Geometry of the localized σσ∗ excited state of n-tetrasilane

Full optimization of the geometry of the lowest excited singlet of n-tetrasilane, Si4H10, at several levels of CASSCF theory yielded a floppy C2 structure with an SiSiSiSi dihedral angle of 105° that can be increased to 180° at the cost of only 4 kcal/mol, SiSiSi valence angles of 90°, central SiSi bond stretched to 2.55 Å and terminal ones to 2.42 Å. The σσ∗ excitation is localized in the central bond.