ELECTRON SPIN-ECHO STUDIES OF NITROXIDE FREE-RADICALS IN LIQUIDS

Direct measurements of the electron transverse (T2) and longitudinal (T1) relaxation times of two nitroxide free radicals in liquid solution have been carried out over a range of concentrations and temperature by the method of electron spin-echoes (ESE). Reorientational correlation times and the Heisenberg exchange rate for these radicals in toluene were obtained from the ESE data and compared with results from continuous wave (CW) EPR experiments. The radicals studied were perdeuterated Tempone (PDT) which is roughly spherical in shape and deuterated (in the nitroxide moiety) N-(p-methoxyybenzylidene)-4-amino-2,2,6,6-tetramethylpiperidinyl-1-oxy (DMBATPO) which is rod shaped. The Heisenberg exchange rates are approximately equal for the two radicals in toluene whereas the correlation times as well as the anisotropy of rotational diffusion are quite different for the two spin probes. The inhomogeneous broadening of nitroxide line shapes from unresolved intramolecular proton or deutron hyperfine interactions must be considered when extracting relaxation data from CW spectra whereas the ESE measurements are independent of any source of inhomogeneous broadening. The electron spin-lattice relaxation rates for these two radicals in toluene and the liquid crystal MBBA were measured and are far less sensitive to the motion of the radicals than is the electron spin-spin relaxation rate. © 1979 American Chemical Society.