Probing delocalization in stable silylenes:: Core excitation spectra of Si(NRCH=CHNR), Si(NRCH2CH2NR), H2Si(NRCH=CHNR), and H2Si(NRCH2CH2NR) (R = tBu)

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 tetravalent dihydridosilane molecules, H2Si(NRCH=CHNR) and H2Si(NRCH2CH2NR) (R = Bu-t). The nature of the excited states arising from one-electron core --> pi* and core --> sigma* excitations at different sites on the silylene ring are examined by relating the changes in the spectra of these compounds to differences in their molecular structure. The variations in the intensities of core --> pi* transitions in these spectra probe the extent of delocalization in the pi-electron manifold and, thus, allow us to investigate the nature of pi stabilization in heterocycles containing divalent silicon. Low-lying transitions to a pi*(Si-N) level observed in all core edge spectra strongly support the existence of pi-delocalization in the C=C unsaturated silylene. Ab initio calculations are used to confirm spectral, assignments.