Solid‐state ¹⁴N MAS NMR of ammonium ions as a spy to structural insights for ammonium salts

Abstract

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 (14)N MAS NMR study of some ammonium salts: CH(3)NH(3)Cl, (NH(4))(2)(COO)(2) x H(2)O, (CH(3))(3)(C(6)H(5)CH(2))NCl, (CH(3))(3)(C(6)H(5))NI, [(n-C(4)H(9))(4)N](2)Mo(2)O(7), (NH(4))(2)HPO(4), and NH(4)H(2)PO(4). It is shown that the high-quality (14)N MAS NMR spectra, which can be obtained for these salts, allow determination of the (14)N quadrupole coupling parameters, i.e. C(Q) (the quadrupole coupling constant) and eta(Q) (the asymmetry parameter), with very high precision. In particular, it is shown that precise C(Q), eta(Q) parameters can be determined for at least two different (14)N 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 (14)N 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 (nu(r) = 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 (14)N quadrupole coupling constants for the ammonium salts studied here are in the range from C(Q) approximately 20 kHz to 1 MHz while the asymmetry parameters span the full range 0 < or = eta(Q) < or = 1. Clearly, the (14)N quadrupole coupling parameters (C(Q), eta(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 (14)N (or (15)N) chemical shifts.

Direct detection of nitrogen-14 in solid-state NMR spectroscopy

“…These parameters compared poorly with values calculated from a crystal structure, but after optimization of the atomic positions using DFT (with fixed lattice parameters), the predicted values matched the experimental parameters far more closely, thereby confirming the accuracy of the newly optimized structure. In a further 14 N MAS NMR study of tetraalkylammonium salts, this time with multiple sites and larger C Q values, the importance of the spinning rate was highlighted [114]. A MAS spectrum of trimethylbenzylammonium chloride recorded at a spinning rate of 6 kHz could be fitted accurately with a single site, but at 1 kHz the presence of two distinct sites became clear, and was subsequently confirmed by X-ray diffraction (XRD).…”

High-resolution 14N-edited 1H–13C correlation NMR experiment to study biological solids

“…In solid-state samples, the large electric quadrupole moment results in spectra having a width of several MHz wide due to the first-order quadrupole interaction. The 14 N MAS study of powdered solids has therefore been reduced to samples with quadrupole coupling constants of C Q $ 1 MHz or less [1][2][3][4][5]. Tycko et al [6][7][8] have demonstrated overtone NMR spectroscopy, where spectra are only influenced by the much smaller second-order quadrupole interaction.…”

3D 1H–13C–14N correlation solid-state NMR spectrum

“…In solid-state, the large quadrupolar moment for this spin-1 nucleus often results in NMR spectra of several MHz wide, with powder samples, due to the first-order quadrupole interaction. The study of 14 N MAS spectra was thus reduced to samples with quadrupolar coupling of C Q $ 1 MHz or less [1][2][3][4][5]. In 1986, Tycko et al [6][7][8], inspired by the work of Bloom and Legros [9], showed that using overtone NMR, the spectra would only be influenced by the much narrower second-order quadrupole interaction.…”