Novel pentagonal silicon rings and nanowheels stabilized by flat pentacoordinate carbon(s).

It is predicted by accurate density functional and coupled-cluster theory that planar Si(5)C and Si(5)C rings can be stabilized by flat pentacoordinate carbon-silicon bonds. The energy difference of the Si(5)C dianion from the lowest energy three-dimensional isomer is about 12.2 kcal∕mol at the level of the density functional theory using the Becke 3-parameter (exchange), Lee, Yang and Parr functional, and the triple-ζ doubly polarized basis sets. Stable composite Si(5)C structures are formed either as nanowheels with axial C-C bonds of 1.51 Å or as isoenergetic pentagonal graphiticlike layers with double C-C distance (3.02 Å) and almost double aromaticity index, based on nucleus independent chemical shifts. Both of these structures are at least 12 kcal∕mol lower in energy than the lowest energy Si(10)C(2) structure reported in the literature, but about 5 kcal∕mol higher than the lowest energy structure found here.