Chiral molecules chirality

In most common chiral molecules, chirality arises from chiral tetravalent atoms. A conformation-independent chirality code (CICC) was developed that encodes the molecular chirality originating from a chiral tetravalent atom [42], For more generality, a conformation-dependent chirality code (CDCC) is used [43]. CDCC ti cats a molecule as a rigid set of points (atoms) linked by bonds, and it accounts for chirality generated by chirality centers, chirality axes, or chirality planes. 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

Metalaxyl and most of its active analogues are chiral molecules. Chirality is caused by the asymmetric carbon atom in the alkyl side chain of the alanine moiety. The two optically pure enantiomers S (+) and R (-) differ widely in their biological activity both in vitro and in vivo. In all experiments, the R (-) enantiomer was more active than its antipode S (+) (22, 24, 30). The main characteristics of metalaxyl have been discussed in detail by several authors (J, 21, 28, 29, 32> 38). Of particular value is the rapid uptake of metalaxyl by the plant tissue, especially under the wet conditions that favor foliar Oomycete diseases. Acylalanines are easily translocated in the vascular system of the plant after foliar, stem or root treatment (35, 47). The predominant route of transport is the transpiration stream, thus apoplastic (12, 35). Symplastic transport occurs but is much less evident (35, 47). In potatoes treated by foliar sprays of metalaxyl concentrations (0.02-0.04 ppm), Bruin et al. (SO were able to demonstrate protection of harvested tubers from late blight. 

Polavarapu PL (2008) Why is it important to simultaneously use more than one chiroptical spectroscopic method for determinating the structures of chiral molecules Chirality 20 664-672... 

Chiral crystal from chiral molecule Chiral molecule Moderate-high diastereodifferentiation... 

Molecules satisfying the requirements for optical activity are also known as chiral molecules. Chirality arises in pyramidal and tetrahedral phosphorus compounds when the substituent groups are all different (Chapter 3.2). It may also arise in trigonal bipyramidal and octahedral compounds when at least three different kinds of substituent molecules are present. 

Cholesteric liquid crystal phase is obtained when a nematic phase is doped with chiral molecules. Chiral molecules are optically active and are known to show optical rotary dispersion in the order of l7cm. However in the cholesteric phase they induce rotation of the long axes of the liquid crystal molecules (the director n) about a helix as shown in Figure 1. 

Polavarapu PL. Why is it Important to Simultaneously Use More Than One Chiroptical Spectroscopic Method for Determining the Structures of Chiral Molecules Chirality 2008 20 664-672. 

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. 

The R, S convention is a scheme which has largely superseded the D, i. system to denote configuration about a chiral centre in a molecule. The convention allows unequivocal designation of the absolute configuration in a description of the positions in space of ligands attached to a chiral centre, in relation to an agreed standard of chirality like a right-hand helix. 

Marquardt R and Quack M 1996 Radiative excitation of the harmonic oscillator with applications to stereomutation in chiral molecules Z. Rhys. D 36 229-37... 

Quack M 1989 Structure and dynamics of chiral molecules Angew. Chem. Int. Ed. Engl. 28 571-86... 

Koroteev N I 1995 BioOARS—a novel nonlinear optical technique to study vibrational spectra of chiral biological molecules in solution Biospectroscopy 1 341-50... 

An important distinction among surfaces and interfaces is whether or not they exliibit mirror synnnetry about a plane nonnal to the surface. This synnnetry is particularly relevant for the case of isotropic surfaces (co-synnnetry), i.e. ones that are equivalent in every azunuthal direction. Those surfaces that fail to exliibit mirror synnnetry may be tenned chiral surfaces. They would be expected, for example, at the boundary of a liquid comprised of chiral molecules. Magnetized surfaces of isotropic media may also exliibit this synnnetry. (For a review of SFIG studies of chiral interfaces, the reader is referred to [68]. ... 

Given the interest and importance of chiral molecules, there has been considerable activity in investigating die corresponding chiral surfaces [, and 70]. From the point of view of perfomiing surface and interface spectroscopy with nonlinear optics, we must first examhie the nonlinear response of tlie bulk liquid. Clearly, a chiral liquid lacks inversion synnnetry. As such, it may be expected to have a strong (dipole-allowed) second-order nonlinear response. This is indeed true in the general case of SFG [71]. For SHG, however, the pemiutation synnnetry for the last two indices of the nonlinear susceptibility tensor combined with the... 

