Enantiomorphic elements

Let us now differentiate between structures which are asymmetric and dissymmetric. The word asymmetric conveys the idea that the molecule is completely devoid of the elements of symmetry. Dissymmetric on the other hand means not completely devoid of elements of symmetry but possessing so few elements of symmetry that on the whole it will posses two structures which will be the mirror images of each other. Therefore to avoid confusion the term asymmetric is used to cover examples which rotate the plane polarized light. The two forms of an optically active compound are called enantiometers or enantiomorphs or optical antipodes. They are also said to have enantiomeric relationship to each other. 

Prelog and Helmchen (5) defined pseudoasymmetry as the duality resulting from the two ways with which one can combine two enantiomorphic ligands with two enantiotopic spaces (Fig. 4). This represents an innovation because it limits pseudoasymmetry to achiral compounds (35). This follows from the fact that enantiotopic spaces can exist only if the molecule possesses an element of... 

A second problem that has repeatedly concerned us is the inability of the Sequence Rule to provide descriptors for some elements of stereoisomerism. When Cahn et al. (16) first encountered this problem with the all-cis and all-trans isomers of inositol, they attributed it to the fact that the symmetry has become so high that they have no asymmetric, nor even a pseudo-asymmetric atom. This interpretation, we believe, is incorrect. If the two ring ligands of any carbon atom of m-inositol were not heteromorphic, their exchange could not yield an isomer, as it clearly does. Each atom is a center of stereoisomerism with a pair of enantiomorphic ligands (Cg+g hi) and indistinguishable from the traditional pseudoasymmetric atom. The description of cu-inositol as all-5 could be accomplished by the same device that would allow one to specify the configurations of C(l) and C(4) of 4-methylcyclohexanol. 

Symmetry elements of the second kind other than ct may generate enantiotopic ligands. Thus compound 42 in Fig. 16 (F and T are enantiomorphic, i.e. mirror-... 

In order to ascertain which symmetry elements are present in raeso-tartaric acid, it is necessary to look at the various conformations of the molecule. The symmetrical highest energy conformer, i.e. the synperiplanar conformer (sp), has a plane of symmetry in which both enantiomorphic halves of the molecule are reflections of each other. No other symmetry elements are present in this conformation (point group Cs). In the ap conformation of raeso-tartaric acid the only symmetry element present is a centre of symmetry (disregarding the fact that the centre of symmetry is equivalent to any of the infinite number of S2 axes). The symmetry point group is therefore Cj. All other conformations, e.g. the +synclinal conformation (+sc) of raeso-tartaric acid shown below, are chiral and do not possess any symmetry elements and therefore belong to the point group C. ... 

Chirality is a concept well known to organic chemists and to all chemists concerned in any way with structure. The geometric property that is responsible for the nonidentity of an object with its mirror image is called chirality. A chiral object may exist in two enantiomorphic forms that are mirror images of one another. Such forms lack inverse symmetry elements, that is, a center, a plane, and an improper axis of symmetry. Objects that possess one or more of these inverse symmetry elements are superimposable on their mirror images they are achiral. All objects belong to one of these categories. 

There is a further element of arbitr iness in our insistence on right-handed co-ordinates, unless the molecules are enantiomorphic, or enantiomorphically... 

The common structural elements (A) and (B) are liot identical with their mirror-images. This corresponds with the observation that the enantiomorphous forms of the sugare and alcohols of the two groups have not been found in nature. Even if some such forms should be detected later, the conclusion that the structural elements (A) and (B) are the overwhelmingly preferred ones would not be changed. It is correct and more definite, therefore, to name the two sets of natural carbohydrates the D-mannitol and D-perseitol groups. 

A review of hydroxycarbonates of the lanthanide elements covering much of the earlier literature has appeared. It has been shown that weloganite crystallizes as discrete enantiomorphs in space group PI and exhibits a remarkable variety of coordination geometries in its structure. Carbonate ions and water molecules link the six cations such that the Zr ion is nine-coordinate with three oxygen donors each lying above, below, and in the plane with the metal ion. The... 

The influence of a surface on an adsorbed species is well-accepted. The TA/Ni(l 10) system demonstrates how much the molecule can influence the behaviour of the surface. How far can an adsorbate like tartaric acid induce such effects Work by Switzer and co-workers on the electrodeposition of CuO films in the presence of tartaric acid showed that chirality could be induced in a normally achiral inorganic material [25]. In a standard electrochemical cell, a Au(OOl) crystal is placed in a solution containing Cu(II) ions, tartrate ions and NaOH. At a certain potential, CuO will deposit, as a thin-film on the Au Surface. Characterization by diffraction revealed that the deposited CuO film has no mirror or inversion elements, i.e. it is chiral. The chirality of the film is controlled by the chirality of the tartrate ions in the solution (/ ,/ )-tartrate yielding a chiral CuO(-lll) fihn while presence of (S,S )-tartrate produces the mirror Cu(l-l-l) enantiomorph. Switzer et al, by catalyzing the oxidation of tartaric acid, demonstrate that not only the bulk, but also the surface of the CuO film is chiral the CuO electrode surface grown in the presence of (/ ,/ )-tartrate is more effective at oxidizing (/ ,/ )-TA, while the surface deposited in the presence of (S,S )-tartrate is more effective at oxidizing (S,S )-TA. 

Figure 26.14 Supramolecular chirality provided by the tilt of the elements, (a) An enantiomorphic pair of polyhedrons,...

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