Natural Abundance ²⁵Mg Solid‐State NMR of Mg Oxyanion Systems: A Combined Experimental and Computational Study

Abstract

Solid-state (25)Mg magic angle spinning nuclear magnetic resonance (MAS NMR) data are reported from a range of organic and inorganic magnesium-oxyanion compounds at natural abundance. To constrain the determination of the NMR interaction parameters (delta(iso), chi(Q), eta(Q)) data have been collected at three external magnetic fields (11.7, 14.1 and 18.8 T). Corresponding NMR parameters have also been calculated by using density functional theory (DFT) methods using the GIPAW approach, with good correlations being established between experimental and calculated values of both chi(Q) and delta(iso). These correlations demonstrate that the (25)Mg NMR parameters are very sensitive to the structure, with small changes in the local Mg(2+) environment and the overall hydration state profoundly affecting the observed spectra. The observations suggest that (25)Mg NMR spectroscopy is a potentially potent probe for addressing some key problems in inorganic materials and of metal centres in biologically relevant molecules.

Recent technique developments and applications of solid state NMR in characterising inorganic materials

“…The use of variable magnetic fields to constrain simulations of NMR spectra by comparing with experimental data collected at several magnetic fields illustrated by (a) 25 Mg in MgSO 6 .6H 2 O at three magnetic fields showing the two inequivalent magnesium sites, and (b) 93 Nb from YNbO 4 at four magnetic fields. Reproduced from [9] and [10] respectively with permission of the copyright owner. Figure 2.…”

“…1. 25 Mg MAS at three magnetic fields (11.7, 14.1 and 18.8 T) is used to constrain and determine the isotropic chemical shift ( iso ), the quadrupolar coupling constant ( Q ,) and quadrupolar asymmetry parameter ( Q , see refs 3, 5 for definition of these parameters) in MgSO 4 .6H 2 O [9]. Despite the strong overlap between these two sites the NMR parameters are well constrained by this approach.…”

“…This situation has very recently moved forward significantly with two studies reporting extensive sets of 25 Mg NMR interactions from relatively simple inorganic magnesium-containing compounds, comparing the results with DFT calculations [9,66]. The experimental and calculated parameters show excellent correlation, generally indicating that 25 Mg NMR can indeed be used to probe structure.…”

Magnesium incorporation into hydroxyapatite

“…The PBE generalised gradient approximation was used and the valence electrons were described by norm conserving pseudopotentials [72] 43 Ca and 25 Mg, the quadrupolar parameters C Q and η were also analysed. [38] It should be noted that based on previous studies and our own experience, the error on calculated 1 H and 31 P isotropic chemical shifts is estimated at ~0.4 [77] and 0.7 ppm, [78] respectively, whereas it is ~3 to 5 ppm on 43 Ca and 25 Mg isotropic chemical shifts, and ~0.5 to 2 MHz on 43 Ca and 25 Mg C Q values, [37,39] respectively. The four Ca(I) ions in the unit cell are usually referred to as "columnar calcium sites", because they form single atomic columns perpendicular to the basal plane.…”

“…240000 transients were acquired, with a recycle delay of 0.3 s. 25 Mg NMR chemical shifts were referenced using an aqueous solution of magnesium sulfate. [38,39] Ca K-edge EXAFS and XANES EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near…”