Dissymmetric molecule

Krow, G., The Determination of Absolute Configuration of Planar and Axially Dissymmetric Molecules, 5, 31. 

Absolute Conflguiation of Planat and Axially Dissymmetric Molecules VOL. PAGE... 

Atomic Inversion, Pyramidal (Lambert) Axially and Planar Dissymmetric Molecules, Absolute Configuration 6 19... 

Piperidines, Quaternization Stereochemistry of (McKenna) Planar and Axially Dissymmetric Molecules, Absolute Connguration of 5 275... 

Absolute Configuration of Planar and Axially Dissymmetric Molecules... 

The application of exciton coupling between the benzoate and the s-trans-diene or -enoate chromophore in axially dissymmetric molecules derived from hydroxy-substituted adaman-tanone allowed determination of not only the absolute but also the relative configuration. All benzoates 2-5 of 4R configuration show negative exciton Cotton effects, with amplitudes lower for (2 )-adamantylidene compounds 2, 4 compared with 2Z-isomers 3, 5, in which the two interacting chromophores are at a closer distance143. 

Molecules that are not superimposable on their mirror images are termed dissymmetric. This term is used rather than asymmetric, since the latter means, literally, have no symmetry that is, it is applicable only to a molecule belonging to point group Cx. All asymmetric molecules are dissymmetric, but the converse is not true. Dissymmetric molecules can and often do possess some symmetry. It is possible to give a very simple, compact rule expressing the relation between molecular symmetry and dissymmetric character ... 

In the older literature, chiral centers often are called asymmetric centers and you may be confused by the difference between asymmetric and dissymmetric. Both asymmetric and dissymmetric molecules (or objects) are chiral. An asymmetric object has no symmetry at all and looks different from all angles of view. Formulas 3 and 4 represent asymmetric molecules. A dissymmetric molecule is chiral, but looks the same from more than one angle of view. A helical spring is dissymmetric—it looks the same from each end. We will encounter dissymmetric molecules later. 

The One-Electron Mechanism. Both the electric and magnetic dipole transitions reside in the same chromophore. The rest of the dissymmetric molecule acts as a perturbing field which partially breaks down the symmetry of the chromophore, and therefore mixes the two transitions. The one-electron theory is also known as the Condon, Altar, and Eyring theory. 

The terms erythro and threo are generally used only with molecules that do not have symmetric ends. In symmetric molecules such as 2,3-dibromobutane and tartaric acid, the terms meso and (d,l) are preferred because these terms indicate the diastere-omer and tell whether or not it has an enantiomer. Figure 23-4 shows the proper use of the terms erythro and threo for dissymmetric molecules, as well as the terms meso and (.d,l) for symmetric molecules. 

The terms erythro and threo are used with dissymmetric molecules whose ends are different. The erythro diastereomer is the one with similar groups on the same side of the Fischer projection, and the threo diastereomer has similar groups on opposite sides of the Fischer projection. The terms meso and ( ) [or (4,/)] are preferred with symmetric molecules. 

Generally, optical activity is observed when electrons are displaced along chiral paths by an applied electric field. This does not necessarily require the involvement of dissymmetric molecules. Quartz crystals, in which the first observation < of optical activity was achieved, have no chiral molecules or ions, and their optical activity arises simply from the helical placement of atoms in the crystal. Likewise, a crystal composed of achiral untwistable molecules may become optically active if there are strong helical interactions between neighboring molecules. 

In Section 4.3 we mentioned two important stereochemical terms, enantiomers and enantiotopic nuclei. Enantiomers are structures related as the left hand is related to the right nonsuperimposable mirror images. Any chiral (dissymmetric) molecule can exist in two (and only two) enantiomeric forms. For example, chiral alcohol 10-11 has two enantiomeric configurations, labeled R and S.10 Enantiotopic nuclei are those related by a plane of symmetry. The methylene hydrogens of benzyl alcohol (10-12) are enantiotopic and are labeled pro-R and pro-S10 ... 

Molecules that are not superposable on their mirror images are called dissymmetric. This term is preferable to asymmetric, which means without symmetry, whereas dissymmetric molecules can and often do possess some symmetry, as will be seen. ... 

Dissymmetric molecules either have no symmetry at all, or they belong to one of the groups consisting only of proper rotation operations, that is, the CR or D groups. (Groups T, O, and 1 are, in practice, not encountered, though molecules in these groups must also be dissymmetric.) Important examples are the bischelate and trischelate octahedral complexes (A5-VIII), (A5-X), and (A5-XIX). 

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