Orientational motion of nitroxides in molecular glasses: dependence on the chemical structure, on the molecular size of the probe, and on the type of the matrix.

Echo detected electron paramagnetic resonance (EPR) study of orientational molecular motion of nitroxide spin probes in glassy solvents was performed by evaluating the anisotropic transverse relaxation rate 1/T(2) at different positions of the EPR spectrum. Experiments were done on nitroxides of different sizes and shapes, in different solvent glasses, with different deuteration degree, and at different temperatures. We found that the properties of the solvent glass have a much stronger impact on the relaxation rate than the size and shape of the nitroxide have. We concluded that the anisotropic relaxation is induced by reordering of the solvent cage and not by small angle fluctuations of the nitroxide in the cage or intramolecular motion of nitroxide. (c) 2004 American Institute of Physics.