Modified Hartmann–Hahn double NMR in solids for high resolution at low gyromagnetic ratio: CaF2 and quadrupole interaction in MgF2

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Abstract
Moderately high resolution NMR of rare spins in solids is described, using a modified Hartmann–Hahn double resonance experiment. Resolution is achieved by high power resonant decoupling of abundant spins, and the Hartmann–Hahn condition is achieved by irradiation of the rare spins off resonance, which preserves their chemical shift in the rotating frame. The rare spins are saturated with an additional weak variable frequency rf field. Following this cross‐polarization period is a period when strong gated rf is applied off resonance to the abundant spins. This permits time‐shared observation of the abundant spin magnetization, which reflects saturation of the rare system during the previous cycle. At the same time it serves to replenish the rare spin magnetization because it is off resonance. Transition between the two modes is adiabatic. This method distorts the rare‐spin line shape, but a related model experiment is also described which would not do so. This method is expected to be more useful than that of Pines, Gibby, and Waugh for studies of rare low gyromagnetic ratio species (lower than 13C) of spin 1/2 nuclei. The sensitivity limit is estimated to be in the millimolar range and is expected to be independent of gyromagnetic ratio. In CaF2 the 43Ca gyromagnetic ratio is measured to be 286.545±0.01 Hz/G. In MgF2 the 25Mg quadrupolar interaction is inferred to be e2qQ/h=3.2±0.3 MHz, η=0.36±0.04, with the uncertainty arising from lack of a fourth order theory.