ABSTRACT

The energy levels of a nitroxide spin label are linked by spin–lattice relaxation pathways for flipping the electron spin, and the nuclear spin, and also by cross-relaxation pathways that involve simultaneous flips of both spins. Additional relaxation pathways are afforded by Heisenberg exchange between electron spins or similarly by exchange of a spin label between different physical or chemical environments. Also, interaction with fast-relaxing paramagnetic species provides an additional spin–lattice relaxation pathway or sink. In this chapter, rate equations for population differences between levels are used to determine the effects of these different pathways on the effective T 1-relaxation times in progressive-saturation CW-EPR and on the reduction factors in CW-ELDOR. A valuable summary is given in the concluding section of this chapter.