The Disilane Chromophore: Photoelectron and Electronic Spectra of Hexaalkyldisilanes and 1,(n+2)-Disila[n.n.n]propellanes

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Abstract

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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