Ab initio quantum chemical investigation of several isomers of anionic Si6

Eight isomers (planar hexagon, benzvalene, Dewar benzene, triangular prismane, bicyclopropenyl, octahedron, chair form, and twist boat form) of Si6, Si 6 2 - , Si 6 4 - , and Si 6 6 - , have been searched for by the MP2 and B3LYP electronic structure calculations. Totally 14 isomers were found: two Si6, six Si 6 2 - , five Si 6 4 - , and one Si 6 6 - . Two of them are different from the eight isomers: deformed triangle Si 6 2 - ; pentagonal pyramidal Si 6 4 - . We discovered that the predicted stable shapes of Si 6 2 - , Si 6 4 - , and Si 6 6 - are octahedral, pentagonal pyramidal, and hexagonal, respectively, which agrees well with Wade rule.