The making and breaking of σ bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of σ electrons are not nearly as well understood as the states of π-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of σ delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.
Photoelectron spectra and solution UV absorption and magnetic circular dichroism (MCD) of hexamethyldisilane (1), hexaethyldisilane (2), hexa-tert-butyldisilane (3), and the 1,(n+2)-disila[n.n.n]propellanes [n =
4 (4) and 5 (5)] were measured, as was the linear dichroism (LD) of 3 and 4 partially aligned in stretched
polyethylene. The results support the assignment of the lowest energy electronic absorption band of the disilanes
1−5 to a doubly degenerate σSiSi(HOMO) → π*SiC(LUMO) transition and of the next band, observed in the
solution spectra of 2−4 and in the gas-phase spectrum of 1, to a σSiSi → σ*SiSi transition. MP2/VTZ optimized
geometries of 1−5 and ab initio molecular orbital energies (HF/VTZ//MP2/VTZ) and ionization potentials
(ROVGF/VTZ//MP2/VTZ) of these disilanes reproduce the reported geometries and the trends observed in
the photoelectron spectra, respectively. B3LYP/6-31G(d,p) calculations of the Kohn−Sham orbital energies
and TD B3LYP/6-31G(d,p) calculations of transition energies and intensities of 1 as a function of Si−Si
bond length suggest that many of the features of the UV absorption spectrum of 3, including the small energy
difference between the two transitions observed and the large extinction coefficient of the band peaking at
higher energy (σSiSi → σ*SiSi), are due to its very long Si−Si bond.
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