Optical activity chiral molecules

Keywords chiral molecules optical activity pseudoscalars liquids second-order nonlinear optics ... 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

Suitably Substituted Adamantanes. Adamantanes bearing four different substituents at the bridgehead positions are chiral and optically active, and 14, for example, has been resolved. This type of molecule is a kind of expanded tetrahedron and has the same symmetry properties as any other tetrahedron. 

The reasons for the increasing acceptance of enzymes as reagents rest on the advantages gained from utilizing them in organic synthesis Isolated or wholecell enzymes are efficient catalysts under mild conditions. Since enzymes are chiral materials, optically active molecules may be produced from prochiral or racemic substrates by catalytic asymmetric induction or kinetic resolution. Moreover, these biocatalysts may perform transformations, which are difficult to emulate by transition-metal catalysts, and they are environmentally more acceptable than metal complexes. 

Problem 5.25 (a) What is the necessary and sufficient condition for the existence of enantiomers (6) What is the necessary and sufficient condition for measurement of optical activity (c) Are all substances with chiral atoms optically active and resolvable (d) Are enantiomers possible in molecules that do not have chiral carbon atoms (e) Can a prochiral carbon ever be primary or tertiary (/) Can conformational enantiomers ever be resolved ... 

Hund, one of the pioneers in quantum mechanics, had a fundamental question of relation between the molecular chirality and optical activity [78]. He proposed that all chiral molecules in a double well potential are energetically inequivalent due to a mixed parity state between symmetric and antisymmetric forms. If the quantum tunnelling barrier is sufficiently small, such chiral molecules oscillate between one enantiomer and the other enantiomer with time through spatial inversion and exist in a superposed structure, as exemplified in Figs. 19 and 24. Hund s theory may be responsible for dynamic helicity, dynamic racemization, and epimerization. 

A quick test for chirality is to look for the presence of an internal mirror plane. A molecule that lacks such a plane is most always chiral or optically active and is often designated by the R rectus, or right-handed)/ sinister, or left-handed) scheme of nomenclature. 

Chiro-optical properties. The optical properties specific of chiral molecules (optical rotation, circular dichroism, Raman optical activity (ROA)) may also be enhanced by the interaction with plasmons. This very interesting field is much less developed than others, although theoretical contributions have already been published (e.g. for surface-enhanced Raman optical activity, SEROA) [48,49]. 

Chirality is usually seen optically in the form of rotation of the optical axis of a linearly polarized light crossing a chiral material (optical activity). In isotropic liquids optical activity requires chiral molecules, which results in typically of about 1 degree rotation of a light crossing 1 cm slab. In liquid crystals molecular chirality leads to helical structure, which enhances the optical activity so that the optical rotation (OR) can be as large as 100deg/p,m in some short pitch cholesteric or SmC materials. 

Another interesting group of compounds where chirality often occurs is spir-anes (Figure 2.17). It is sufficient to substitute any two hydrogen atoms in both spirane rings with any substituents (they can be identical) to make this molecule optically active. 

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