Accurate decay rate measurements for longitudinal modes are extent in some measurements, such as those of longitudinal essential for many of the methods proposed for investigating momagnetization, by ensuring that only the species of interest lecular dynamics by NMR. However, the effects of cross relaxation is perturbed from equilibrium (10). This allows the assumpoften make it impossible to determine accurate values for these tion to be made that, initially at least, the decay of the species quantities. A method is presented that enables the effects of cross of interest is unaffected by cross relaxation. However, the relaxation to be largely eliminated from such measurements. Its perturbation of other species from equilibrium is often an reliability is assessed by comparing the values for internuclear unavoidable side effect of exciting the species of interest.
distances that can be determined from the resulting relaxation
This is usually true for all spin orders between two or more rates with values obtained using other methods. Other consespins, where the longitudinal magnetization of one or more quences of using this technique include an increased robustness of the spins involved in the mode will be perturbed from of experiments to short (i.e., Γ΅5T 1 ) relaxation times and the ability to make multiple ''selective'' relaxation measurements simultane-equilibrium during its excitation. For nuclei such as 1 H,
ously. α§ 1997 Academic Press
where chemical-shift anisotropy is small, this does not usually affect the decay of longitudinal two-spin order since it does not cross relax with longitudinal magnetization as a result of purely dipolar relaxation. However, for three-spin 265