A schematic diagram of the surface of a liquid of non-chiral (a) and chiral molecules (b) is shown in figure Bl.5.8. Case (a) corresponds to oom-synnnetry (isotropic with a mirror plane) and case (b) to oo-symmetry (isotropic). For the crj/ -synnnetry, the SH signal for the polarization configurations of s-m/s-out and p-m/s-out vanish. From table Bl.5.1. we find, however, that for the co-synnnetry, an extra independent nonlinear susceptibility element, is present for SHG. Because of this extra element, the SH signal for... 

Figure Bl.5.13 Spectra of the various non-chiral [p-in/p-oiit (filled circles) and s-in/p-oiit (filled diamonds)] and chiral [p-in/s-oiit (triangle)] SHG signals of (R)-BN molecules adsorbed at the air/water interface. (From [80].)...

With relatively simple spectra, it is usually possible to extract the individual coupling constants by inspection, and to pair them by size in order to discover what atoms they coimect. However, the spectra of larger molecules present more of a challenge. The multiplets may overlap or be obscured by the presence of several unequal but similarly sized couplings. Also, if any chiral centres are present, then the two hydrogens in a... 

Related results of promotion (catalysis) and inliibition of stereonuitation by vibrational excitation have also been obtained for the much larger molecule, aniline-NHD (CgH NHD), which shows short-time chirality and stereonuitation [104. 105]. This kind of study opens the way to a new look at kinetics, which shows coherent and mode-selective dynamics, even in the absence of coherent external fields. The possibility of enforcing coherent dynamics by fields ( coherent control ) is discussed in chapter A3.13. 

Altliough in figure C2.2.2 they are sketched witli rodlike molecules, botli nematic and chiral nematic phases can also be fonned by discotic molecules. 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

The polymers described so far have relatively flexible main chains which can result in complex confonnations. In some cases, tliey can double back and cross over tliemselves. There are also investigations on polymers which are constrained to remain in a confonnation corresponding, at least approximately, to a straight line, but which have amphiphilic properties tliat ensure tliat tliis line is parallel to tire water surface. Chiral molecules are one example and many polypeptides fall into tliis class [107]. Another example is cofacial phtlialocyanine polymers (figure C2.4.9). 

The first line of the connection table, called the counts line (see Figure 2-21), specifies how many atoms constitute the molecule represented by this file, how many bonds arc within the molecule, whether this molecule is chiral (1 in the chiral flag entry) or not, etc. The last-but-onc entry (number of additional properties) is no longer supported and is always set to 999. The last entry specifics the version of the Ctab format used in the current file. In the ease analyzed it is V2000". There is also a newer V3000 format, called the Extended Connection Table, which uses a different syntax for describing atoms and bonds [50. Because it is still not widely used, it is not covered here. 

If the tctravalcnt carbon atom has three difFcrcnt substituents, the molecule is chiral and it is not possible to superimpose it onto its mirror image. Our feet are also chiral objects the right foot is a mirror image of the left one and does not ftt into the left shoe. 

An example of a chiral compound is lactic acid. Two different forms of lactic acid that are mirror images of each other can be defined (Figure 2-69). These two different molecules are called enantiomers. They can be separated, isolated, and characterized experimentally. They are different chemical entities, and some of their properties arc different (c.g., their optical rotation),... 

Figure 2-70. Examples of chiral molecules with different types of stereogenic units.

Clearly, the next step is the handling of a molecule as a real object with a spatial extension in 3D space. Quite often this is also a mandatory step, because in most cases the 3D structure of a molecule is closely related to a large variety of physical, chemical, and biological properties. In addition, the fundamental importance of an unambiguous definition of stereochemistry becomes obvious, if the 3D structure of a molecule needs to be derived from its chemical graph. The moleofles of stereoisomeric compounds differ in their spatial features and often exhibit quite different properties. Therefore, stereochemical information should always be taken into ac-count if chiral atom centers are present in a chemical structure. 

Figure 5-28. Reaction query for Example 2 "R indicates that the bond must be part of a ring system, and "53 represents an atom with three non-hydrogen attachments. The Chiral" flag is necessary to retrieve only molecules with the identical absolute stereoconfiguration.

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40]. 

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. 

The two values, e and c, calculated for all combinations of four atoms, are then combined to generate a conformation-dependent chirality code. fc )QO using Eq. (30), where n is the number of atoms in each molecule, and r introduces the conformation dependence ... 

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