Carbosilanes Conformations

In contrast to solid dendrimers [71, 84, 88], molecular resolution was not achieved for a closed film of the carbosilane dendrimers. However, one could extract structural information about the molecular conformation from the film thickness and wetting edges on a solid substrate and from surface pressure/area... 

Complementary information about the interfacial conformation of the carbosilane dendrimers was obtained from u-A isotherms measured during compression of molecular films at the air/water interface [74]. The full reversibility of the isotherm in Fig. 13 a indicated that the experiment was done under equilibrium conditions, and that the data give direct evidence on the phase behavior. Two transitions, marked by I and II in Fig. 13 a, were observed for the OH terminated carbosilane dendrimer (dendrimer 2 in Fig. 8). The first transition (I) was... 

The interpretation of the Langmuir experiments with the carbosilane den-drimers is supported by the results of molecular dynamics simulation. Figure 14 shows snapshots of a dumbbell-like conformation of carbosilane dendximers observed during lateral compression of a dendrimer monolayer on a polar sub-... 

P. R. Sundararajan, Macromolecules, 23, 3179 (1990). Conformational Features of the Carbosilane Polysilapropylene and Comparison with Propylene. 

Ab initio calculations of rotational barriers of single bonds between elements of group 14 predict a linear relationship between the bond lengths and the barrier, for instance for the series H3SiXH3 with X = C, Si, Ge, Sn and Pb [1]. The barriers decrease from about 6.7kJ/mol for Si-C to about 3.8 for Si-Si and 1.7 kJ/mol for Si-Pb bonds. For the spectroscopist interested in molecular conformations, barriers that lie well above RT at room temperature (2.48 kJ/mol) support the expectation that the interconversion of rotamers will be slow on the time scale typical for Raman vibrational spectroscopy (= lo sec) and that the rotamers can be distinguished by their individual vibrational spectra. It is our objective to compare conformational stabilities of carbosilanes (Si-C bonds) with those of disilanes (Si-Si bonds) bearing identical substituents. Fore this purpose, we have prepared the title compounds 1-4 and investigated their conformational compositions by variable temperature Raman spectroscopy. 

Associated with the determination of the exact molecular structures of the carbosilanes was the question of the chemical behavior of the compounds, which in turn is a question of the influence of the substituents on conformation and bond behavior. The most important results of these studies are now presented. 

Carbosilanes that have been clarified structurally thus far are of two classes either carborundanes (Si—C six-membered rings existing in the chair conformation) or scaphanes (Si—C six-membered rings in the boat conformation). Compound represents a further type of carbosilane, namely, one in which the six-membered rings exist in both the chair and boat conformations. The results of the X-ray crystal structure analysis are presented in Fig. 27 and Table 71. 

Totally unexpected is the finding that the conformation of the six-membered rings in the hexasilapropellane 405 is practically the same as that in octasiladodeca-scaphane and in tetrasilabarrelane 198. These are two basic examples for polycyclic six-membered ring systems in the boat conformation. The twist conformation is practically identical in all three molecules. This creates the impression that this particular conformation is typical for boat-shaped carbosilane six-membered rings. 

Most cyclic carbosilanes can be classified in two groups the carborundanes and the Si-scaphanes. Compounds belonging to the carborundane class maintain Si—C six-membered rings in the boat conformation. 

Boiko N, Zhu X, Vinokur R, Rebrov E, Muzafarov A, Shibaev V (2000) New carbosilane ferroelectric liquid crystalline dendrimers. Mol Cryst Liq Cryst 352 342—350 Botiz I, Stingelin N (2014) Influence of molecular conformations and microstructure rat the optoelectronic properties of conjugated polymers. Materials 7 2273-2300 Bumiing TJ, Natarajan LV, Tondiglia VP, Sutherland RL (2000) Holographic polymer dispersed liquid crystals (HPDLCs). Armu Rev Mater Sci 30 83-115 Busch K, John S (1999) Liquid-crystal photonic-band-gap materials the tunable electromagnetic vacuum. Phys Rev Lett 83 967-970... 

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