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Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain
Author(s) -
Jovanovic Milena,
Antic Dean,
Rooklin David,
Bande Annika,
Michl Josef
Publication year - 2017
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201700226
Subject(s) - delocalized electron , coiled coil , elongation , helix (gastropod) , chain (unit) , physics , electromagnetic coil , crystallography , chemistry , materials science , biophysics , biology , quantum mechanics , snail , metallurgy , ultimate tensile strength , ecology
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 ‐Si n R 2 n +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]‐Si n Me 2 n +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).