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Great efforts have been exercised during the past few years in applying magic-angle spinning (MAS) NMR spectroscopy to the highly abundant 14 N quadrupolar nucleus (99.6% natural abundance, spin I ) 1) in polycrystalline solids. [1][2][3] Obviously a successful outcome of these efforts would be highly appreciated in particular if the resulting spinning sideband (ssb) spectra would be of sufficient quality to allow analysis in a straightforward manner, thereby yielding useful spectral parameters. For example, this could involve the procedures and versatile software programs already developed for the analyses of spinning sideband spectra (the complete manifold and/or individual ssb line shapes) for halfinteger spin (I ) 3 / 2 , 5 / 2 , 7 / 2 , ...) quadrupolar nuclei and 2 H. [4][5][6][7][8][9][10] The potential of a routine determination for the 14 N quadrupole coupling constant (C Q ) and asymmetry parameter (η Q ) from such spectra would constitute a valuable parameter set characterizing the environment of the nitrogen atom in addition to the isotropic/ anisotropic 15 N chemical shift data, oftentimes determined using 15

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The first application of 14N MAS NMR spectroscopy and the associated 14N quadrupole coupling data for characterizing a phase transition between two solid phases is presented. For NH4NO3, the transition between the phases IV and III at about 35 °C and the phases themselves are studied by 14N MAS NMR using the NH4 + as well as the NO3 - ion as probes. Spectra are acquired employing spinning speeds in the range from 2000 to 7000 Hz thereby taking advantage of the frictional heating as the heat source. The 14N quadrupole coupling constants and asymmetry parameters for both the NH4 + and NO3 - ion determined from these spectra show large changes upon transition between the two phases. Also the transition exhibits the effect of hysteresis with a deviation of about 10 °C for the transition temperature. The results show great potential for 14N MAS NMR being a useful tool in characterizing phase transitions for nitrogen compounds from 14N quadrupole coupling data.

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