MRC, Jakobsen, 14N MAS NMR, ammounium salts

Magnetic Resonance in Chemistry
Vol: 44, Issue: 3, March 2006
pp. 348 - 356

Title: Solid-state 14N MAS NMR of ammonium ions as a spy to structural insights for ammonium salts
Authors: Jakobsen, Hans J.a; Hove, Anders R.a; Hazell, Rita G.b; Bildsøe, Henrika; Skibsted, Jørgena
Affiliations: a. Instrument Centre for Solid‐State NMR Spectroscopy, Department of Chemistry, University of Aarhus, DK‐8000 Aarhus C, Denmark
b. Division of Inorganic Chemistry, Department of Chemistry, University of Aarhus, DK‐8000 Aarhus C, Denmark
Keywords: NMR; 14N; solid state; MAS; ammonium salts; quadrupole coupling; crystal structure; trimethyl benzylammonium chloride
Abstract (English):

The high resolution offered by magic-angle spinning (MAS), when compared to the static condition in solid-state NMR of powders, has been used to full advantage in a 14N MAS NMR study of some ammonium salts: CH3NH3Cl, (NH4)2(COO)2·H2O, (CH3)3(C6H5CH2)NCl, (CH3)3(C6H5)NI, [(n-C4H9)4N]2Mo2O7, (NH4)2HPO4, and NH4H2PO4. It is shown that the high-quality 14N MAS NMR spectra, which can be obtained for these salts, allow determination of the 14N quadrupole coupling parameters, i.e. CQ (the quadrupole coupling constant) and Q (the asymmetry parameter), with very high precision. In particular, it is shown that precise CQ, Q parameters can be determined for at least two different 14N sites in case the individual spinning-sideband (ssb) intensities arise from a single manifold of ssbs, i.e. the ssbs for the two sites cannot be resolved. This feature of 14N MAS NMR, which is the first demonstration for manifolds of ssb in MAS NMR without the potential information from a central transition, becomes especially useful at the slow spinning frequencies (nr = 1000–1500 Hz) applied to some of the ammonium salts studied here. The detection of the number of sites has been confirmed by the corresponding crystal structures determined from single-crystal X-ray diffraction (XRD), either in this work for the unknown structure of benzyl trimethylammonium chloride or from reports in the literature. The magnitudes of the 14N quadrupole coupling constants for the ammonium salts studied here are in the range from CQ 20 kHz to 1 MHz while the asymmetry parameters span the full range 0 £ Q £ 1. Clearly, the 14N quadrupole coupling parameters (CQ, Q) for ammonium ions appear highly sensitive toward crystal structure and therefore appreciably more informative for the characterization of ammonium salts in comparison to the isotropic 14N (or 15N) chemical shifts.