Optimized ladder C and ladder H models for sigma conjugation: chain segmentation in polysilanes

Abstract

The parameters of the Hückel type ladder C and ladder H models of permethylated oligosilanes have been optimized to reproduce orbital energies obtained from ab initio Hartree–Fock (HF) calculations of 84 conformers with 1–8 silicon atoms at their optimized geometries. With the optimized parameters, the ladder models reproduce the top ∼5% of the ab initio occupied orbital energies with a mean error of 0.03–0.05 eV. Using Koopmans' theorem, trends in the ionization potentials of very long oligosilanes and polysilanes, including their conformational dependence, can now be predicted with the accuracy of the HF/TZ ab initio method. We use the ladder models to examine the chain‐length dependence of conformational effects in chains with up to 40 silicon atoms, with particular attention to the issue of conformationally induced chain segmentation. Copyright © 2002 John Wiley & Sons, Ltd.

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

“…The surrounding grid points were assigned avalue of the RGB code. The parameters used in the Hückelc alculations were (in eV): [19] Ladder C, a Si = À6.1,…”

“…The use of standard Ladder Cp arameters for oligosilanes [19] predicts that the all-syn limit will be s-localized, as indicated in Figures10, 12, and 13. The orbital level correlation between the w = 08 and 1808 is shown in Figure 12 Ba nd provides as imple explanation of the main features of Figure 9.…”

“…[16] In long polysilane chains, conformational segmentation into individual chromophores is important for optical absorption, emission, and charge-transport properties. [17][18][19] The effect of backbone conformation on the electronic excitation energies of peralkylated oligosilanes is nicely illustrated by the stark contrastb etween the reported effects of chainlength extension in chainsk ept in an all-transoid conformation [t]a nd those held in an alternating cisoid,anti conformation [ca] ( Figure 1; in either measurement, the sense of chain helicity varies randomly along the chain). As shown in Figure2,a n extension of the SiSiSiSib ackbone from four to ten silicon atoms lowers the excitation energy of the first ss*e xcitation by 9000 cm À1 (232 to 293 nm) in the former case, [20] whereas in the latter,i tm akes no differencea ta ll, and the excitation energy remains steady at % 41 000 cm À1 ( % 243 nm).…”

Geometric localisation in Möbius π systems

“…The fact that the occupied molecular orbitals of small Mobius annulenes can be partitioned into those of a polyene and a twisted ethene fragment [10] is consistent with the dominance of r factors. The energetic cost of such twisting can be estimated by a modified version of the Sandorfy C model [26] taking explicitly into account 1,3-interactions [27,28].…”

The tunability of the electronic structures for poly(carbosilylsilanes): a theoretical study

“…Electronic structures of poly(alkysilanes) have been discussed extensively using molecular orbital theory, Sandorfy C/H model and ''ladder C'' model, respectively [2,38,39]. In them, each silicon atom on main chain has four sp 3 hybrid orbitals.…”