Intuitive Understanding of sigma Delocalization in Loose and sigma Localization in Tight Helical Conformations of an Oligosilane Chain

0476131 - ÚOCHB 2018 RIV DE eng J - Článek v odborném periodiku
Jovanovic, M. - Antic, D. - Rooklin, D. - Bande, A. - Michl, Josef
Intuitive Understanding of sigma Delocalization in Loose and sigma Localization in Tight Helical Conformations of an Oligosilane Chain.
Chemistry-An Asian Journal. Roč. 12, č. 11 (2017), s. 1250-1263. ISSN 1861-4728. E-ISSN 1861-471X
Institucionální podpora: RVO:61388963
Klíčová slova: electron delocalization * electron localization * electronic states * oligosilanes * sigma conjugation
Obor OECD: Organic chemistry
Impakt faktor: 3.692, rok: 2017

Conformational effects on the sigma-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--SinR2n+2) up to n=16 and for backbone dihedral angles =55-180 degrees. The extent of sigma 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 approximate to 90 degrees from the sigma-delocalized limit at =180 degrees toward and close to the physically non-realizable sigma-localized tight-helix limit =0 with entirely different properties. The distinction is also obtained in the Huckel Ladder H and C models of sigma 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 (sigma hyperconjugation), one consisting of the odd and the other of the even sigma(SiSi) bonds, and then allow the two subchains to interact by geminal interactions (sigma conjugation).
Trvalý link: http://hdl.handle.net/11104/0272659