Rigid aromatic structure

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

Jang et al. (2004) observed that ABA triblock copolymers composed of a docosyl chain, a rigid aromatic segment, and a flexible PEO dendrimer assemble into a hexagonal columnar or body-centered cubic structure in the solid state for the first- and second-generation dendrons, respectively (Fig. 11.27). 

Whereas a center or an axis of chirality can by clearly defined 2> 7) there is still some ambiguity in respect to the specification of a plane of chirality. It therefore seems somewhat difficult to define the scope of planar chiral compounds which at a first glance include mostly rather rigid aromatic compounds of special interest from synthetic, structural, spectroscopic and especially chiroptical points of view. 

Recent computer modeling clearly shows that for molecules such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), the methyl groups interfere with catalyst-molecule interactions as the sulfur atom adsorbs primarily through a one-point attachment and the dibenzothiophene ring system is nearly perpendicular to the catalyst surface. Hydrogenation of one ring of the 4,6-DMDBT causes the rigid planar aromatic structure to pucker and allow much better interaction between the sulfur atom and the catalyst surface (77). 

The benzopyrene has a rigid polycondensed aromatic structure, typical of... 

The GCM approach has been applied to the estimation of Tm for organic compounds containing functional groups with O, S, N, and halogen atoms [10], for rigid aromatic compounds [11], and for organic polymers with various possible substituents [12]. The latter method employs various corrections that account for special structural features in the polymer molecule. The first two methods are described below. 

Non therm oprocessihle Condensation Polyimides. These are obtained from condensation of aromatic dianhydrides with aromatic diamines. They are linear noncross-linked resins but their rigid chain structure and strongly hydrogen-bonded character leads to systems which do not melt or soften before decomposition. 

The several methods of energy dissipation, including fluorescence, are strongly dependent on the structure of the excited molecule. The existence of rigid planar aromatic structures is usually favourable to fluorescence. The size of the aromatic system also directly affects the fluorescence intensity and the excitation and emission wavelengths. For the series benzene, naphthalene, anthracene, tetracene and pentacene the emission and excitation wavelengths increase from SOS to S80 nm and from 278 to 640 nm, respectively [28]. 

The methods used for complete structure determination cannot be reviewed in any detail here. It is well known that the central problem is that of determining the relative phase relations of the diffracted waves. As only the intensities can be observed, the problem may be approached by trial and error calculations until a good measure of agreement with the observed values is obtained. This method has in fact been used for a great many of the aromatic structures dealt with in this article. The rigid molecular frameworks with at most only a few degrees of freedom lend themselves to such a treatment. A more systematic approach along the same lines is possible by the method of Fourier transforms (Lipson and Taylor, 1958). 

Most useful polyesters have need for the strong, rigid aromatic ring in their structure since they lack the hydrogen bonding prevalent in polyamides. 

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