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Conformational dependence of sigma-electron delocalization in linear chains: permethylated oligosilanes.

The effects of sigma-electron delocalization on optical properties of saturated linear chains of permethylated oligosilanes are strongly conformation dependent. We analyze the origin of the conformational dependence of the energies of molecular orbitals and of electronic excitations in simple intuitively understandable terms by using a first-order approximation to the Hückel version of the "Ladder C" model. The analysis is supported by comparison with results of numerical calculations from time-dependent density functional theory, which agree well with experiment. To facilitate the comparison, a simple procedure has been developed that defines the overall and local fractional sigma and pi characters of a backbone molecular orbital and a fractional overall and local sigma sigma* and sigma pi* characters of an excited state for any conformation of a linear chain.