PULSED-GRADIENT SPIN-ECHO MEASUREMENTS OF ANISOTROPIC DIFFUSION BY DIPOLE-DECOUPLED C-13 NUCLEAR-MAGNETIC-RESONANCE

A new approach for monitoring diffusion in anisotropic phases is proposed and demonstrated. The method relies on the observation of dilute spins (e.g., C-13) in the presence of heteronuclear high-power dipolar decoupling, a procedure which can yield time-domain nuclear magnetic resonance (NMR) signals lasting over three orders of magnitude longer than their H-1 counterparts. This allows one to apply conventional 90-180 degrees pulsed-gradient spin-echo (PGSE) schemes on organic systems without having to employ complex and highly sensitive multiple-pulse H-1 irradiation schemes. Instrumental aspects of this C-13 method are discussed, and an application to the variable-temperature determination of anisotropic self-diffusion in a thermotropic liquid crystal is illustrated.