Chiral indicators

Three-dimensional (3-D) descriptors of molecules quantify their shape, size, and other structural characteristics which arise out of the 3-D disposition and orientation of atoms and functional groups of molecules in space. A special class of 3-D indices is quantitative descriptors of chirality. If a molecule has one or more chiral centers, the spatial disposition of atoms can produce enantiomers, many of which will have the same magnitude of calculated and experimental physicochemical properties having, at the same time, distinct bioactivity profiles. Basak and coworkers [22] have developed quantitative chirality indices to discriminate such isomers according to their structural invariants which are based on the Cahn-Ingold-Prelog (CIP) rules. 

Relative chirality indices (vRCl) for a pair of enantiomers are calculated as... 

In addition to the spectroscopic investigations, there have been attempts to obtain structural and stereochemical information about radicals by chemical means.25 The approach generally taken is to generate radicals by one of the methods discussed in the next section at a carbon where stereochemistry can be determined. As an example, we may cite the experiment shown in Equation 9.6, in which an optically active aldehyde is heated in the presence of a source of radicals.26 The reaction follows the chain pathway indicated in Scheme 1 the loss of chirality indicates that the radical is either planar or, if pyramidal, undergoes inversion rapidly with respect to the rate (on the order of 10s sec-1) at which it abstracts a hydrogen atom from another molecule of aldehyde. 

Two synonyms, two definitions, and an expression of admiration. The word norepinephrine is synonymous with noradrenalin, and the word epinephrine is synonymous with adrenalin. The distinctions are that the first in each case is American and the second British. And the term chiral indicates a potential asymmetry in a molecule that would allow eventual separation into two optical isomers. The term racemic refers to a mixture of these two isomers which has not yet been separated into the individual components. A racemic mixture is called a racemate and, from the point of view of the human animal (which is completely asymmetric), must be considered as a mixture of two structurally identical but... 

Figure 3. Calculated adsorption energies ads of a single hydrogen atom on the zigzag BNNTs (n,0) with n ranging from 5 to 10 versus the radius R of the tube. The heavy dots and triangles refer to the adsorption on the B atom and the N atom, respectively.The numbers in parenthesis stand for the tube chirality indices.The solid and dashed lines are intended as a guide to the eye.

Double-walled carbon nanotubes (DWNTs), first observed in 1996, constitute a unique family of carbon nanotubes (CNTs). -2 DWNTs occupy a position between the single-walled carbon nanotubes (SWNTs) and the multiwalled carbon nanotubes (MWNTs), as they consist of two concentric cylinders of rolled graphene. DWNTs possess useful electrical and mechanical properties with potential applications. Thus, DWNTs and SWNTs have similar threshold voltages in field electron emission, but the DWNTs exhibit longer lifetimes.3 Unlike SWNTs, which get modified structurally and electronically upon functionalization, chemical functionalization of DWNTs surfaces would lead to novel carbon nanotube materials where the inner tubes are intact. The stability of DWNTs is controlled by the spacing of the inner and outer layers but not by the chirality of the tubes 4 therefore, one obtains a mixture of DWNTs with varying diameters and chirality indices of the inner and outer tubes. DWNTs have been prepared by several techniques, such as arc discharge5 and chemical vapor depo-... 

Chirality indices indicated residual chirality in the solvent as a consequence of the presence of a chiral solute. Wang and Cann proposed the application of such indices for the prediction of the solvent contribution in optical activity [294],... 

The CD spectral investigation of optically active bi[10]paracyclophanes (47), in which two planar and one axial chirality elements are incorporated, was reported in the literature [57]. The experimental CD spectrum of enantiomerically pure (RV,SP)-47, which was derived from the corresponding diastereomeric tetra-(S )-camphanoate, was found to be in good agreement with that calculated for the (aA)-isomer, but approximately the mirror image of the spectrum computed for the (aSHsomer (Fig. 11). Thus, the CD spectrum can be interpreted mostly in terms of the axial chirality, indicating that the effects from the planar chirality of cyclophane units were more or less cancelled out. 

A word of caution should be added with respect to Chemical Abstracts, as far as chirality assignments of homoannular substituted ferrocene derivatives are concerned. Until the 8th collective index, only (-f) and (—) are found as chirality indicators. For quite a long time, no descriptors were given at all, only the remark stereoisomer , followed by the registry number, which does not allow identification of a compound easily. This fact is in sharp contrast to the claims of Chemical Abstracts Service authors that they would consequently use Schlogl s central descriptors [20, 21]. Since volume 114, the (R, S ) nomenclature for ferrocene derivatives begins to appear, but its application is not very consequent, at least at the time where the book was written, and it is advisible to examine the orginal article rather than trust Chemical Abstract s descriptors. 

For achiral molecules, all the chirality indices (and the corresponding vector elements) equal zero. 

Chiavaro, E., Caligiani, A., and Palla, G. Chiral indicators of ageing in balsamic vinegars of Modena, Ital. J. Food Sci., 10, 329,1998. 

Relative chirality indices and chirality correction factors forthe two enantiomers shown below, together with valence vertex degrees and first-order valence connectivity index of the functional groups. 

In the example presented below, the unique element that should be changed to calculate the chiral indices is the element (2,1) in all the three matrices, that is, 1.73. 

The methyl ester of acid 76 was adsorbed onto NaY zeolite, in which optically pure inductors like ephedrine and diethyl tartrate had been preloaded. In this instance, however, the results of photoreaction were disappointing [84]. Diethyl tartrate proved to be the best chiral inductor (12% ee) all the rest gave ee values of 5% or less. Photoisomerization of the methyl ester of the diphenylben-zoyl acid 78 using zeolites modified with chiral indicators also gave low ee values [85]. Norephedrine was found to be the best chiral indicator, resulting in 20% ee. 

Samples of (3R)- and (3S)-[3- Hi]tryptophans 331 have been converted to the corresponding kynurenines 336 using the enzymes tryptophan dioxygenase (EC 1.13.11.11) and formylkynurenine formamidase (EC 3.5.1.9). These have been used to investigate the fission of kynurenine 336 to anthranilic acid 300 and alanine 337 in H20 (337) (Scheme 85). Conversion of the alanine to acetate and assessment of sense of chirality indicated that... 

Show that the diameter of an (n, m) SWNT is related to the chiral indices by the expression ... 

The electrical properties of CNTs are related to the electronic properties of graphene, since a nanotube can be considered a rolled sheet of graphene (Figure 10.9). Therefore, depending on the CNT chirality, several options are possible related to the specific chiral indices n,m) of the chirality vector C = + mz.2 of hexagonal lattice metallic behavior (m-CNT), when n = m ... 

Electronic structure of zigzag carbon nanotubes is characterized by chiral indices ( , 0) and described by the well-known dispersion relation which obtained in the fiamewoik of Huckel jt-electron approximation [3] ... 

Many of the properties of nanotubes (e.g., zonal structure or space group of symmetry) strongly depend on the value of the ehiral vector. Chirality indicates what property has a nanotube - a semieonductor or metallicheskm. For example, a nanotube (10, 10) in the elementary cell contains 40 atoms and is the type of metal, whereas the nanotube (10, 9) has already in 1084 and is a semiconductor (Fig. A. 13). 

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