Core Excitation Spectroscopy of Phenyl- and Methyl-Substituted Silanol, Disiloxane, and Disilane Compounds: Evidence for π-Delocalization across the Si−Cphenyl Bond

The Si 1s and 2p solid state photoabsorption (total electron yield) spectra of triphenylsilanol, hexaphenyldisiloxane, and hexaphenyldisilane and the Si 1s spectra (total ion yield) of gaseous trimethylsilanol, hexamethyldisiloxane, hexamethyldisilane, and trimethylmethoxysilane have been recorded using synchrotron radiation. These spectra are compared to inner shell electron energy loss spectra of gaseous triphenylsilanol, hexaphenyldisilane, trimethylmethoxysilane, hexamethyldisiloxane, and hexamethyldisilane in the Si 2p and C 1s regions, measured under scattering conditions where electric dipole transitions dominate (2.5 keV residual energy, θ ≤ 2°). Comparison 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 in the Ph3Si−X species. These transitions are attributed to (Si 1s-1,π*Si - Ph) and (Si 2p-1,π*Si - Ph) states in which the core excited electron is delocalized across the Si−C(phenyl) bond into the π* levels of the phenyl ring. Extended Hückel and ab initio molecular orbital calculations of the core excitation spectra support this interpretation. Transitions characteristic of Si−Si and Si−O bonds are also identified.