Chirality of molecules

The relationship between a chiral object and its mirror-related object is called enantiomerism. A knowledge of the existence of enantiomers was one of the reasons that van t Hoff and Le Bel proposed, as described in Chapter 1, that the four valences of carbon are spatially directed to the corners of a regular tetrahedron.The only difference between a pair of enantiomers is that, if one can be described as a left-handed form, the other will be a right-handed form they have identical chemical formulae. The major physical property that allows one to distinguish between enantiomers is the direction in which they, or their solutions, rotate the plane of polarized light, that is, when they are studied in the chiral environment provided by the polarized light. It is important to note that a molecule is not necessarily chiral just because it contains an asymmetric center, or that it is necessarily achiral because it lacks such an asymmetric center. These are not the criteria for molecular chirality. The test for chirality in a molecule is the nonsuperimposability of the object on its mirror image. 

Many compounds contain more than one chiral center. If a molecule contains n chiral centers, there will be a maximum of 2 stereoisomers. If there is only one chiral center, there are two stereoisomers (enantiomers), while if there are two asymmetric carbon atoms, then there will be four isomers (two pairs of mirror-image-related molecules). While two chiral centers lead, theoretically, to four stereoisomers, these stereoisomers may not all be different. Some pairs may be identical, containing mirror-image symmetry because the two chiral centers are 

In this Chapter we will describe chiral molecules and their representations, then ways of determining absolute structure, and finally extensions of the experimental methods used. 

FIGURE 14.2. The stereoisomers of tartaric acid molecules (a) and (b) are enantiomers, related by miror symmetry. Molecules (c) and (d) are equivalent, termed mesocompounds, because they have two identical chiral centers. Thus there are onl three stereoisomers [(a), (b), and (c) = (d)]. 

FIGURE 14.3. Comparison of (a) enantiomers that are mirror images of each other, and (b) diastereomers, which are not. 

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Molecules which exhibit optical activity are molecules which have a handedness in their structure. They are chiral . Chemists often have reasons to obtain chemical pure aliquots of particular molecules. Since the chirality of molecules can influence biological effect in pharmaceuticals, the chiral purity of a drug substance can pose a challenge both in terms of obtaining the molecules and in assaying the chiral purity by instrumental methods. While diastereomers can have different physical properties including solubility, enantiomers have the same physical properties and the same chemical composition. How then to separate optically active molecules ... 

It is evident that two hydrogen bonds between adjacent molecules and the chirality of molecule contribute to one dimensional molecular alignment in spite of the strong dipole-dipole interactions and that -conjugated system of DAD molecule extend from amino groups to cyclobutenedione ring enhance second order nonlinearity of DAD molecular crystal. 

The principal goal is to define those factors which lead to the macroscopic chirality of the dendrimer. Despite numerous studies on this topic, the relation between the molecular chirality of the dendritic building blocks and the macroscopic chirality of molecules has still not been completely elucidated [12]. Yet an understanding of this relation is important for the development of new materials, including polymers, whose properties and function depend upon their macroscopic chirality [13]. 

Optical chirality of molecules is a characteristic measure of stereo-chemical property of biological, pharmaceutical, and metal coordination compounds. Choral structures of amino acids, proteins, DNAs, and various drugs in solutions have been determined from the measurement of circular dichroism (CD). However, small amount of molecules at the liquid-liquid interfaces has never been measured before CLM/CD method [19] and SHG/CD method have been reported [20],... 

Virtually everything that exists or happens in real space has a corresponding property or effect in diffraction space, and vice versa. The correspondences are established through the Fourier transform, which, as we have seen, operates symmetrically in both directions, getting us from real space into reciprocal space and back again. It may occasionally appear that this rule is violated, but in fact it is not. For example, the chirality of molecules and the handedness of their arrangement in real space would seem to be lost in reciprocal space as a consequence of Friedel s law and the addition of a center of symmetry to reciprocal space. If, however, we could record phases of reflections in reciprocal space, we would see that in fact chirality is preserved in phase differences between otherwise equivalent reflections. The phases of Fhu, for example, are 0, but the phase of F-h-k-i are —0. Fortunately the apparent loss of chiral information is usually not a serious problem in the X-ray analysis of proteins, as it can usually be recovered at some point by consideration of real space stereochemistry. 

