Ashbrook Sharon E, Le Pollès Laurent, Pickard Chris J, Berry Andrew J, Wimperis Stephen, Farnan Ian
School of Chemistry and EaStCHEM, University of St Andrews, North Haugh, St Andrews, UK KY16 9ST.
Phys Chem Chem Phys. 2007 Apr 7;9(13):1587-98. doi: 10.1039/b618211a. Epub 2007 Feb 12.
The nuclear magnetic resonance (NMR) shielding and electric field gradient (EFG) tensors of three polymorphs of Mg(2)SiO(4), forsterite (alpha-Mg(2)SiO(4)), wadsleyite (beta-Mg(2)SiO(4)) and ringwoodite (gamma-Mg(2)SiO(4)), have been calculated using a density functional theory (DFT) approach with a planewave basis set and pseudopotential approximation. These Mg(2)SiO(4) polymorphs are the principal components of the Earth down to depths of 660 km and have been proposed as the hosts of water in the Earth's upper mantle and transition zone. A comparison of our calculations with single-crystal spectroscopic data in the literature for the alpha-polymorph, forsterite, shows that both the magnitude and orientation of the shielding and EFG tensors for O and Si can be obtained with sufficient accuracy to distinguish subtle differences in atomic positions between published structures. We compare calculated (17)O MAS NMR quadrupolar powder lineshapes directly with experimental lineshapes and show that we are able to reproduce them within the precision with which the NMR parameters may be determined from multi-parameter fitting. The relatively small amounts of sample available for the beta- and gamma-polymorphs, arising from the high pressures required for synthesis, has hindered the extraction of NMR parameters in previous work. The application of DFT calculations to these high-pressure polymorphs confirms previous spectral assignments, and provides deeper insight into the empirical correlations and observations reported in the literature. These first-principles methods are highly promising for the determination of local bonding in more complex materials, such as the hydrated forms of Mg(2)SiO(4), by aiding analysis of their multinuclear NMR spectra.
利用平面波基组和赝势近似的密度泛函理论（DFT）方法，计算了硅酸镁（Mg₂SiO₄）三种多晶型物——镁橄榄石（α-Mg₂SiO₄）、瓦兹利石（β-Mg₂SiO₄）和林伍德石（γ-Mg₂SiO₄）的核磁共振（NMR）屏蔽张量和电场梯度（EFG）张量。这些Mg₂SiO₄多晶型物是地球直至660千米深处的主要成分，并被认为是地球上地幔和过渡带中水的宿主。将我们的计算结果与文献中关于α-多晶型物镁橄榄石的单晶光谱数据进行比较，结果表明，O和Si的屏蔽张量和EFG张量的大小及方向都能以足够的精度获得，从而区分已发表结构中原子位置的细微差异。我们将计算得到的¹⁷O MAS NMR四极粉末线形直接与实验线形进行比较，结果表明，我们能够在通过多参数拟合确定NMR参数的精度范围内重现它们。由于合成所需的高压，β-和γ-多晶型物可得的样品量相对较少，这阻碍了先前工作中NMR参数的提取。DFT计算应用于这些高压多晶型物，证实了先前的光谱归属，并为文献中报道的经验关联和观察结果提供了更深入的见解。这些第一性原理方法对于通过辅助分析更复杂材料（如Mg₂SiO₄的水合形式）的多核NMR光谱来确定局部键合极具前景。
