Inherent dissymmetry

The physical reason for the inherent lack of incentive for mixing in a polymer-polymer system is related to that already cited in explanation of the dissymmetry of the phase diagram for a polymer-solvent binary system. The entropy to be gained by intermixing of the polymer molecules is very small owing to the small numbers of molecules involved. Hence an almost trivial positive free energy of interaction suffices to counteract this small entropy of mixing. 

Online CD detectors are now commercially available for use with HPLC that are inherently more sensitive than corresponding OR detectors and not affected by solvent changes to the same extent and are thus more gradient compatible [121]. Provided Ae and the concentration of an analyte are known with good precision/accuracy, the measurement of CD will allow the determination of enantiomeric purity. In addition, with CD-based detection systems, both chiroptical and ordinary absorbance can be determined simultaneously allowing the measurement of the g-factor (or dissymmetry factor), which is defined as the ratio of the CD to the absorbance (AA/A) [122]. The g-factor is concentration independent and its measurement allows a more reliable determination of enantiomeric purity (without using a CSP) with reference to standards of known enantiomeric composition irrespective of their concentration [123]. A small number of recent literature examples have suggested the potential use of achiral HPLC with online CD detection for the determination of extreme enantiomeric ratios [121, 124-126] however, chiral separation techniques currently provide a more reliable measurement of enantiomeric purity. 

Extrinsic Cotton effects are due to the inherent dissymmetry of the enzyme-bound chromophore (an inherent effect) and/or to the interactions of the chromophore with the encompassing dissymmetric environment (interactive effects). The inherent effects are those which the free chromophore would exhibit if its conformation were identical with that of the enzyme-bound form. The interactive effects result from protein-ligand interactions or ligand-ligand interactions. The main problem in interpretation of die CD of enzyme-bound chromophores is distinguishing between the inherent and the interactive effects. 

Due to the inherent dissymmetry of monosubstituted triquinacenes, dimerization of such compounds will generate a pair of coupling products (meso and dl) if the monomer is racemic. Only when bonding between optically pure triquinacenes of the same chirality results can it be guaranteed that the formation of meso dimer will be precluded. In their study of the pinacolic reduction of <2/-2,3-dihydrotriquinacen-2-one, Paquette and co-workers showed that approximately equal amounts of the dl diol 458 and meso isomer 459 resulted.405 When enantiomerically pure (+)-380 was used, 458 was the exclusive reductive coupling product because of enforced enantio-... 

A chiral substance is defined by the International Union of Pure and Applied Chemistry (lUPAC) as one that interacts differently with left and right circularly polarized light. Two types of molecular optical activity are recognized inherent dissymmetry characterized by large rotational strengths and inherently symmetrical, but asymmetrically perturbed, molecules for which rotational strengths are less by a factor of a thousand or so. 

Multiple Cotton effects in the visible region were observed for the algal chromoprotein phycocyanin (265, 266). Interpretations included inherent dissymmetry of the tetrapyrrole chromophore (267), possibly induced by binding to the protein, interactions between chromophores of different monomeric units or between these chromophores and protein transitions (265, 266). The chemistry and mode of attachment of the conjugated molecules to the protein will have to be elucidated in more detail in order to enable a better evaluation of the origin of the optical activity observed. 

The design of viable, highly enantioselective epoxidation catalysts based on porphyrin ligands is confronted by the inherent difficulty associated with inducing dissymmetry from remote parts of an sp -hybridized coordination sphere, and the relative difficulties in constructing the chiral porphyrin rings. As research in this field has been the subject of an insightful review [90], only more recent developments will be covered here. 

All chemical bonds, except for symmetrically substituted ones, are inherently dissymmetric in two respects charge dissymmetry and hard-soft dissymmetry. The reversal of charge dissymmetry by modifying the structural unit (umpolung) has been well documented and has become one of the important principles in synthetic organic chemistry (16). On the other hand,... 

A more subtle way to generate a chiral calixarene is to create dissymmetry or asymmetry within the molecule itself One of the means for doing this with a calix[4]arene is to establish a substitution pattern of AABC or ABCD on the upper or lower rim, producing a compound that is inherently chiral in all possible conformations. Tabulations of the cone, partial cone, 1,2-alternate, and... 

The fourth, the so-called inherent dissymmetric chromophore, does not possess local symmetry. Therefore, the transitions belonging to these chromophores are magnetically and electrically allowed. Inherent dissymmetric chromophores are often found with so-called form chiral molecules for which atropisomers like binaphthols are typical representatives. Further examples are chromophores that come into being by exciton coupHng. In both cases the dissymmetry factor g is in the order of 10 to 10 and the CD is easily measurable in spite of the fact that the absorption coefficients of these compounds are often very high (e between 10" and 10 ). 

Polyamides with inherent dissymmetry and no asymmetric carbon. Although they do not fall formally in this paragraph, it seems convenient to report here data on polyamides which are chiral by atropoisomerism due to restricted chain rotation in the repeating unit such polymers may assume preferential asymmetric conformations in solution. This could be achieved by polycondensation of either optically active cyclic or biphenyldicarboxylic acids with acyclic and cyclic symmetrical diamines (Scheme XXVI). Thus, optically active (-)-(5 )-2,2 -dinitro-6,6 -dimethylbiphenyl-4,4 -dicarboxylic acids (XXVIa) were chosen as diacid components, a series of phenylene diamines (o, m, p) and cyclic diamines such as piperazine, trans-2,5-dimethylpiperazine and 1,2-pyrazolidine were used in order to main-... 

Here, the optical activity is due to an inherent dissymmetry by atropoisomerism, as is well knovm for low molecular weight compounds. This difficult kind of synthesis, mainly to obtain the monomer, has been only mentioned for polyamides [55—57] and also for modified polyvinylalcohol [181]. 

---