¹⁷O and ²⁹Si NMR parameters of MgSiO₃ phases from high-resolution solid-state NMR spectroscopy and first-principles calculations

Ashbrook, S.E., Berry, A.J., Frost, D.J., Gregorovic, A., Pickard, C.J., Readman, J.E. and Wimperis, S. (2007) ¹⁷O and ²⁹Si NMR parameters of MgSiO₃ phases from high-resolution solid-state NMR spectroscopy and first-principles calculations. Journal of the American Chemical Society, 129(43), pp. 13213-13224. (doi: 10.1021/ja074428a)

Abstract

The ²⁹Si and ¹⁷O NMR parameters of six polymorphs of MgSiO₃ were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO₃ is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the ¹⁷O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO₃ (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first ¹⁷O NMR data for these materials and confirm previous ²⁹Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing ¹⁷O chemical shift with increasing Si−O bond length, the shift observed also depends upon the exact coordination environment.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Gregorovic, Dr Alan and Wimperis, Professor Stephen
• Authors: Ashbrook, S.E., Berry, A.J., Frost, D.J., Gregorovic, A., Pickard, C.J., Readman, J.E., and Wimperis, S.
• Subjects: Q Science > QD Chemistry
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Journal of the American Chemical Society
• Publisher: American Chemical Society
• ISSN: 0002-7863
• ISSN (Online): 1520-5126
• Published Online: 09 October 2007