Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK.
Scientific Computing Department, Rutherford Appleton Laboratory, Harwell Campus, Didcot, OX11 0QX, UK.
Solid State Nucl Magn Reson. 2018 Feb;89:1-10. doi: 10.1016/j.ssnmr.2017.11.002. Epub 2017 Nov 28.
Two different axial symmetries of the Sn chemical shift anisotropy (CSA) in tin dioxide with the asymmetry parameter (η) of 0 and 0.27 were reported previously based on the analysis of MAS NMR spectra. By analyzing the static powder pattern, we show that the Sn CSA is axially symmetric. A nearly axial symmetry and the principal axis system of the Sn chemical shift tensor in SnO were deduced from periodic scalar-relativistic density functional theory (DFT) calculations of NMR parameters. The implications of fast small-angle motions on CSA parameters were also considered, which could potentially lead to a CSA symmetry in disagreement with a crystal symmetry. Our analysis of experimental spectra using spectral simulations and iterative fittings showed that MAS spectra recorded at relatively high frequencies do not show sufficiently distinct features in order to distinguish CSAs with η ≈ 0 and η ≈ 0.4. The example of SnO shows that both the MAS lineshape and spinning sideband analyses may overestimate the η value by as much as ∼0.3 and ∼0.4, respectively. The results confirm that a static powder pattern must be analysed in order to improve the accuracy of the CSA asymmetry measurements. The measurements on SnO nanoparticles showed that the asymmetry parameter of the Sn CSA increases for nm-sized particles with a larger surface area compared to μm-sized particles. The increase of the η value for tin atoms near the surface in SnO was also confirmed by DFT calculations.
先前基于 MAS NMR 谱的分析，报道了二氧化锡中锡化学位移各向异性（CSA）的两个不同的轴向对称性，其不对称参数（η）分别为 0 和 0.27。通过对静态粉末图案的分析，我们表明锡 CSA 是轴对称的。通过对 NMR 参数的周期性标量相对论密度泛函理论（DFT）计算，推导出了 SnO 中 Sn 化学位移张量的近轴向对称性和主轴系统。还考虑了快速小角度运动对 CSA 参数的影响，这可能导致 CSA 对称性与晶体对称性不一致。我们使用光谱模拟和迭代拟合对实验光谱进行分析的结果表明，在相对较高的频率下记录的 MAS 光谱没有足够明显的特征，无法区分 η ≈ 0 和 η ≈ 0.4 的 CSA。SnO 的例子表明，MAS 谱线形状和旋转边带分析都可能高估 η 值高达约 0.3 和 0.4。结果证实，为了提高 CSA 不对称性测量的准确性，必须分析静态粉末图案。对 SnO 纳米颗粒的测量表明，与 μm 尺寸的颗粒相比，具有较大表面积的 nm 尺寸的颗粒的 Sn CSA 不对称参数增加。DFT 计算还证实了 SnO 中表面附近锡原子的 η 值增加。
