SENSITIVITY-ENHANCED QUADRUPOLAR-ECHO NMR OF HALF-INTEGER QUADRUPOLARNUCLEI - MAGNITUDES AND RELATIVE ORIENTATION OF CHEMICAL SHIELDING AND QUADRUPOLAR COUPLING TENSORS

A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupo lar nuclei in static powders is analyzed. By acquisition of the QE spe ctrum during a Carr-Purcell-Meiboom-Gill (CPMG) train of selective pi pulses, the second-order quadrupolar line shape for the central transi tion is split into a comb of sidebands leading to a considerable incre ase in the sensitivity compared to a conventional QE spectrum. The app licability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experi mental Rb-87 (RbClO4 and RbVO3) and Co-59 spectra (Co(NH3)(5) Cl-3), i t is demonstrated that the quadrupolar CPMG experiment represents a us eful method for studying half-integer quadrupolar nuclei exhibiting la rge quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 a re observed for the samples studied.