PROBING DELOCALIZATION IN STABLE SILYLENES - CORE EXCITATION-SPECTRA OF SI(NRCH=CHNR), SI(NRCH2CH2NR), H2SI(NRCH=CHNR), AND H2SI(NRCH2CH2NR) (R = BU-T)

The silicon 1s, 2p, and 2s, carbon 1s, and nitrogen 1s gas-phase core excitation spectra are reported for two stable divalent silylenes, Si( NRCH=CHNR) and Si(NRCH2CH2NR) (R = Bu-t), and for the two analogous te travalent dihydridosilane molecules, H2Si(NRCH=CHNR) and H2Si(NRCH2CH2 NR) (R = Bu-t). The nature of the excited states arising from one-elec tron core --> pi and core --> sigma* excitations at different sites o n the silylene ring are examined by relating the changes in the spectr a of these compounds to differences in their molecular structure. The variations in the intensities of core --> pi transitions in these spe ctra probe the extent of delocalization in the pi-electron manifold an d, thus, allow us to investigate the nature of pi stabilization in het erocycles containing divalent silicon. Low-lying transitions to a pi( Si-N) level observed in all core edge spectra strongly support the exi stence of pi-delocalization in the C=C unsaturated silylene. Ab initio calculations are used to confirm spectral, assignments.