1H and 29Si NMR investigation of SinHn polysilanes with n≤60: A DFT study

Density functional theory (DFT) calculations were performed to investigate the electronic features of the most stable structures of SinHn polysilanes (n=4, 6, 8, 10, 12, 20, 24, 28, 30, 36, 50, and 60). To this aim, 29Si and 1H CS tensors as well as natural charge analyses are calculated for the optimized structures. 29Si CS parameters detect equivalent electronic environment for silicon atoms within SinHn polysilanes with n<20 while 29Si chemical shifts δiso of SinHn polysilanes with n≥20 are separated to few peaks. 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). The silicon atoms of Si60H60, similar to Si20H20, are equivalent but unlike Si20H20 (with Siα sites) have Siγ local structure and present only one peak in chemical shift value of −154.5 ppm. The similar values (26–28 ppm) of 1H calculated chemical shieldings obtained for all the polysilanes means the same tendency of the silicon atoms on the surfaces of all cages for contribution to chemical bonding with hydrogen atoms. Although the total magnitude of charge transfer from silicon atoms on the surface of the cages to their chemically bonded hydrogen atoms increase in larger cages, charge transfer per SiH units remains constant in good agreement with the similarity of chemical shieldings for hydrogen atoms.