ABSTRACT
Experiments ................................................................................................ 211 References ............................................................................................. 214
In this chapter, cross-relaxation phenomena will be investigated. For spin 1/2 nuclei such as
H,
C, and
N in organic molecules, relaxation seldom involves only one spin because the dipole-dipole interaction with other nearby spins is the dominating relaxation mechanism. Thus, multispin contributions to relaxation become important. In these cases, the nuclear Overhauser effect (NOE), which was earlier seen to be related to the cross-relaxation rate, provides a measure of a third relaxation parameter, often as a complement to
T
and
T
measurements for heteronuclei. Perhaps the most widely studied relaxation phenomenon, however, is
homonuclear proton-proton dipole-dipole cross-relaxation in organic and biological macromolecules, which contains an important source of information. Cross-relaxation studies offer possibilities to investigate the geometry of a molecule through the internuclear distance dependence of the dipole-dipole interaction. Another application of relaxation involves dynamical investigations, which are often carried out by combining measurements of heteronuclear
T
,
T
, and heteronuclear NOE relaxation parameters. In Section 9.1, the NOE measurements for a two-spin system of unlike spins will be discussed. Homonuclear proton cross-relaxation applications will be discussed in Section 9.2 through Section 9.4.
