Molecular chirality structures

A molecule is chiral if it cannot be superimposed on its mirror image (or if it does not possess an alternating axis of symmetry) and would exhibit optical activity, i.e. lead to the rotation of the plane of polarization of polarized light. Lactic acid, which has the structure (2 mirror images) shown exhibits molecular chirality. In this the central carbon atom is said to be chiral but strictly it is the environment which is chiral. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

In chemoinformatics, chirality is taken into account by many structural representation schemes, in order that a specific enantiomer can be imambiguously specified. A challenging task is the automatic detection of chirality in a molecular structure, which was solved for the case of chiral atoms, but not for chirality arising from other stereogenic units. Beyond labeling, quantitative descriptors of molecular chirahty are required for the prediction of chiral properties such as biological activity or enantioselectivity in chemical reactions) from the molecular structure. These descriptors, and how chemoinformatics can be used to automatically detect, specify, and represent molecular chirality, are described in more detail in Chapter 8. 

Structural isomers have identical molecular formulas, but their atoms are linked to different neighbors. Geometrical isomers have the same molecular and structural formulas but different arrangements in space. Molecules with four different groups attached to a single carbon atom are chiral they are optical isomers. 

The study of the cholesteric mesophases obtained by doping thermotropic nematics with chiral, nonracemic compounds, has lead to relevant information about the stereochemistry of the dopants. Chiral interactions change the structure of the phase and therefore molecular chirality can be mapped onto an achiral (nematic) phase to yield a superstructural phase chirality. In 1984 Sol-ladie and Zimmermann published the first review summarizing the state of the art at that time.52 Later on, several review articles updated this subject.53-55... 

Although biaryl-based chiral molecules are the most extensively studied, also other molecular frameworks are associated to high twisting powers and thus suitable for stereochemical studies. One of the first chiral structures used in... 

Since the walls between heterochiral domains are unacceptable defects in an LC display, enantiomericafly enriched dopants are added to the LC to favor one sign of twist over the other in actual devices, providing a monodomain in the TN cell. It should be noted, however, that the chirality of the structure derives from the interaction of the LC director with the surfaces the molecular chirality serving simply to break the degeneracy between mirror image domains to favor one over the other. 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

IV. A BRIDGE BETWEEN CRYSTAL STRUCTURE, CRYSTAL MORPHOLOGY, AND MOLECULAR CHIRALITY... 

Finally, reference must be made to the important and interesting chiral crystal structures. There are two classes of symmetry elements those, such as inversion centers and mirror planes, that can interrelate. enantiomeric chiral molecules, and those, like rotation axes, that cannot. If the space group of the crystal is one that has only symmetry elements of the latter type, then the structure is a chiral one and all the constituent molecules are homochiral the dissymmetry of the molecules may be difficult to detect but, in principle, it is present. In general, if one enantiomer of a chiral compound is crystallized, it must form a chiral structure. A racemic mixture may crystallize as a racemic compound, or it may spontaneously resolve to give separate crystals of each enantiomer. The chemical consequences of an achiral substance crystallizing in a homochiral molecular assembly are perhaps the most intriguing of the stereochemical aspects of solid-state chemistry. 

A hypothetical doubly-bridged allene 123 of D2 symmetry was first formulated in Cahn, Ingold, and Prelog s classical paper 11 > in which they summarized their novel proposal for specifying various molecular chiralities. Despite the close structural resemblance between 123 and [m.m]betweenanene, Cahn, Ingold and Prelog s rules shows that the chirality of these two classes of compounds are to be specified as axial and planar respectively. 

Unsymmetrically disubstituted (and hence chiral) metallocenes (with point group C,) such as the ferrocene 122) provide a rather special problem they were first defined as planar chiral 19) and the specification of molecular chirality (descriptors Rp and Sp) applied accordingly19). Several authors still classify these structures as planar chiral21 23-24). 

Finally, it should be noted that other spectral analyses such as Raman, nuclear magnetic resonance, and fluorescence spectroscopies, which are correlated to more local chemical entities or individual atoms in the molecular systems, complement the CD data. Although other spectral approaches may provide more effective tools for analyzing a local structure or individual atoms in the molecular systems than the CD does, the CD approach is indispensable for the estimation of change or population in chiral structures on the average. 

There are two different kinds of sources of molecular chirality central chirality and axial chirality (Fig. 1). Central chirality is due to the existence of chiral carbon, whereas axial chirality originates from twisted structures of molecules, between which a sufficiently high energy barrier exists, preventing the chiral conformational interconversion in ambient conditions. Surprisingly, however, the introduction of nonchiral molecules to chiral liquid crystalline environments sometimes enhances the chirality of the systems [3-5]. This means that inherently nonchiral molecules act as chiral molecules in chiral environments. This occurs in the following way. Molecules with axial chirality behave as nonchiral molecules when the potential barrier is low enough for chiral conformational interconversion. But when such... 

The book provides systematic and detailed descriptions of the numerous approaches to chiral resolution. The first chapter is an introduction to basic concepts of molecular chirality and liquid chromatography. Chapters 2 through 9 discuss the chiral resolution of various classes of chiral stationary phases. Chapter 10 deals with chiral resolution using chiral mobile phase additives. These discussions elaborate the types, structures, and properties of the chiral phases,... 

---