Probing Crystal Structures and Transformation Reactions of Ammonium Molybdates by 14N MAS NMR Spectroscopy
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- 作者：Anders R. Hove ; Henrik Bilds
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- 刊名：Inorganic Chemistry
- 出版年：2006
- 出版时间：December 25, 2006
- 年：2006
- 卷：45
- 期：26
- 页码：10873 - 10881
- 全文大小：539K
- 年卷期：v.45,no.26(December 25, 2006)
- ISSN：1520-510X
文摘
The unique high-resolution feature offered by 14N magic-angle spinning (MAS) NMR spectroscopy of ammoniumions has been used to characterize the crystal structures of various ammonium molybdates by their 14N quadrupolecoupling parameters, i.e., CQ, the quadrupole coupling constant, and 
Q, the asymmetry parameter. Two polymorphsof diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction(XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two setsof characteristic 14N CQ and Q values have been obtained. Similarly, the well-characterized ammoniumpolymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24·4H2O, and (NH4)6Mo8O27·4H2O also give rise to distinct and characteristic14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24·4H2O spectrumrequires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (
r = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity.These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessmentof the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experimentsperformed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 andmP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.
Q, the asymmetry parameter. Two polymorphsof diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction(XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two setsof characteristic 14N CQ and Q values have been obtained. Similarly, the well-characterized ammoniumpolymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24·4H2O, and (NH4)6Mo8O27·4H2O also give rise to distinct and characteristic14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24·4H2O spectrumrequires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (r = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity.These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessmentof the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experimentsperformed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 andmP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.