Electronic structure of a stable silylene: photoelectron spectra and theoretical calculations of Si(NRCHCHNR), Si(NRCH2CH2NR) and SiH2(NRCHCHNR)

The He I and He II photoelectron spectra are reported and assigned for three closely related molecules, Si(ButNCHCHNBut), Si(ButNCH2CH2NBut) and SiH2(ButNCHCHNBut), providing evidence for an energetically significant contribution from the silicon 3pπ orbital to the ring π system of Si(ButNCHCHNBut). Theoretical calculations on the model compounds Si(HNCHCHNH), Si(HNCH2CH2NH) and SiH2(HNCHCHNH) have been carried out and there is good agreement between the calculated and experimental geometries. Trends in the orbital energies corresponded closely with the trends found for the ionization energies of the photoelectron bands. Calculated singlet–triplet splittings predict singlet ground states for the model silyienes. Calculated heats of hydrogenation at the ring carbons of Si(HNCHCHNH) and SiH2(HNCHCHNH) show the latter to be more exothermic by 13 kcal mol–1 providing evidence for aromatic cyclic (4n+ 2)-π delocalization in Si(ButNCHCHNBut). Estimates of Si–N double-bond character are obtained by ab initio calculations on rotamers of the model compound Si(NH2)2 and compared with analogous values for C(NH2)2.