VIBRATIONS OF H8SI8O12, D8SI8O12, AND H10SI10O15 AS DETERMINED BY PNS, IR, AND RAMAN EXPERIMENTS

A detailed study of the vibrational structure of the silasesquioxanes H8Si8O12 and H10S10O15 based on INS, IR, and Raman spectra and on a no rmal-coordinate analysis is reported. The inelastic neutron scattering (INS) spectrum of crystalline H8Si8O12 is in good agreement with the optical data and allowed the assignment of optically forbidden transit ions. The previously published force field, determined from IR and Ram an data of H8Si8O12 and D8Si8O12, provided a good fit to the INS frequ encies and intensities except in the O-Si-H bending region, which is s ensitive to intermolecular interactions. The INS spectrum, a compariso n of the Raman spectra of dissolved and solid H8Si8O12, and published diffraction data were used to analyse the influence of crystal packing . A modified force field was developed which is adapted to crystalline H8Si8O12 and distinguishes between O-Si-H bending force constants for the axial and the equatorial hydrogen atoms. Excellent agreement with all spectra was thus obtained. This force field was also applied to t he INS spectra of H10Si10O15 and D8Si8O12. Based on a normal-coordinat e analysis of the INS data, the total mean-square displacements (msd) of the hydrogen atoms of H8Si8O12 were extracted as well as limits for the total msd of the silicon atoms. The msd's and the neutron diffrac tion values are in good agreement, showing that no or very little stat ic disorder is present in crystalline H8Si8O12. Based on the msd's, th e lowest internal torsional frequency was estimated to be 41 +/- 7 cm( -1).