Determination of the Full 207Pb Chemical Shift Tensor of Anglesite, PbSO4, and Correlation of the Isotropic Shift to Lead–Oxygen Distance in Natural Minerals

Otto Zeman,Rupert Hochleitner,Jennifer Steinadler,Thomas Bräuniger

doi:10.3390/cryst9010043

Abstract

The full 207 Pb chemical shift (CS) tensor of lead in the mineral anglesite, PbSO 4 , was determined from orientation-dependent nuclear magnetic resonance (NMR) spectra of a large natural single crystal, using a global fit over two rotation patterns. The resulting tensor is characterised by the reduced anisotropy Δ δ = ( - 327 ± 4 ) ppm, asymmetry η C S = 0 . 529 ± 0 . 002 , and δ i s o = ( - 3615 ± 3 ) ppm, with the isotropic chemical shift δ i s o also verified by magic-angle spinning NMR on a polycrystalline sample. The initially unknown orientation of the mounted single crystal was included in the global data fit as well, thus obtaining it from NMR data only. By use of internal crystal symmetries, the amount of data acquisition and processing for determination of the CS tensor and crystal orientation was reduced. Furthermore, a linear correlation between the 207 Pb isotropic chemical shift and the shortest Pb–O distance in the co-ordination sphere of Pb 2 + solely surrounded by oxygen has been established for a large database of lead-bearing natural minerals.

Highlights

For the structural characterization of periodic solids, either in single or multicrystalline form, X-ray crystallography is an invaluable tool [1]
That the chemical shift tensor dCRY we want to calculate from the rotation patterns possesses only four independent tensor elements (Equation 10), due to constraints imposed by the crystal symmetry
Since the rotation was carried out around one physical goniometer axis ~g, the two harmonics of the magnetically inequivalent lead atoms are linked by one constraint [14], and only five linear independent parameters may be extracted from one rotation pattern

Summary

Introduction

For the structural characterization of periodic solids, either in single or multicrystalline form, X-ray crystallography is an invaluable tool [1]. As a complementary technique for the elucidation of structure and dynamics in solids, including amorphous ones, nuclear magnetic resonance (NMR). In the context of NMR crystallography, many questions (e.g., the determination of asymmetric units, assignment of space groups) may already be answered by considering only the isotropic chemical shift δiso of the NMR-observed nuclide [2]. The deceptively simple scalar δiso is, the result of a contraction (of the isotropic part) of the second-rank chemical shift (CS) tensor: δiso = (δ + δ22 + δ33 ) (1). The full CS tensor δ reflects the spatial distribution of electrons around the observed nucleus, which ’shield’ it from the external magnetic field to a certain extent. A more complete picture becomes available when the full chemical shift tensor is known, which provides possible information on co-ordination, the influence of electron lone pairs, etc. The eigenvalues δ11 , δ22 , and δ33 may be determined by measuring the NMR spectrum of a Crystals 2019, 9, 43; doi:10.3390/cryst9010043 www.mdpi.com/journal/crystals

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Results

Conclusion