Ultrahigh-Field ²⁵Mg NMR and DFT Study of Magnesium Borate Minerals

Abstract

A series of well-characterized magnesium borate minerals and synthetic analogues have been studied via ultrahighfield 25 Mg solid-state NMR spectroscopy. Correlations between 25 Mg NMR parameters and the local structure at the magnesium site(s) are highlighted and discussed. First-principles density functional theory calculations of 25 Mg NMR parameters carried out with the WIEN2k software package support our experimental 25 Mg NMR data. Experimental 25 Mg C Q values range from 0.7 ± 0.1 MHz in hungchaoite to 18.0 ± 0.5 MHz in boracite-type Mg 3 B 7 O 13 Br. To our knowledge, the latter C Q value is the largest reported 25 Mg nuclear quadrupole coupling constant. In general, C Q values correlate positively with the degree of geometrical distortion at the Mg site, despite the diversity in nearest-neighbor ligands (O 2− , OH − , H 2 O, F − , Cl − , and Br − ) across the series of magnesium borates. Experimental δ iso values range from 0.2 ± 0.5 ppm in hungchaoite to 23 ± 3 ppm in grandidierite, which are within the expected chemical shift range for diamagnetic magnesium borates.

Ultrahigh-Resolution 7Li Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy by Isotopic Dilution

“…For the borates studied here, four-coordinate lithiums, [4] Li, are found above 0.7 ppm, whereas [5] Li atoms appear between −0.2 and 0.8 ppm. This follows the trend documented for many other cations, such as 29 Si, 27 Al, 11 B, 73 Ge, 25 Mg, and 133 Cs, whereby a decrease in coordination number, or a decrease in average bond length (Figure b), is associated with a shift to a higher frequency. ,− Further partitioning of the [5] Li region to reflect specific geometric arrangements of the oxygen ligands is tempting, with trigonal-bipyramidal-like geometries appearing in the lower part of the [5] Li chemical shift range and square-pyramidal-like geometries at higher chemical shifts. Using the τ parameter previously defined to reflect a structural transition from idealized trigonal-bipyramidal (τ 5 = 1) to square-pyramidal (τ 5 = 0) geometry yields a suggestive trend (Figure S4), but the small data set ( n = 6) renders any conclusions speculative.…”

Recent progress in solid‐state nuclear magnetic resonance of half‐integer spin low‐γ quadrupolar nuclei applied to inorganic materials

“…A combination of 20 T, DFS and QCPMG was used to enhance the sensitivity, and the NMR parameters 25 Mg MAS NMR spectra from some natural and synthetic magnesium borates. [146] (b) 25 Mg QCPMG NMR spectrum from MgAl 2 O 4 with no (0) and low level of lithium doping (5%) showing the presence of both MgO 6 and MgO 4 sites. [147] Reproduced from the respective references with permission of the copyright holder extracted were compared with DFT calculations.…”

“…[145] An extensive study of a series of natural and synthetic magnesium borates at 21.1 T reported both experimental and DFT determinations of the NMR interaction parameters, with Figure 7A showing some example spectra. [146] δ iso from the experiments (Table 4) lay between 0 and 23 ppm, the expected range for 25 Mg, and T A B L E 4 A compilation of experimentally determined 25 Mg NMR interaction parameters since 2013 from inorganic materials (nd-not determined, *P Q determined) C Q varied from 0.7 to 18.0 MHz. There was good agreement between experiment and DFT calculations and C Q correlated positively with the modified longitudinal strain.…”

Impact of magnesium on the structure of aluminoborosilicate glasses: A solid‐state NMR and Raman spectroscopy study

“…For nuclei such as 29 Si, 27 Al, or 11 B, a decrease in chemical shift typically results from an increase in the coordination number. For 25 Mg, most studies on the subject have focused on crystalline phases, melts or very simple glasses 19,20,30,44,[44][45][46][47][48][49] ; therefore, there is a lack of 25 Mg NMR data, particularly for borosilicate glasses. Indeed measuring the NMR spectrum of 25 Mg is difficult because of its low natural abundance (10%) and low Larmor frequency.…”