pDTEMPONE, with C<70 mM. The spectra were fitted to theoretical expression based on modified Bloch equations, which describe motion for transversal magnetizations of three subensambles of radicals with different projections of 14N nuclear spin (I=1). In these equations, the spin dephasing rate w2C and the coherence transfer rate v2C are introduced to take into account the effect of Heisenberg spin exchange (HSE) and dipole-dipole interaction (DD) between radicals [2]. The best-fit values of the spin dephasing and coherence transfer rates obtained from the spectra were subsequently fitted to theoretical linear concentration dependence. Thus, we found total rate constants of spin dephasing w2=w2HSE+w2DD and coherence transfer v2=v2HSE+v2DD. Using the facts that v2HSE/w2HSE=1/2 and that v2DD/w2DD≈−4/19 holds when correlation times of DD are much longer than inverse Zeeman frequency, the HSE and DD contributions to the rate constants were separated. Then, applying theoretical equation for w2HSE and w2DD in the continuous-diffusion model, the long-time diffusion constants D(HSE) and D(DD) of the radical were obtained as a function of temperature. Additionally, we analyzed the effective 14N hyperfine splitting frequency a, which is expected to vary with concentration as a=a0–b2C due to HSE [2]. This concentration dependent shift in frequency is induced by spin precession during the time tauc spent by two colliding radicals in the HSE interaction zone and the total time tauRE between their re-encounters. Using the best-fit values of a obtained from the EPR spectra, we found the rate constants b2 from the concentration dependence of a and the re-encounter times tauRE from the equation b2/w2HSE=0.43(a0*tauRE)^1/2. This equation follows from the continuous-diffusion model when tauc<<tauRE and it is expected that tauRE≈2r^2/D, where 2r is the closest distance between colliding radicals [2]. The results show that the long-time diffusion constants D(HSE) and D(DD) correlate well with each other, which justifies the separation method of HSE and DD contributions and the continuous-diffusion model. By comparing temperature dependence of diffusion constants with hydrodynamic Stokes-Einstein equation, we found that the effective hydrodynamic radius varies within 15% from the van der Waals radius of pDTEMPONE (~3.5 Å). Temperature dependence of hydrodynamic radius is somewhat stronger in the supercooled than in normal region. On the other hand, the temperature dependence of re-encounter time tauRE is unusual. In the normal region tauRE increases with decreasing temperature, as expected, but in the supercooled region it starts to decrease with decreasing temperature. This indicates that the re-encounter dynamics of radicals after first contact within the solvent cage differs markedly from the continuous diffusion in the supercooled region. References: 1. I. Peric, D. Merunka, B. L. Bales and M. Peric, submitted to J. Phys. Chem. B 2. K.M. Salikhov, Appl. Magn. Reson. 38 (2010) 237.

water; EPR; diffusion

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