Accurate Gas-Phase Experimental Structures of Octasilsesquioxanes (Si8O12X8; X = H, Me)

The accurate structures of silsesquioxanes Si 8 O 12 H 8 and Si 8 O 12 Me 8 have been determined by gas-phase electron diffraction methods in order to obtain experimental data on single molecules unconstrained by a crystal lattice for comparison with data obtained by theoretical methods. For Si 8 O 12 H 8 the experimentally determined structure shows ideal O h symmetry with Si−O distances and Si−O−Si angles of 161.41(3) pm and 147.9(2)° [ r e, uncertainties (σ) in parentheses] compared with 162.9 pm and 147.8° for theoretical results from MP2/6-311++G(3df,3pd) calculations. In Si 8 O 12 Me 8 a similar O h symmetry model gave experimental values of 161.74(5) pm and 148.9(2)° for the Si−O distances and Si−O−Si angles compared with calculated values of 163.2 pm and 148.6°, respectively.