title: Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain.
abstract: 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-SinR2n+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]-SinMe2n+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).
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309-0215, USA.|Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 16610, Prague 6, Czech Republic.
authors.forename: Milena
authors.initials: M
authors.lastname: Jovanovic
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309-0215, USA.
authors.forename: Dean
authors.initials: D
authors.lastname: Antic
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309-0215, USA.
authors.forename: David
authors.initials: D
authors.lastname: Rooklin
authors.affiliation: 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.
authors.forename: Annika
authors.initials: A
authors.lastname: Bande
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309-0215, USA.|Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 16610, Prague 6, Czech Republic.
authors.forename: Josef
authors.identifier: http://orcid.org/0000-0002-4707-8230
authors.initials: J
authors.lastname: Michl
keywords: electron delocalization; electron localization; electronic states; oligosilanes; sigma conjugation