Claudia Marcolli, Philippe Lainé, René Bühler, and Gion Calzaferri
Institute of Inorganic and Physical Chemistry, University of Berne, Freiestrasse 3, 3000 Berne 9, Switzerland
John Tomkinson
ISIS Facility Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom
Abstract
A detailed study of the vibrational structure of the silasesquioxanes
H8Si8O12 and
H10Si10O15 based on INS, IR and Raman spectra
and on a normal 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 transitions. The previously published force field,
determined from IR and Raman data of H8Si8O12
and D8Si8O12, provided a good fit to the INS
frequencies and intensities except in the O–Si–H bending region, which
is sensitive to intermolecular interactions. The INS spectrum, a comparison 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 the INS spectra of
H10Si10O15 and
D8Si8O12. Based on a normal coordinate 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 diffraction
values are in good agreement, showing that no or very little static disorder is
present in crystalline H8Si8O12. Based on the
msd's, the lowest internal torsional frequency was estimated, 41 ± 7
cm-1.
published in: J. Phys. Chem. B 1997, 101, 1171.
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