Tin chemical shift anisotropy in tin dioxide: On ambiguity of CSA asymmetry derived from MAS spectra.

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.