MULTIPLE-QUANTUM NMR COHERENCE GROWTH IN SINGLE-CRYSTAL AND POWDERED CALCIUM-FLUORIDE

Powdered and oriented single-crystal calcium fluoride (CaF2) samples w ere used to determine the orientational dependence of F-19 multiple-qu antum (MQ) NMR coherence development in the absence of variations in l attice spacing and molecular motion on the experimental time scale. Th e normalized n-quantum coherence intensities and resultant effective s pin-cluster size [N(tau)] of the powdered sample were found to be repr oducible and insensitive to the cycle time chosen for the underlying e ight-pulse MQ excitation sequence. Oscillations in the time-dependent 0Q and 2Q coherence intensities are seen in the oriented single-crysta l samples. However, these oscillations are absent in the powdered CaF2 sample, indicating damping by its orientational-based dipolar-couplin g distribution. However, monotonic growth of the effective number of c orrelated nuclei, N(tau), is still observed. In addition, the applicab ility of an average product-operator model of coherence growth based o n the dimensionality of spin distribution, and previously only applied to powders, is tested for oriented single-crystal CaF2 samples. The p owdered and (100)-oriented single crystal show good agreement with the model, while the growth from the (111) orientation is slightly faster than predicted. This difference most likely arises from the unusual d ipole structure of the (111)-oriented crystal, where nearest-neighbor couplings become zero. In addition, the refocused MQ signal intensity (f(MQ)) is adversely effected by increasing the number of pulses appli ed in a fixed MQ preparation time. An empirical prediction of f(MQ) fo r samples with a static dipolar field is developed and serves as a bas is for predicting the effects of motion on this MQ refocusing fraction . This relationship may also provide some insight into the possibility of extending the MQ technique to study correlations over larger lengt h scales as the relatively rapid decay of f(MQ) typical limits the pre sent experiment to similar to 10 Angstrom length scales. (C) 1994 Acad emic Press, Inc.