FULL DETERMINATION OF THE ROTATIONAL DIFFUSION TENSOR BY ELECTRON-PARAMAGNETIC RESONANCE AT 250 GHZ

High-frequency (250 GHz) electron paramagnetic resonance (EPR) spectra in the limit of motional narrowing have been studied for a nitroxide spin probe diffusing in a low-viscosity isotropic solvent. The enhanced sensitivity of the 250-GHz spectrum to rotational modulation of the g tensor reveals new details of the microscopic motion of the spin probe. Specifically, it is shown how independent linear constraints imposed by line-width measurements at 250 GHz and a lower frequency may be used to fully determine the diffusion tensor R in the general case R(x) not-equal R(y) not-equal R(z). This procedure is demonstrated experimentally for the perdeuterated Tempone (PDT) probe diffusing in toluene-d8, where analysis of the line widths obtained at 250 and 9.5 GHz gives values for the anisotropy parameters rho(x) = R(x)/R(z) and rho(y) = R(y)/R(z) of 1.8 +/- 0.2 and 1.5 +/- 0.3, respectively. Thus, this molecule is found to exhibit small deviations from spherically symmetric reorientation. The enhanced accuracy of determining the g tensor from rigid-limit spectra obtained at 250 GHz is an important feature of such high-frequency studies. Existing theoretical expressions for motionally narrowed nitroxide line widths have been modified appropriately for the high-field case by including a completely anisotropic diffusion tensor and the N-14 nuclear Zeeman interaction, which becomes important at fields above about 5 T.