STRUCTURAL AND VIBRATIONAL PROPERTIES OF THE OCTANUCLEAR SILASESQUIOXANE C6H13(H7SI8O12)

The crystal structure of hexylheptahydrooctasilasesquioxane C6H13(H7Si 8O12) has been determined by single-crystal X-ray diffraction and comp ared with the structures of H8Si8O12, [Co(CO)4(H7Si8O12)] and (CH3)8Si 8O12. The monosubstituted silasesquioxane molecules have a crystallogr aphic C1 symmetry. The local symmetry of their octanuclear Si cage, ho wever, is close to C3v. They are closer to C3v than H8Si8O12 which has an effective molecular symmetry of T(h) in the crystal. The major dis tortions originate in intermolecular O ... Si contacts. The larger the number of such contacts per molecule, the larger is the distortion of the local symmetry of the cage. The structure of C6H13(H7Si8O12) comp ares well with that of [Co(CO)4(H7Si8O12)], and it is clearly the leas t distorted of the two due to a reduced influence of the n-hexyl ligan d on the cage geometry and due to a smaller number of intermolecular i nteractions. The IR spectrum of C6H13(H7Si8O12) has been interpreted b y correlating it with the three molecules H8Si8O12, C6H13[Si(OSiMe3)3] and HSi(OSiMe3)3.