Advancements in Natural Abundance Solid-State ¹⁴N and ³³S MAS NMR with Applications to Inorganic/Organic Materials

Beskrivelse

The ¹⁴N (I = 1) and ³³S (I = 3/2) quadrupolar nuclei are neighbors in the low-frequency end of the NMR-frequency table. For that reason we have taken full advantage of all experimental and theoretical experiences gained from our recent achievements in improving the instrumental and experimental strategies for ¹⁴N MAS NMR [1,2] to potentially make a similar positive contribution to the status of natural abundance solid-state ³³S MAS NMR spectroscopy. These two spin isotopes have found limited use in solid-state NMR spectroscopy for obvious reasons which are related to a combination of their spin quantum numbers, their low gyromagnetic ratios, and the natural abundance of ¹⁴N (99.63%) and ³³S (0.76%). Considering the absolute receptivities for ¹⁴N and ³³S relative to ¹³C at natural abundance (5.69 and 0.097, respectively), the factor of 60 higher receptivity for ¹⁴N relative to ³³S would seem to indicate a much higher sensitivity for ¹⁴N as is also observed in liquid-state NMR studies. However, in solid-state NMR the experimental difficulties for ¹⁴N and ³³S become more similar for different reasons. The main reason which makes solid-state ¹⁴N NMR a low-sensitivity technique is because its integer spin (I = 1) precludes the detection of a central transition as is usually observed for half-integer quadrupolar nuclei. This restricts ¹⁴N observation to the detection of the (1 ↔ 0) and (0 ↔ -1) transitions and thereby makes the sensitivity extremely dependent on the width of these transitions, i.e., on the first-order quadrupolar broadening, directly proportional to the magnitude of the ¹⁴N quadrupolar coupling constant. On the other hand, in solid-state ³³S NMR its spin I = 3/2 allows observation of the central (1/2 ↔ -1/2) transition, a transition not broadened by he first-order quadrupole interaction but only to second-order. An overview of the achievements gained for these two low-frequency nuclei, particularly in relation to ¹⁴N, will be presented.

In addition, ¹⁴N MAS NMR is used to characterize the crystal structures of some organic materials (amino acids and templates for inorganic synthesis) and in particular for a series of important precursors in heterogeneous catalysis. These include the two polymorphs of (NH₄)₂MoO₄, mS60 and mP60, recently distinguished by synthesis and structure determination. Their unique and characteristic ¹⁴N MAS NMR spectra allow the transformation reactions of (NH₄)₂MoO₄ to be followed both qualitatively and quantitatively, while fingerprint ¹⁴N MAS NMR spectra for the polymolybdates allow product identification. Natural-abundance ³³S MAS NMR spectra for the central and satellite transitions are obtained for (NH₄)₂MoS₄ and (NH₄)₂WS₄. Analysis of these spectra allows determination of the principal elements for both the quadrupole coupling and chemical shift tensors as well as the relative orientation of the tensors for these isostructural (Pnma) tetrathiometallates.