Calcium-43 Chemical Shift Tensors as Probes of Calcium Binding Environments. Insight into the Structure of the Vaterite CaCO₃ Polymorph by ⁴³Ca Solid-State NMR Spectroscopy

Abstract

Natural-abundance (43)Ca solid-state NMR spectroscopy at 21.1 T and gauge-including projector-augmented-wave (GIPAW) DFT calculations are developed as tools to provide insight into calcium binding environments, with special emphasis on the calcium chemical shift (CS) tensor. The first complete analysis of a (43)Ca solid-state NMR spectrum, including the relative orientation of the CS and electric field gradient (EFG) tensors, is reported for calcite. GIPAW calculations of the (43)Ca CS and EFG tensors for a series of small molecules are shown to reproduce experimental trends; for example, the trend in available solid-state chemical shifts is reproduced with a correlation coefficient of 0.983. The results strongly suggest the utility of the calcium CS tensor as a novel probe of calcium binding environments in a range of calcium-containing materials. For example, for three polymorphs of CaCO3 the CS tensor span ranges from 8 to 70 ppm and the symmetry around calcium is manifested differently in the CS tensor as compared with the EFG tensor. The advantages of characterizing the CS tensor are particularly evident in very high magnetic fields where the effect of calcium CS anisotropy is augmented in hertz while the effect of second-order quadrupolar broadening is often obscured for (43)Ca because of its small quadrupole moment. Finally, as an application of the combined experimental-theoretical approach, the solid-state structure of the vaterite polymorph of calcium carbonate is probed and we conclude that the hexagonal P6(3)/mmc space group provides a better representation of the structure than does the orthorhombic Pbnm space group, thereby demonstrating the utility of (43)Ca solid-state NMR as a complementary tool to X-ray crystallographic methods.

Exploring the limits of 73Ge solid-state NMR spectroscopy at ultrahigh magnetic field

“…However, for quadrupolar nuclei, the CS tensor parameters often cannot be accurately extracted from the spectrum acquired at low-and moderate-field strength and, in many cases, the small CSA cannot be directly measured at all because when the large quadrupolar interaction dominates, the small CSA cannot manifest itself in the spectrum. As mentioned earlier and shown in several reports, 21,23,24 this problem can be significantly alleviated by working at very high field due to the fact that the second-order quadrupolar interaction scales inversely and the CS interaction is proportional to the field strength.…”

Magnesium incorporation into hydroxyapatite

“…[37,84,85] In order to investigate this point, the local environment of calcium was analyzed using X-ray absorption spectroscopy, because Ca K-edge XANES (X-ray absorption near edge structure) can provide information about the local geometry around calcium, [86] and Ca K-edge EXAFS (Extended X-ray absorption fine structure) on the average Ca•••O bond distance in the first sphere. [46] As shown in Figure 5, the Ca K-edge XANES spectrum of Mg10-HA is nearly identical to the one of non-substituted HA, especially in the region of the pre-edge, which is sensitive to the local distortion around calcium sites.…”

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

“…With an increasing range of observations of 43 Ca [68] it is clear that iso , which is the most readily determined parameter from MAS NMR spectra, could provide information not only on the average Ca---O bond distances, but also about the nature of the next nearest neighbours around the calcium, as noted for borates and aluminosilicates [69].…”