ABSTRACT
The theory of nuclear relaxation is the basis for various relaxation applications that provide accurate measurements of quadrupolar and dipolar coupling constants or chemical shift anisotropy, in connection with the symmetry of nuclear environments, structural features of compounds, and characterization of molecular mobility, including the type of motions, identication of weak intermolecular interactions, and even determination of interatomic distances in solutions. In addition to sophisticated relaxation experiments directed toward solving very specic problems, the measurement of relaxation times T1 and T2, utilizing standard hardware and software (or knowledge of their values) to assist in recording one-dimensional (1D) and two-dimensional (2D) NMR spectra, can reduce potential errors, such as perturbation of integral intensities, missing signals, or the appearance of artifacts. They can also help with the assignment of signals based on their unusual widths. In addition, the modication of pulse sequences to account for relaxation behavior can help to overcome difculties in the accumulation of signals with long relaxation times. The driven equilibrium Fourier transform (DEFT) pulse sequence, for example, is effective for 29Si NMR in solutions of biological systems.1 Finally, there are pulse sequences that allow detection of small amounts of compounds in the presence of large amounts of other compounds when their T2 relaxation times are strongly different. Some aspects of such applications are considered in this chapter to demonstrate the methods and strategies of studies for which the choice of nuclei (high or low sensitivity, quadrupolar or not) and the experimental techniques applied play major roles. The techniques should be adequate and correspond well to the specied goals, as relaxation measurements generally represent long-term experiments.
