Chiral charge

Chiral Charge Density Waves in Quasi One-Dimensional Organic Conductors 303... 

CHIRAL CHARGE DENSITY WAVES IN QUASI ONEDIMENSIONAL ORGANIC CONDUCTORS... 

We discuss the interaction of a partially filled electronic conduction band in a segregated donor-acceptor stack system with libra-tional modes of the solid. The orientational Peierls instability predicted by us earlier leads to the formation of chiral charge density waves, which interact and phase-lock below the metal-insulator transition via the Coulomb interaction. The effect of the resulting order on the physical properties of the system and the implications for the understanding of the recent neutron scattering data for the occurrence of several transitions in TTF-TCNQ will be discussed. 

We propose in this paper that the observed Peierls instability in TTF-TCNQ and the richness of structural transitions arises from the interaction of the tt electron and hole system on separate TCNQ and TTF chains, respectively, with orientational modes (librons) of the solid. We suggest, that the chiral charge density waves (CCDW), which result from the orientational distortion of the TCNQ and TTF stacks account for the observed continuous increase in the unit cell dimension from a = 2a at 54° to a = 4a at 38°K. 

We consider the Coulomb potential due to a helical charge distribution at some distance d larger than the molecular dimensions of the assumed chiral charge distribution (see Fig. 2)... 

By incorporating into the stationary phase a chiral charge transfer reagent also capable of hydrogen bonding, such as 2,2,2-trifluoro-l-[(10-methyl)-9-anthryl] ethanol, Pirkle and coworkers [18] have resolved substances which either contain a CT complexing moiety in their molecule, e.g. aromatic sulphoxides and lactones, or possess a function through which such a moiety can be introduced, e.g. alcohols, mercaptans, amines, amino alcohols, amino acids, hydroxy acids and esters. 

Internally, GEMINI uses a rich set of data types with well-defined relationships which can include redundancy or ambiguity. Such information includes atomic number, connectivity, bond order, chirality, charge, co-ordinates, etc. Fortunately, there are very few different representations that are used for each kind of information, and algorithms are readily available for their interconversion. 

The chirality code of a molecule is based on atomic properties and on the 3D structure. Examples of atomic properties arc partial atomic charges and polarizabilities, which are easily accessible by fast empirical methods contained in the PETRA package. Other atomic properties, calculated by other methods, can in principle be used. It is convenient, however, if the chosen atomic property discriminates as much as possible between non-equivalent atoms. 3D molecular structures are easily generated by the GORINA software package (see Section 2.13), but other sources of 3D structures can be used as well. 

Appllca.tlons. MCA is used for the resolution of many classes of chiral dmgs. Polar compounds such as amines, amides, imides, esters, and ketones can be resolved (34). A phenyl or a cycloalkyl group near the chiral center seems to improve chiral selectivity. Nonpolar racemates have also been resolved, but charged or dissociating compounds are not retained on MCA. Mobile phases used with MCA columns include ethanol and methanol. 

The 260 nm band of chiral thiiranes is optically active and a Cotton effect is observed R) (+)-methylthiirane shows a negative Cotton effect at ca. 250 nm followed by a positive effect below 200 nm. An MO analysis indicates that charge transfer contributions are most important in determining the optical activity of the transition (81JCS(F2)503). The... 

Keller, H. J., and Soos,-Z. G. Solid Charge-Transfer Complexes of Phenazines. 127, 169-216 (1985). Kellogg, R. M. Bioorganic Modelling — Stereoselective Reactions with Chiral Neutral Ligand Complexes as Model Systems for Enzyme Catalysis. 101, 111-145 (1982). 

In principle this is the method that gives rise to the strongest support-complex interaction. We have considered in this category all the methods in which the support compensates for at least one of the charges of the complex, usually due to the metal, although without considering the exact nature of the metal-support bond, i.e., purely ionic or polarized covalent. In any case, the only possible covalent bond between support and complex would be estabhshed with the metal center, not with the chiral hgand. 

Fig. 14 Chiral salen ligands with charged substituents...

In general the metal complexes are charged. It is thus possible to convert the racemic mixture of such a complex into a pair of diastereoisomeric species with different physico-chemical properties, in particular solubihty, by association with an enantiomerically pure chiral coimterion [19]. Examples of frequently used such ions are shown in Fig. 3. Then the separation can be achieved by ... 

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