Assignment of photoelectron spectra of silatranes: first ionization energies and the nature of the dative Si←N contact

Abstract

The problematic experimental photoelectron spectra of fluoro- and ethoxy-silatranes, XSi[OCH₂CH₂]₃N (X = F and OEt), were assigned using theoretical spectra obtained by combining the OVGF//CCSD vertical ionization energies with the vibrational widths of the electronic transitions (linear vibronic coupling formalism, LVC). Taking into account the overlapping of the silatrane bands with the bands of probable impurities, bicyclic amines, (OH)XSi(OCH₂CH₂)₂NCH₂CH₂OH, allowed us to reliably determine the position of the low-energy bands (at ∼9.7 eV for F- and at ∼9.2 eV for EtO-silatrane) associated with the ionization from a nitrogen lone pair level. For XSi[OCH₂CH₂]₃N (X = F, H, OEt, Me), the correlation between the first vertical ionization energies, VIEs₁, and the geometrical, electronic and orbital characteristics of the Si←N bonding was found. Its analysis suggests that the Si←N coordination in silatranes is orbital-controlled rather than charge-controlled.