Not only rigidness can be important for chirality. Indeed, the definition pass over observation time scale, physical conditions (temperature, pressure), state of aggregation, solvation, isotopic composition etc. which are, for obvious reasons, important for chirality of molecules. The time scale is especially crucial for spectroscopic observations of non-rigid chiral molecules, because, the effects observed. 

Note that (pro) -chirality is a property of the entire molecule, not just a particular atom within the molecule. Consider the example of triptycene. 111, in Figure 2.46. The structure can be desymmetrized by several pathways, one of which will produce a chiral structure in only one step. However, the desymmetrization can be accomplished in at most three steps if each step involves a different symmetry element, so it is said to be (prop-chiral. Table 2.3 lists the (prop-chirality of molecules as a function of their symmetry. ... 

Non-chiral nanoparticles were also of interest for sensing of optical activity through the enhancement of the chirality of molecules in the presence of the plasmonic field at the edge of, for example nanocubes or gold structures on surfaces. ... 

The chirality of molecules can be demonstrated with relatively simple compounds. Consider, for example, 2-butanol ... 

The chirality of molecules breaks the mirror symmetry C2h of the achiral smectic C phase. The only symmetry element left is a twofold rotation axis C2, and the point symmetry group becomes C2 instead of C2h- The structure of a single smectic C layer is shown in Fig. 4.34. As in achiral smectic C, the molecules in the layer obey head-to-tail symmetry, the director n coincides with average orientation of molecular axes and form angle 1 with the smectic normal h. 

Year 1975 has been marked off by an outstanding publication of R. Meyer and his French co-workers [4]. As has been discussed in Section 4.9, chirality of molecules removes the mirror symmetry of any phase. The idea of Meyer was to apply this principle to the SmC phase by making it chiral. He believed that if chiral molecules formed a tilted smectic phase, its point group symmetry would reduce from to C2... 

How Do We Describe the Chirality of Molecules with Three or More Stereocenters ... 

The necessary conditions for its existence (P 0) are a finite tilt angle 6 0, chirality of molecules, resulting in the hindered rotation of molecules... 

Objects like atoms, molecules, and ensembles of molecules are chiral. Thus, properties connected to the chirality of an object may have very different origins. Therefore, it makes sense to introduce a concept in which four levels of chirality exist. The first level of chirality is the chirality of the atoms [30] caused by weak interaction which is of no interest for the discussion of liquid crystal properties. The second level is the chirality of molecules, while the third level is derived from the ordering of atoms, ions, or molecules in isotropic or anisotropic phases by long-range positional and long-range orientational order. The fourth level of chirality is the form of a macroscopic object which can be, e.g., an enantiomorphic crystalline form (habitus of the crystal). 

Cholesteric liquid crystals (CLCs) show very distinctly that molecular structure and external fields have a profound effect on cooperative behavior and phase structure (see also Chapters 2 and 3). CLCs possess a supermolecular periodic helical structure due to the chirality of molecules. The spatial periodicity (helical pitch) of cholesterics can be of the same order of magnitude as the wavelength of visible light. If so, a visible Bragg reflection occurs. On the other hand, the helix pitch is very sensitive to the influence of external conditions. A combination of these properties leads to the unique optical properties of cholesterics which are of both scientific and practical interest. 

Main quantities characterizing the properties of FLCs (the values of the tilt angle d, the spontaneous polarization P, the dielectric constant , the critical frequencies for particular collective modes) depend on intermolecular interactions caused by the chirality of molecules. " Thus the ferroelectricity of LCs must be sensitive to the intermolecular distance it must then be pressure dependent. The pressure studies of FLCs have been undertaken in a few labora-tories. - ... 

The analysis of the CPL spectra constitutes a straightforward method for the study of the chirality of molecules in their luminescent excited states. By means of comparative CD/CPL measurements one can investigate the geometrical differences between the ground and excited states. The observation of CPL has the problems and limitations already described in the previous sections. In particular, the molecular or supramolecular species must contain a luminophore exhibiting a sufficiently high emission quantum yield. CPL spectroscopy, however, has a number of advantages in terms of specificity and selectivity that can be extremely useful in supramolecular chemistry, namely ... 

Chirality of molecules is an important aspect of their structure in all fields, no less among insect substances. Many pheromones and hormones are chiral, and the behavioural response they produce may vary greatly with the chiral form. For example, the unnatural enantiomer of a pheromone can inhibit completely the response to the natural enantiomer, the natural pheromone may be a blend of unequal proportions of... 

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