X-RAY-DIFFRACTION STUDY OF THE MOLECULAR-STRUCTURE OF A SPHEROHYDRIDOSILASESQUIOXANE, H10SI10O15, A FLEXIBLE ASSEMBLY OF RIGID TETRAHEDRA

The X-ray structure of H10Si10O15 has been determined at 180 K and at 295 K. It crystallizes in the monoclinic space group I2/a, with a = 14 .141(8) angstrom, b = 8.953(3) angstrom, c = 15.022(8), beta = 94.91(3 )-degrees, and V = 1894.9 A3 at 180 K, a = 14.254(5) angstrom, b = 9.0 53(2) angstrom, c = 15.062(5) angstrom, beta = 94.87(l)-degrees, and V = 1936.6 angstrom3 at 295 K, Z = 8, D(x) = 1.86 g cm-3 (180 K), 1.82 g cm-3 (295 K), and fw = 530.92. Single-crystal diffraction data were collected with lambda(Mo Kalpha) = 0.710 73 angstrom and mu(Mo Kalpha) = 7.45 cm-1 (180 K) and with lambda(Ag Kalpha) = 0.560 83 angstrom mu (Ag Kalpha) = 3.78 cm-1 (295 K). Refinement of 2089 observed reflectio ns gave R = 0.0294 (R(w) = 0.0287) for the 180 K data, and refinement of 1575 observed reflections gave R = 0.0486 (R(w) = 0.0661) for the 2 95 K data. Using principal-component as well as rigid-body analysis, t he molecular deformations from D5h symmetry and the anisotropic displa cement parameters are interpreted in terms of a molecular model built from rigid tetrahedra and connected through flexible oxygen atom joint s. Results of the deformation analysis are compared to those in the st ructure of (CH3)10Si10O15.