Cyclic pentamer

GPC traces of poly(phenylmethylsilylenes) prepared in the ultrasonication bath are shown in Fig. 1. In contrast to thermal condensation, monomodal high molecular weight polymer is formed. Oligomeric cycles (mostly cyclic pentamer), formed usually in high yield (cf. Table 1), can be very easily separated from the reaction mixture by precipitation with isopropanol. The molecular weight of polysilanes decreases and polydispersity increases with temperature. 

Kipping experiment of the coupling of diphenyldichlorosilane had to be very low because in addition to cyclic tetramer, large quantities of cyclic pentamer and hexamer were formed. Only very recently was existance of polysilanes with two aryl substituents proved by Miller who prepared... 

Figure 5.23 Model (HF) clusters (n = 3-5), with calculated net binding energies in parentheses. Only the cyclic pentamer (d) is a true equilibrium structure (see the text).

Table 5.19. Energies ofsingle monomers (/f mon) and dimer pairs (/s pair) leading to evaluation ofV(pair), VPW, and %npw (Eqs. (5.51) and (5.52)) for the open trimer and cyclic pentamer o/ HF (note [see note 61] that, for these comparisons, V(pair), Vtrue, and Vpw are all expressed with respect to the energies of monomers frozen in the geometry of the complex)...

The actual natural charge in the cyclic pentamer is qnat = 0.570, which makes the Coulombic point-charge estimate entirely unrealistic.) Thus, no matter how q is chosen, a simple Coulombic point-charge model will give >10% errors for one or the other of these clusters. 

Table 5.20. Cooperative (n-dependent) properties of linear formamide chains (see Figs. 5.19 and 5.27), showing the incremental binding energy AEn, average H-bonded Ron and Rmi distances, and partial charges Q and Qn on terminal monomers of the (H2NCHO) chain (for comparison, the cyclic pentamer in Fig. 5.27(c) has Ro u = 1.879 A, Rm = 1-024 A, and average H-bond energy 7.92 heal mol x)...

Two of the water clusters that appear to exhibit particular thermodynamic stability under ambient conditions of the liquid phase are shown in Fig. 5.29 a cyclic pentamer (Wsc) and a cyclic hexamer (W6C). The characteristic factors (F1)-(F3) that distinguish these clusters from other possible isomeric forms can be identified as follows ... 

On enthalpic grounds alone, under ambient conditions the Boltzmann factor for the chain pentamer would be reduced by about 1010 relative to that for the cyclic pentamer, showing that the former topology contributes negligibly to the equilibrium population distribution. 

Most of the reactions that can be used to prepare distannanes can be extended to the preparation of oligo- and polystannanes, which have attracted interest for potential use in electronic and optical devices.455 The structures of the products are not necessarily those of completely linear catenanes, (R2Sn) , and different degrees of branching confer different properties. The formation of polymers is also frequently accompanied by the formation of cyclic pentamers or hexamers, and samples prepared by different methods may show substantially different properties. 

We first confirmed the formation of these macrocycles in the polymerization of THF by using coupled gas chromatography/mass spectrometry ( 2). Macrocyclic ethers containing up to 8 THF units could be separated and identified by this method (23). The two predominant macrocyclic species found in THF polymerization mixtures are a cyclic tetramer and a cyclic pentamer. In analogy to the "crown ether" nomenclature, we proposed the name 20-crown-4 for the cyclic tetramer and 25-crown-5 for the cyclic pentamer (22). 

Evaporation of the xylene filtrate and trituration of the residue with methylene chloride yields ca. 11 g of solid which consists mainly of cyclic hexamer and cyclic tetramer. The methylene chloride contains inter alia, cyclic pentamer, cyclic heptamer, and bishomo compound, and these can be obtained as pure samples in low yield by fractional crystallization procedures. The compositon of the reaction mixtures can be qualitatively established by TLC by means of the following values in 9 1 petroleum... 

The formation of individual cycloborazanes can be achieved, in some cases, by transition metal-catalysed dehydrocoupling reactions. By using this strategy, secondary amine-boranes are converted to the four-membered ring (H2BNMe2)2 under mild conditions, whereas primary amine-boranes produce borazines. The cyclic pentamer is obtained as the exclusive product from ammonia-borane adduct H3B NH3 employing an iridium(fll) catalyst [see eqn (2.13) in Section 2.3)]. 

The most intriguing difference between the chemical properties of cyclopolysilanes and those of cycloalkanes is the ability of the former to form either anion or cation radicals upon one-electron reduction or oxidation, respectively. For example, the cyclic pentamer (Mc2Si)5 is reduced to the corresponding radical anion by sodium-potassium alloy in diethyl ether [see eqn (4.1) in Section 4.1.3], whereas the hexamer (Me2Si)6 is oxidised by aluminium trichloride in dichlor-omethane to the corresponding cation radical. In both cases the EPR spectra of the radical ions can be interpreted in terms of a-electron delocalisation over the entire polysilane ring (see Section 10.1.4.1). In this respect, the cyclosilanes resemble aromatic hydrocarbons rather than their aliphatic analogues. 

Computer simulation studies by Stillinger and Rahman (1974) suggest that the pentamer is the most likely structure to spontaneously arise in water at many temperatures, followed in frequency by hexamers, and squares. In a review of water, Frank (1970) noted that closed rings of bonds are always more stable than the most stable open chains of the same cluster number, due to the extra energy of the hydrogen bond. Through molecular dynamics studies of many five-molecule clusters, Plummer and Chen (1987) argued that the cyclic pentamer that comprises many hydrate cavities is the only stable five-member cluster above 230 K. 

The first [5] super-cyclodextrin whose nano-sized cyclo-pentameric array is held only by a mechanical bond was synthesized by the pentakis-azo coupling of a new hermaphrodite monomer with 2-naphthol as a stopper, isolated by chromatography, and characterized by MS, 2D NMR, and visible spectral methods with the help of computer simulation [60], Cyclic pentamer (as red film, 15% yield) accompanied with the corresponding monomer (20%) (Figure 22) was obtained. 

For the large-scale synthesis of 125, the crude reaction product was a mixture of cyclic hexamer and cyclic pentamer (9 1) and the compounds were separated by column chromatography. 

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