Measuring T 1 and T 2 Relaxation Rates

This chapter examines the specific methods used to determine relaxation rates and begins with the simple, single-spin relaxation properties related to the Bloch equations. The measurement of nuclear spin relaxation times has been an important and active field of research since the early days of Fourier transform nuclear magnetic resonance. As in all fields of Nuclear Magnetic Resonance, the methodological development has been enormous, from direct measurements on low abundant nuclei to indirect multidimensional techniques applied to biological macromolecules. The measurement of spin-lattice, T₁, and spin-spin, T₂, relaxation time constants has found particular use in characterising the reorientational and local motion of molecules. Alternative approaches to the measurements of spin-lattice relaxation times include the method known as the constant relaxation period experiment. An important aspect in longitudinal relaxation measurements concerns the fact that longitudinal magnetisation during the relaxation period can be transferred among the possible longitudinal spin states.