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. 2020 Feb 19;25(4):933.
doi: 10.3390/molecules25040933.

Computational and Experimental 1H-NMR Study of Hydrated Mg-Based Minerals

Eric G Sorte  1 , Jessica M Rimsza  2 , Todd M Alam  1
Affiliations

Computational and Experimental 1H-NMR Study of Hydrated Mg-Based Minerals

Eric G Sorte et al. Molecules. .

Abstract

Magnesium oxide (MgO) can convert to different magnesium-containing compounds depending on exposure and environmental conditions. Many MgO-based phases contain hydrated species allowing 1H-nuclear magnetic resonance (NMR) spectroscopy to be used in the characterization and quantification of proton-containing phases; however, surprisingly limited examples have been reported. Here, 1H-magic angle spinning (MAS) NMR spectra of select Mg-based minerals are presented and assigned. These experimental results are combined with computational NMR density functional theory (DFT) periodic calculations to calibrate the predicted chemical shielding results. This correlation is then used to predict the NMR shielding for a series of different MgO hydroxide, magnesium chloride hydrate, magnesium perchlorate, and magnesium cement compounds to aid in the future assignment of 1H-NMR spectra for complex Mg phases.

Keywords: 1H-NMR; DFT; GIPAW; chemical shift; hydroxylated; magnesium minerals; magnesium oxide.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
Examples of different MgO conversion reactions.
Figure 1
Figure 1
Deconvolution of the 1H- magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra for (a) brucite, Mg(OH)2, (b) nitromagnesite, Mg(NO3)2·6H2O, (c) bischofite, MgCl2·6H2O, (d) epsomite, MgSO4·7H2O, (e) magnesium bromide, MgBr2·6H2O, (f) hydromagnesite, 4MgCO3·Mg(OH)2·4H2O, (g) artinite, MgCO3·Mg(OH)2·3H2O, and (h) dypingite, 4MgCO3·Mg(OH)2·5H2O.
Figure 2
Figure 2
2D double quantum (DQ)-single quantum (SQ) 1H-MAS-NMR correlation spectra for (a) hydromagnesite, 4MgCO3·Mg(OH)2·4H2O and (b) artinite, MgCO3·Mg(OH)2·3H2O. The auto-correlation peaks between protons in the same environment (chemical shift) are identified by solid circles, and the colored lines identify correlation between different proton environments.
Figure 3
Figure 3
Snapshots of four different MgO-containing crystal structures (a) MgCl2·6H2O, (b) Mg(OH)2, (c) MgSO4·7H2O, and (d) MgNO3·6H2O. Atom colors: Mg (green), O (red), H (white), Cl (purple), S (yellow), and N (blue).
Figure 4
Figure 4
Correlation (R2 = 0.9268) between the average computational 1H-NMR shielding and experimental chemical shift for select MgO crystalline structures.
Figure 5
Figure 5
Calculated 1H-NMR isotropic chemical shift (δ) values versus OH…O hydrogen bond lengths (Å) for refence compounds. The dashed line serves as a guide line to the reader.
See this image and copyright information in PMC

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