The Monetite Structure Probed by Advanced Solid-State NMR Experimentation at Fast Magic-Angle Spinning.

Yang Yu,Hua Guo,Michael Pujari-Palmer,Mattias Edén,Håkan Engqvist,Baltzar Stevensson

doi:10.3390/ijms20246356

Abstract

We present a solid-state nuclear magnetic resonance (NMR) spectroscopy study of the local P and H environments in monetite [CaHPO; dicalcium phosphate anhydrous (DCPA)], as well as their relative spatial proximities. Each of the three H NMR peaks was unambiguously assigned to its respective crystallographically unique H site of monetite, while their pairwise spatial proximities were probed by homonuclear H–H double quantum–single quantum NMR experimentation under fast magic-angle spinning (MAS) of 66 kHz. We also examined the relative H–P proximities among the inequivalent {P1, P2} and {H1, H2, H3} sites in monetite; the corresponding shortest internuclear H–P distances accorded well with those of a previous neutron diffraction study. The NMR results from the monetite phase were also contrasted with those observed from the monetite component present in a pyrophosphate-bearing calcium phosphate cement, demonstrating that while the latter represents a disordered form of monetite, it shares all essential local features of the monetite structure.

Highlights

The natural mineral monetite (CaHPO4), referred to as dicalcium phosphate anhydrous (DCPA), is the anhydrous form of brushite (CaHPO4·2H2O; dicalcium phosphate dihydrate; DCPD)
The progressive peak-narrowing for increasing spinning speed stems from the suppression of broadenings from 1H–1H dipolar interactions, which at the higher rates readily resolve the resonances from the two crystallographically inequivalent H2 (13.4 ppm) and H3 (12.9 ppm) sites, whereas the nuclear magnetic resonance (NMR) signal from the H1 counterpart (15.8 ppm) is well-separated from the H2/H3 resonances at all magic-angle spinning (MAS) rates
Note that the 1H MAS NMR spectrum recorded at 66.00 kHz MAS was obtained at B0 = 14.1 T, whereas all other 31P and 1H NMR spectra were acquired at B0 = 9.4 T

Summary

Introduction

The natural mineral monetite (CaHPO4), referred to as dicalcium phosphate anhydrous (DCPA), is the anhydrous form of brushite (CaHPO4·2H2O; dicalcium phosphate dihydrate; DCPD). The latter is of interest for biomineralization, both as a potential precursor phase of bone mineral [1], as well as its appearance under acidic conditions associated with pathological bone mineralization pathways that for instance lead to dental calculus and kidney stone [2]. The P1:P2A:P2B multiplicities are 2:1:1, meaning that P1:P2 exhibit equal multiplicities [6]

Methods

Results

Discussion

Conclusion