CORE EXCITATION SPECTROSCOPY OF PHENYL-SUBSTITUTED AND METHYL-SUBSTITUTED SILANOL, DISILOXANE, AND DISILANE COMPOUNDS - EVIDENCE FOR PI-DELOCALIZATION ACROSS THE SI-C-PHENYL BOND

The Si 1s and 2p solid state photoabsorption (total electron yield) sp ectra of triphenylsilanol, hexaphenyldisiloxane, and hexaphenyldisilan e and the Si 1s spectra (total ion yield) of gaseous trimethylsilanol, hexamethyldisiloxane, hexamethyldisilane, and trimethylmethoxysilane have been recorded using synchrotron radiation. These spectra are comp ared to inner shell electron energy loss spectra of gaseous triphenyls ilanol, hexaphenyldisilane, trimethylmethoxysilane, hexamethyldisiloxa ne, and hexamethyldisilane in the Si 2p and C 1s regions, measured und er scattering conditions where electric dipole transitions dominate (2 .5 keV residual energy, theta less than or equal to 2 degrees). Compar ison of the Si 1s and Si 2p spectra of the Ph3Si-X and Me3Si-X species shows there are low-lying transitions at Si which occur exclusively i n the Ph3Si-X species. These transitions are attributed to (Si 1s(-1), piSi-Ph) and (Si 2p(-1),pi*Si-Ph) states in which the core excited el ectron is delocalized across the Si-C(phenyl) bond into the pi levels ofthe phenylring. Extended Huckel and ab initio molecular orbital cal culations of the core excitation spectra support this interpretation. Transitions characteristic of Si-Si and Si-O bonds are also identified .