# 19F MAS NMR chemical shift tensor and the crystal structure

$\ce{^{19}F}$ MAS NMR is measured on fluorine-doped $\ce{TiO2}$. $\ce{F}$ is to substitute $\ce{O}$ and the crystal structure is anatase, which has a tetragonal crystal system with a space group $\ce{I 41/a m d S(141)}$. Using the observed isotropic chemical shift value, a chemical shift anisotropy (CSA) analysis is carried out using Simpson and gives a uniaxial chemical shift tensor ($\ce{δ11 = δ22 ≠ δ33, etha = 0}$). This result dictates a three-fold symmetry at the fluorine site. In anatase with a tetragonal system, however, the oxygen ions occupy position 8e with a point group of ${mm}$ and no three-fold axis.

How can the uniaxial chemical shift tensor, indicating three-fold symmetry, correlate to the crystal structure of anatase which does not possess any three-fold axis?

• You had the isotropic chemical shift and from that somehow back-calculated the CSA using Simpon? I wouldn't have thought that it is possible... Can you post the spin system section of your Simpson input file? – selkie222 Jun 1 '16 at 8:56