K. C. Wong-Moon

Dipolar dephasing between quadrupolar and spin-12 nuclei. REDOR and TEDOR NMR experiments on VPI-5

Abstract In this work we examine the use of dipolar-dephasing magic-angle spinning NMR to study mixed pairs of quadrupolar and spin- 1 2 nuclei in solids. Dipolar connectivities are examined in both directions between the 27 Al ( I =5/2) spins in the very large pore aluminophosphate VPI-5, which is considered representative of a range of inorganic framework materials. Rotational-echo double-resonance (REDOR) and transferred-echo double-resonance (TEDOR) experiments are demonstrated, as well as a two-dimensional extension of the TEDOR experiment. These experiments restore the heteronuclear dipolar coupling under MAS conditions via a train of radiofrequency pulses, thereby providing information about connectivities and distances between coupled nuclei in these inorganic solids.

1D and 2D solid state NMR investigations of the framework structure of As-synthesized AlPO4-14

The framework structure of As-synthesized AlPO4-14 has been investigated with a combination of different one-dimensional 27Al and 31P solid state NMR techniques and 27Al/31P double resonance methods. The results are found to be fully consistent with the assumed structural model. 27Al MAS and DOR experiments at three different magnetic field strengths together with simulations show the presence of two tetrahedral sites, one pentacoordinated and one octahedral aluminum site. The 27Al quadrupolar coupling constants and the 31P isotropic chemical shifts of the tetrahedral sites correlate well with tetrahedral shear-strain parameters and mean P-O-Al bond angles, respectively. These correlations allow one to assign all of the NMR resonances to specific T-sites in the proposed framework structure. The assignments are then further confirmed by the application of three different two-dimensional heteronuclear correlation methods (i.e., 27Al-->31P TEDOR, CP, and INEPT) which reveal the connectivities between AlOx and PO4 polyhedra. The two-dimensional INEPT experiment is applied here for the first time in the solid state.

27Al/31P solid-state n.m.r. structural investigations of AlPO4-5 molecular sieve

27 Al/ 31 P solid-state n.m.r. connectivity experiments were used to investigate the structure of the molecular sieve AIPO 4 -5 hydrated with D 2 O. Hydration with D 2 O resulted in an increased efficiency of coherence transfer from octahedral 27 Al to 31 P compared with hydration with H 2 O. The experiments prove directly that both the tetrahedral aluminum and the octahedral aluminum formed upon hydration are connected to phosphorus in the framework. The two-dimensional 27 Al → 31 P transferred-echo double-resonance and cross-polarization experiments show a downfield offset of the octahedral 27 Al correlation from the tetrahedral 27 Al correlation, consistent with the growth of a downfield shoulder on the 31 P resonance as water is incorporated into the lattice.

Structural investigations of SAPO-37 molecular sieve by coherence-transfer and dipolar-dephasing solid-state nuclear magnetic resonance experiments

In this work, dipolar-based coherence-transfer and dipolar-dephasing solid-state nuclear magnetic resonance (NMR) experiments were performed to investigate the structure of the silicoaluminophosphate molecular sieve SAPO-37. The 27Al→29Si transferred-echo double-resonance (TEDOR) experiment directly proves, for the first time, that silicon substitutes for phosphorus atoms in the framework via the SM2 mechanism. The 27Al (31P) and 27Al (29Si) dipolar-dephasing difference experiments and the two-dimensional 27Al→31P TEDOR experiment indicate the presence of three types of aluminum environments: tetrahedral Al surrounded by symmetrical [4P] or [4Si] environments which give rise to a sharp resonance, tetrahedral Al surrounded by asymmetrical [3P, Si], [2P, 2Si] or [P, 3Si] environments which give rise to a broad resonance, and also extra-framework amorphous octahedral Al.

One and two-dimensional solid-state NMR investigations of the three-dimensional structures of zeolite-organic sorbate complexes

Publisher Summary This chapter describes one- and two-dimensional solid-state NMR investigations of the three-dimensional structures of zeolite–organic sorbate complexes. High-resolution solid-state NMR has emerged in recent years as an important complementary technique to XRD in the investigation of zeolite structures, being particularly sensitive to short to medium-range geometries and orderings. For some years, we have worked to develop new approaches in the application of solid-state NMR techniques together with XRD studies to the investigation of zeolite structures with the aim of ultimately being able to determine the 3D structures of their complexes with sorbed organic molecules. The chapter outlines the development of these techniques and their current standing. In the chapter, the application of the CP technique with protons as the source nuclei is described as representative of this class of experiments. In order to localize the polarization source as much as possible, experiments are carried out with the two specifically deuterated p-xylenes.

Cross-Polarization Processes Involving Less Common Pairs of Nuclei

The basic spin-1/2 ↔ spin-1/2 CP technique has been extended from 1H → X to a variety of other combinations, including cases with quadrupolar nuclei and cases with two CP processes in a sequence. Such techniques, including 2D versions, and goal-related techniques such as REDOR and TEDOR, provide valuable information on connectivities and even quantitative measures of internuclear distances in favorable cases.