DIPOLAR-BASED AL-27-]SI-29 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 Al-27 --> Si-29 connectivity experiments for a s eries: of representative zeolite molecular sieve frameworks: zeolites A, X, Y, and Omega. Coherence transfer from quadrupolar Al-27 (I = 5/2 ) to Si-29 (I = 1/2) nuclei through cross polarization and transferred -echo double-resonance (TEDOR) experiments is. observed, and an experi mental investigation of the parameters controlling the efficiency of b oth experiments is presented. Rotational-echo double-resonance (REDOR) and dipolar-dephasing difference experiments are also demonstrated. T hese experiments utilize the heteronuclear dipolar couplings, whose ma gnitudes are proportional to the inverse third power of the internucle ar 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 resonan ce which is approximately linear with the number of aluminum atoms in the neighboring tetrahedral sites. Two-dimensional extensions of the c ross-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.