Silicon-nitrogen bonding in silatranes: assignment of photoelectron spectra

Silicon-nitrogen bonding and the photoelectron spectra of hydro-silatrane and methyl-silatrane, XSi[OCH2CH2]3N (X = H and Me), were studied with ab initio electron propagator theory, many-body methods, and density functional models. A linear vibronic coupling (LVC) model was employed to estimate vibrational widths of the ionization bands and to study the dependence of the ionization energies on the molecular geometry. Particular attention was given to coordinates that change the Si-N distance and the strength of the donor-acceptor interaction between these two atoms. The ionization energy of the highest occupied molecular orbital has a very strong geometrical dependence which leads to an unusually large vibrational width in the corresponding photoelectron band. The assignment of this band in methyl-silatrane, which was controversial for a long time, is resolved by the present study. The calculated photoelectron spectra allow for clear assignment of at least three more bands in the observed spectra. The present results demonstrate the important role of electrostatic interactions in Si <-- N bonding and in the outer-valence ionization energies of the silatranes.