Librational dynamics of nitroxide molecules in a molecular glass studied by echo-detected EPR |
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Authors: | E. P. Kirilina S. A. Dzuba A. G. Maryasov Yu. D. Tsvetkov |
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Affiliation: | 1. Physics Department, Novosibirsk State University, Novosibirsk, Russian Federation 2. Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk, Russian Federation
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Abstract: | Nitroxides 2,2,6,6-tetramethyl-4-piperidone N-oxide (tempone), 3-carboxy-proxyl and potassium peroxylamine disulfonate (Fremy salt) in glycerol solution were studied in a wide temperature range near the glass transition temperatureT g. The echo-detected (ED) electron paramagnetic resonance (EPR) lineshape reveals strong dependence on the time interval τ between the echo-forming microwave pulses which is readily explained by anisotropic phase relaxation. Employing a librational model of molecular motion and the Redfield relaxation theory, spectra were simulated for the τ’s varying in a large interval. The anisotropic relaxation rate increases with temperature increase and it is larger for nitroxide with a larger molecular size. The mean-squared amplitude of motion, obtained from reduced hyperfine splitting in continuous-wave EPR, near Tg linearly depends on temperature which is characteristic of harmonic solids. For tempone in a host crystal 2,2,4,4-tetramethyl-cyclobutan-1, 3-dione the anisotropic spin relaxation rate decreases with temperature increase so the found feature solely belongs to a glassy state. A new approach is proposed for modeling slow wobbling motion in a restricted angular space. |
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