Electronic structure of stable benzodiazasilylenes: Photoelectron spectra and quantum-chemical investigations

Photoelectron spectra of 2,3-dihydro-1,3-di(neopentyl)-1H-1,3,2-benzodiazasilol-2-ylidene were investigated and compared to those of 2,3-dihydro-1,3-di(tert-butyl)-1H-1,3,2-diazasilol-2-ylidene and the related 2,3-dihydro-1,3-di(neopentyl)-1H-1,3,2-benzodiazasilole. The experimental spectra were assigned with the help of quantum-chemical calculations using the MP2/6-31G* level of theory. Trends in the orbital energies correspond well with the observed ionization energies. A 0.41 eV stabilization of the highest occupied molecular orbital level from the silane to the related silylene derivative indicates the incorporation of the silicon empty out-of-plane orbital into the pi system. A good agreement was found between the calculated and experimental geometries. Delocalization stabilization of the investigated molecules is also supported by appropriate isodesmic reactions. The reaction energy indicates significant aromatic stabilization of the silylene derivatives. The stabilization is, however, reduced by a considerable ring strain.