Dipolar-based ²⁷Al → ²⁹Si solid-state NMR connectivity experiments in zeolite molecular sieve frameworks

In this work we present the results of a systematic investigation of a set of dipolar-based ²⁷Al → ²⁹Si connectivity experiments for a series of representative zeolite molecular sieve frameworks: zeolites A, X, Y, and Ω. Coherence transfer from quadrupolar ²⁷Al (I = 5/2) to ²⁹Si (I = 1/2) nuclei through cross polarization and transferred-echo double-resonance (TEDOR) experiments is observed, and an experimental investigation of the parameters controlling the efficiency of both experiments is presented. Rotational-echo double-resonance (REDOR) and dipolar-dephasing difference experiments are also demonstrated. These experiments utilize the 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. In addition, we demonstrate that for these dipolar-based experiments, there is a relative enhancement of a given resonance which is approximately linear with the number of aluminum atoms in the neighboring tetrahedral sites. Two-dimensional extensions of the cross-polarization and TEDOR experiments are also possible and will be of use for complex mixtures or where several resonances are observable in the one-dimensional spectra of the two nuclei.