Multiple-Quantum NMR Coherence Growth in Single-Crystal and Powdered Calcium Fluoride

Abstract

Powdered and oriented single-crystal calcium fluoride (CaF2) samples were used to determine the orientational dependence of 19F multiple-quantum (MQ) NMR coherence development in the absence of variations in lattice spacing and molecular motion on the experimental time scale. The normalized n-quantum coherence intensities and resultant effective spin-cluster size [N(τ)] of the powdered sample were found to be reproducible and insensitive to the cycle time chosen for the underlying eight-pulse MQ excitation sequence. Oscillations in the time-dependent 0Q and 2Q coherence intensities are seen in the oriented single-crystal samples. However, these oscillations are absent in the powdered CaF2 sample, indicating damping by its orientational-based dipolar-coupling distribution. However, monotonic growth of the effective number of correlated nuclei, N(τ), is still observed. In addition, the applicability of an average product-operator model of coherence growth based on the dimensionality of spin distribution, and previously only applied to powders, is tested for oriented single-crystal CaF2 samples. The powdered 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 dipole structure of the (111)-oriented crystal, where nearest-neighbor couplings become zero. In addition, the refocused MQ signal intensity (fMQ) is adversely effected by increasing the number of pulses applied in a fixed MQ preparation time. An empirical prediction of fMQ for samples with a static dipolar field is developed and serves as a basis 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 length scales as the relatively rapid decay of fMQ typical limits the present experiment to ∼ 10 Å length scales.