Of chiral molecules

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

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... 

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

Section 7 2 The most common kind of chiral molecule contains a carbon atom that bears four different atoms or groups Such an atom is called a chirality center Table 7 2 shows the enantiomers of 2 chlorobutane C 2 is a chi rahty center m 2 chlorobutane... 

Techniques for determining the absolute configuration of chiral molecules were not developed until the 1950s and so it was not possible for Eischer and his contemporaries to relate the sign of rotation of any substance to its absolute configuration A system evolved based on the arbitrary assumption later shown to be correct that the enantiomers... 

G. M. Coppola andH. F. Asymmetric Synthesis Construction of Chiral Molecules Using Amino Acids, oBis N d y dn Sons, Inc., New York, 1987. 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

Scheme 2.2. Examples of Chiral Molecules Lacking Asymmetric...

Several more examples of chiral molecules are shown below. Check for yourself that the labeled carbons are chirality centers. You might note that carbons in — CK2—, -CJH3, C—O, C=C, and C=C groups can t be chirality centers. (Why )... 

An interesting phenomenon was observed when the CD of chiral molecules was measured in achiral solvents. The chiral solvent contributed as much as 10-20% to the CD intensity in some cases. Apparently, the chiral compound can induce a solvation structure that is chiral, even when the solvent molecules themselves are achiral. ... 

In the case of representation of chiral molecules, the demands of spatial visualisation ability on understanding become critical. Head and Bucat (2002) have reported the outcomes from interviews with students and lecturers in relation to tasks such as that shown in Fig. 1.11. 

These catalysts were first tested as resin-bound derivatives via HTS, first with metals and then without. Three libraries of chiral molecules, based on three different enantiomerically pure diamines, bulky salicylidene moities and optically active ii-amino acids were used for structure optimisation (Scheme 37 TBSCN = fBuMe2SiCN) [152]. 

Electronic and Magnetic Properties of Chiral Molecules and Supramolecular Architectures... 

A similar convergence study has been reported for the B-spline calculation for trans-2,3 dimethyloxirane molecule, et al. [53] The authors in fact comment that the parameter is no more demanding on basis set size than the p parameter, but their data ([53] Fig. 1) show that when the asymptotic is increased from 10 to 15 there is a significantly greater improvement in the former angular parameter. A value of. (max = 15 was chosen for all subsequent B-spfine calculations for oxiranes, and the same limit tends to be applied in the other reported B-spline calculations of chiral molecule PECD [60, 61]. 

In the case of chiral molecules that are biologically active the desired activity almost always resides in only one of the enantiomers. The other enantiomer constitutes isomeric ballast that does not contribute towards the desired activity and may even exhibit unwanted side effects. Hence, there is a marked trend in pharmaceuticals, agrochemicals and flavours and fragrances towards the marketing of products as enantiomerically pure compounds. This, in turn, has generated a demand for economical methods for the synthesis of pure enantiomers (Sheldon, 1993a). 

The pharmaceutical industry has been giving increased attention to homogeneous asymmetric hydrogenation for the synthesis of chiral molecules due to significant improvements in this technology (1). We recendy synthesized a chiral a-amino acid intermediate using Et-DuPhos-Rh catalyst, obtaining enantiomeric pmities (EP) of... 

A number of specialised stationary phases have been developed for the separation of chiral compounds. They are known as chiral stationary phases (CSPs) and consist of chiral molecules, usually bonded to microparticulate silica. The mechanism by which such CSPs discriminate between enantiomers (their chiral recognition, or enantioselectivity) is a matter of some debate, but it is known that a number of competing interactions can be involved. Columns packed with CSPs have recently become available commercially. They are some three to five times more expensive than conventional hplc columns, and some types can be used only with a restricted range of mobile phases. Some examples of CSPs are given below ... 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

A good working knowledge of stereo-chemistry is certainly a big advantage when looking at the spectra of chiral molecules. Let s start by considering Structure 6.1. 

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