Double resonance experiments in a single resonance probe: detecting 23Na-51V dipolar interactions in sodium vanadates

Heteronuclear correlation experiments of 23Na-27Al in rotating solids

We demonstrated that the heteronuclear correlation experiments between two quadrupolar nuclei, ²³Na and ²⁷Al, with close Larmor frequencies can be achieved via D-HMQC and D-RINEPT approaches by using a diplexer connected to a conventional probe in magic-angle-spinning solid-state NMR. Low-power heteronuclear dipolar recoupling schemes can be applied on ²³Na or ²⁷Al to establish polarization transfers between the central transitions of ²³Na and ²⁷Al for a model compound, NaAlO₂. Further, we showed a practical implementation of the two dimensional ²³Na-²⁷Al dipolar-based heteronuclear correlation experiment on a heterogeneous catalyst, Na₂CO₃/γ-Al₂O₃. This allows to determine spatial proximities between different ²³Na and ²⁷Al sites, thus the surface Na species adjacent to octahedral-coordination Al can be clearly discriminated.

Internuclear (31)P-(51)V distance measurements in polyoxoanionic solids using rotational echo adiabatic passage double resonance NMR spectroscopy

We report the first results establishing rotational echo adiabatic passage double resonance (REAPDOR) experiments for distance measurements between a spin-1/2 (31P) and spin-7/2 (51V) pair in a series of vanadium-substituted polyoxoanionic solids from the Keggin and Wells-Dawson families. We have quantitatively measured 31P-51V distances in monovanadium substituted K4PVW11O40, 1-K7P2VW17O62, and 4-K7P2VW17O62. Numerical simulations of the experimental data yield very good agreement with the averaged P-W/P-V distances determined from the X-ray diffraction measurements in the same or related compounds. REAPDOR is therefore a very sensitive P-V distance probe anticipated to be especially useful in the absence of long-range order. Our results suggest that REAPDOR spectroscopy could be broadly applicable for interatomic distance measurements in other spin-7/2-spin-1/2 nuclear pairs.

1H-13C-27Al triple resonance transfer of populations in double resonance experiments for the detection of 13C-27Al dipolar interactions

A 13C [1H] CPMAS [27Al] TRAPDOR NMR experiment is reported with the aim of detecting 13C-27Al proximities and distances in solids. The 13C and 27Al pulses are applied to the same probe channel, because their resonance frequencies lie extremely close to each other. The study of the heteronuclear dipolar interaction between these two nuclei, which are of fundamental importance in solid state science, is not possible with standard double resonance approaches. Results are presented for the model compound aluminum lactate. The 13C signals for the three lactate carbons show different magnitudes of the TRAPDOR effect, in excellent agreement with their mean Al-C distances, calculated from crystal structure data. The results illustrate the feasibility of this method for the study of systems where the interaction of organic and inorganic fractions is directing the structure (template/zeolite) or controlling the catalytic efficiency (organic reactant/catalytically active sites in zeolites or clays).