SOLID-STATE DOUBLE-RESONANCE NMR EXPERIMENTS INVOLVING QUADRUPOLAR AND SPIN 1/2 NUCLEI

In this work we describe coherence-transfer and dipolar-dephasing double-resonance NMR experiments that demonstrate spatial proximity of quadrupolar and spin 1/2 nuclei (other than protons) in solids. These experiments exploit heteronuclear dipolar couplings, whose magnitudes are proportional to the inverse third power of the internuclear distance and are therefore extremely sensitive to the separation of the coupled spins. Coherence transfer through cross-polarization is observed in both directions between Al-27 (I = 5/2) and P-31 (I = 1/2) nuclei in the inorganic aluminophosphate framework systems VPI-5 and AlPO4-8, and an experimental investigation of the parameters controlling the efficiency of the transfer is presented. Dipolar-dephasing difference experiments, rotational-echo double-resonance (REDOR), and transferred-echo double-resonance (TEDOR) experiments under magic-angle spinning conditions are also demonstrated. Two-dimensional extensions of the cross-polarization and TEDOR experiments reveal the dipolar couplings between specific pairs of Al-27 and P-31 nuclei in two-dimensional correlation spectra, directly mapping connectivities from distinct sites with resolved NMR resonances.