High-precision imaging of an encapsulated Lindqvist ion and correlation of its structure and symmetry with quantum chemical calculations

Low-voltage aberration-corrected transmission electron microscopy (AC-TEM) of discrete Lindqvist [W6O19](2-) polyoxometalate ions inserted from an ethanolic solution of [NBu4](2)[W6O19] into double walled carbon nanotubes (DWNTs) allows a higher precision structural study to be performed than previously reported. W atom column separations within the constituent W-6 tungsten cage can now be visualized with sufficient clarity that reliable correlation with structural predictions from density functional theory (DFT) can be achieved. Calculations performed on [W6O19](2-) anions encapsulated in carbon nanotubes show good agreement with measured separations between pairs of W-2 atom columns imaged within equatorial WO6 polyhedral pairs and also single W atom positions located within individual axial WO6 octahedra. Structural data from the tilted chiral encapsulating DWNT were also determined simultaneously with the anion structural measurements, allowing the influence of the conformation of the encapsulating tubule to be included in the DFT calculation and compared against that of other candidate encapsulating nanotubes. Additional DFT calculations performed using Li+ cations as a model for the [NBu4](+) counterions indicate that the latter may help to induce charge transfer between the DWNT and the [W6O19](2-) ion and this may help to constrain the motion of the ion in situ.