Theoretical study on planar anionic polysilicon chains and cyclic Si ₆ anions with D_6h symmetry

The structures of neutral and anionic hydrogen-terminated oligomers (H-Si_n-H: n = 2, 4, 6, 8) and silicon six-membered rings (c-Si ₆) have been investigated using quantum-chemical methods to find stable planar polysilicon chains and aromatic silicon six-membered rings. Among 18 calculated molecules, planar equilibrium structures have been obtained for seven anionic chains (Si₂H₂^2-, Si ₄H₂^2-, Si₄H₂ ^4-, Si₆H₂^4-, Si₆H ₂^6-, Si₈H₂^4-, Si ₈H₂^6-) and three anionic six-membered rings (c-Si₆^2-, c-Si₆^4-, c-Si ₆^6-). The number of π electrons formally accommodated in the out-of-plane π orbitals is the same as the number of silicon atoms and is independent of the number of doped electrons. An anionic six-membered ring, c-Si₆^2-, shows a large negative value of nucleus-independent chemical shift (NICS), which indicates that c-Si ₆^2- is an aromatic molecule.