A computational investigation of electronic structure as well as 19F and 29Si chemical shielding tensors in the fluorinated silicon fullerenes SinFn (n≤60)

Density functional theory (DFT) calculations are performed to investigate the electronic features of the structures of fluorinated polysilanes SinFn (n=4, 6, 8, 10, 12, 20, 24, 28, 30, 32, 36, 50, and 60). Among all of these fluorinated polysilanes, Si20F20 has the highest binding energy and, thus, stability. The binding energy then shows a very slow (monotonically) decrease as the size of the fluorinated silicon fullerene n≥20 increases which can be related to an increase in fluorine–fluorine repulsion. Following an irregular pattern, the HOMO–LUMO energy gap strongly depends on the size of the cage. On the other hand, 29Si CS parameters detect equivalent electronic environment for silicon atoms within SinHn polysilanes with n≤20 while 29Si NMR pattern indicates a few separated peaks for SinHn polysilanes with n≥20. Seeking correlation between these peaks and local structures around silicon sites, Siα, Siβ, Siγ observed in these models shows that δiso(Siγ)<δiso(Siβ) <δiso(Siα). Obtaining similar values (458.8–478.7 ppm) of 19F calculated chemical shieldings for all the fluorinated polysilanes means the same tendency of the silicon atoms on the surfaces of all cages for contribution to chemical bonding with fluorine atoms.

Graphical abstractDFT study to investigate the electronic features of the structures of fluorinated polysilane cages SinFn.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The same 29Si CS for each SinFn fluorinated polysilane (n≤20). ► Few peaks for 29Si δiso of SinFn polysilanes silicon cages (n>20). ► Correlation between δiso and local structures: δiso(Siγ)<δiso(Siβ)<δiso(Siα). ► Si60H60 consists of Siγ sites compared to Si20H20 consisting of Siα sites. ► The same tendency of Si within all cages for chemical bonding with F atoms.