New PFG NMR Spectrometer with a Rotatable Quadrupole Coil for the Measurement of an Anisotropic Self-Diffusion Coefficient Tensor.

A new pulsed-field-gradient spin-echo NMR spectrometer is reported which is specially designed for the measurement of slow and anisotropic self-diffusion by providing an intense and homogeneous field gradient of 12 T m-1 by a quadrupole coil and a high-quality current stabilizer, and also providing a mechanism by which both the sample cell and the quadrupole coil are rotated from the axis of the external field. Diffusion anisotropy is measured by a combination of coil rotation and magic-angle orientation of the sample. The theoretical basis for the method of measuring diffusion anisotropy, instrumentation details, and several sets of test data are given. The experimental results are reported for the rotation pattern of the diffusion coefficient of a smectic Ad liquid crystal of 4-octyloxy-N-(4-cyanobenzylidene)aniline. The component parallel to the layer normal is shown to be 2.1 times larger than the component within the layer.