MULTIPLE-QUANTUM NMR COHERENCE GROWTH IN POLYCRYSTALLINE SALTS CONTAINING F-19

Multiple-quantum (MQ) nuclear magnetic resonance has been extended to study fluorinated solids. Up to 22 multiple-quantum coherence orders have been observed in CaF2, characteristic of roughly 180 correlated spins. In polycrystalline salts, with only one chemically distinct 19F site, a simple scaling of the spin correlation size as a function of excitation time between materials was observed. For salts having fluoride anions (CaF2, NaF, and CsF) and those with isotropically rotating hexafluoroarsenate anions [NaAsF6 and (C6H5)3SAsF6], the local dipole field strength, determined by the square root of the second moment of the homonuclear dipole couplings, determined the rate of MQ coherence growth. Experimentally, heteronuclear couplings, specifically 1H-19F dipole interactions, did not influence MQ dynamics, in agreement with the zeroth-order average Hamiltonian calculation. This last result allows for the relatively easy application of MQ-NMR experiments to dipolar solids containing heteronuclear dipole couplings. © 1992.