Conformational effects on the σ-electron delocalization in oligosilanes are addressed by Hartree-Fock and time-dependent density functional theory calculations (B3LYP, 6-311G**) at MP2 optimized geometries of permethylated uniformly helical linear oligosilanes (all-ω-Sin R2n+2 ) up to n=16 and for backbone dihedral angles ω=55-180°. The extent of σ delocalization is judged by the partition ratio of the highest occupied molecular orbital and is reflected in the dependence of its shape and energy and of UV absorption spectra on n. The results agree with known spectra of all-transoid loose-helix conformers (all-[±165]-Sin Me2n+2 ) and reveal a transition at ω≈90° from the "σ-delocalized" limit at ω=180° toward and close to the physically non-realizable "σ-localized" tight-helix limit ω=0 with entirely different properties. The distinction is also obtained in the Hückel Ladder H and C models of σ delocalization. An easy intuitive way to understand the origin of the two contrasting limits is to first view the linear chain as two subchains with alternating primary and vicinal interactions (σ hyperconjugation), one consisting of the odd and the other of the even σ(SiSi) bonds, and then allow the two subchains to interact by geminal interactions (σ conjugation).

Department of Chemistry and Biochemistry University of Colorado Boulder CO 80309 0215 USA

Institute of Methods for Material Development Helmholtz Zentrum Berlin für Materialien und Energie GmbH Albert Einstein Str 15 D 12489 Berlin Germany

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic Flemingovo nám 2 16610 Prague 6 Czech Republic

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