ABSTRACT
This chapter deals with analysis of a fundamental property of the spin-label spectrum, namely the dependence on orientation of the nitroxide axes with respect to the static magnetic field. This arises from angular anisotropy of the g-tensor and the nitrogen hyperfine coupling tensor that characterize the nitroxide spin Hamiltonian. For crystals or aligned samples, the spectral positions and splittings therefore depend on orientation to the field. For non-aligned samples, the spectrum is a superposition of individual spectra from all orientations; the resulting envelope gives the powder lineshape. At conventional EPR frequencies, the powder spectrum is dominated by the hyperfine anisotropy; at high frequencies/fields, the powder spectrum is dominated by the g-value anisotropy. Inhomogeneous broadening arises from proton hyperfine splittings that remain unresolved in nitroxides; it is represented by a Gaussian convolution of Lorentzian lines. A valuable summary is given in the concluding section of the chapter. Appendices B, E and G I at the end of the book are directly relevant to this chapter.
