Wednesday, August 24, 2016

Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K


Goslar, J., S.K. Hoffmann, and S. Lijewski, Dynamics of 4-oxo-TEMPO-d16-(15)N nitroxide-propylene glycol system studied by ESR and ESE in liquid and glassy state in temperature range 10-295K. J Magn Reson, 2016. 269: p. 162-75.


ESR spectra and electron spin relaxation of nitroxide radical in 4-oxo-TEMPO-d16-(15)N in propylene glycol were studied at X-band in the temperature range 10-295K. The spin-lattice relaxation in the liquid viscous state determined from the resonance line shape is governed by three mechanisms occurring during isotropic molecular reorientations. In the glassy state below 200K the spin-lattice relaxation, phase relaxation and electron spin echo envelope modulations (ESEEM) were studied by pulse spin echo technique using 2-pulse and 3-pulse induced signals. Electron spin-lattice relaxation is governed by a single non-phonon relaxation process produced by localized oscillators of energy 76cm(-1). Electron spin dephasing is dominated by a molecular motion producing a resonance-type peak in the temperature dependence of the dephasing rate around 120K. The origin of the peak is discussed and a simple method for the peak shape analysis is proposed, which gives the activation energy of a thermally activated motion Ea=7.8kJ/mol and correlation time tau0=10(-8)s. The spin echo amplitude is strongly modulated and FT spectrum contains a doublet of lines centered around the (2)D nuclei Zeeman frequency. The splitting into the doublet is discussed as due to a weak hyperfine coupling of nitroxide unpaired electron with deuterium of reorienting CD3 groups.