Enantiomorphous molecules

The crystal structure of [Cr(S2COEt)3] is composed of enantiomorphous molecules with trigonal symmetry. The short S2C—O bond distance (1.297 A) is taken to mean that the resonance structure S2 —0=5—R contributes considerably.1014... 

FIGURE 14. X-ray crystallographic characterization of 36d-CS2- Left arrangement in the unit cell. Right crystal structure of one of hie two enantiomorphic molecules in the unit cell. The ellipsoids... 

Consider two enantiomorphous molecules, R (right-handed) and L (left-handed), isolated from their surroundings and from external fields but not from each other. That is, they are allowed to interact. In the quantum-mechanical treatment of this system, as two particles in a one-dimensional symmetric double-well potential, the two states are degenerate in energy and are related to wavefunctions TR and 4Y localized in the two potential wells. Superposition of these wavefunctions yields ground and first excited states F+ and P ... 

The molecules Se and Sio in Se-Sio exhibit the same molecular conformations as in the pure crystals of (D i) and of Sio D2). However, the molecular parameters of Se in Se-Sio show a slight variation of less than 1% of the mean values in the case of the bond and torsion angles and less than 0.1% in the case of the internuclear distances. As in pure Sio both enantiomorphic molecules are present in Ss-Sio in equal amounts. The site symmetry of Sio in Se-Sio is C2 as in pure Sio. However, the site symmetry of the Se molecules in Ss-Sio is reduced to C in comparison with Csi in pure S. The mean bond parameters of S and Sio are almost identical in Se-Sio compared to the pure components (Table 10). 

Figure 4.1 The enantiomorphic (mirror image) forms of the molecule alanine, CH3CH(NH2)COOH (a) the naturally occurring form, (S)-(+)-alanine (b) the synthetic form (/ )-(—)-alanine. An enantiomorphic molecule cannot be superimposed upon its mirror image...

If the crystal-structiu-e analysis is made on a derivative containing a heavy atom, with x-rays of wavelength appropriate to the particular heavy atom (that is, Br or I with CuKa radiation), it is possible to determine the absolute configuration of an enantiomorphous molecule. This method was first demonstrated with the rubidium sodium salt of dexiro-tartaric (l-threaric) acid tetrahydrate by Bijvoet and coworkers in 1951. The results confirmed the configuration of dextro-i vi nc acid originally assigned by... 

The simplest way in which optical activity may be observed is doubtless already familiar. It consists in the observation that the plane of polarization of plane-polarized monochromatic light is rotated upon passing through a solution containing one or the other—or an excess of one or the other—of two enantiomorphic molecules. 

A rotative substance (or mixture of substances) may be optically active or inactive. That is to say, the expected optical rotation may be finite, small or even accidentally zero (because of solvent, temperature, pH, or limited sensitivity of the measurement). A rotative substance consists of chiral molecules, and/or unequal numbers of enantiomorphic molecules. A nonrotative substance (or mixture of substances) is always optically inactive (at any wavelength). A nonrotative substance consists of achiral molecules, and/or equal numbers of enantiomorphic sets of molecules. 

The molecular structure and conformational dynamics of dioxadithiazocane (18) <80NJC179> were studied by x-ray and NMR analyses. Compound (18) was found to exist as two pseudo-enantiomorphic molecules. In the solid state, the molecule assumes two approximate boat-chair conformations with transannular sulfur-nitrogen distances of 2.94 A and 2.99 A. In CDCI3 and d(,-benzene solutions, the material exists as an enantiomeric equilibrium of two isoenergetic boat-chair forms involving a pseudorotational path <8UCS(P2)1554>. 

In the latter case the elements of the pair are chiral, and they are said to be enantiomorphic molecules. Each of them is an enantiomorph. 

Pasteur ascribed the existence of this so-called optical isomerism to the formation of enantiomorphous molecules, that is to say, of molecules capable of existing in right- and left-handed forms, and related to each other as an object to its non-coincident image. His researches in this field of chemistry, carried out mostly between 1848 and 1858, led to the conclusion that organic molecules, like tangible objects, fall into two categories, designated by the terms symmetric and asymmetric (or non-symmetric). 

In the present paper we have described an improved synthesis of l,2-6w-crown-5-calix[4]arene (3). The synthesis was achieved by using cesium carbonate. The X-ray structure of (3) indicated the presence of two enantiomorphous molecules in the crystal. These two enantiomorphous molecules were analyzed by molecular mechanics using the GenMol program. It was concluded that while the conformation of (3) in solution is an average of several forms, in the solid state the more stable conformations are the pinched-cone forms which are the limit forms of (3). Calculations on supermolecules (3) complexed to alkali cations indicated that ligand (3) preferentially binds Cs" " compared to the other alkali cations. 

Optical isomers, enantiomorphs or enantiomers, as they are also known, are pairs of molecules... 

Consider a methane molecule CH, and suppose that some or all of its hydrogen atoms are replaced by some other monovalent atom. If the atoms attached to the carbon are all different, that is, the carbon atom is asymmetric, the resulting molecule is chiral and exists in two so-called enantiomorphic forms mirror images of each other. (For further information on chirality see the interesting expository paper [PreV76]). 

If a molecule is nonsuperimposable on its miixor image, the mirror image must be a different molecule, since superimposability is the same as identity. In each case of optical activity of a pure compound there are two and only two isomers, called enantiomers (sometimes enantiomorphs), which differ in structure only in the left-and right-handedness of their orientations (Fig. 4.1). Enantiomers have identical physical and chemical properties except in two important respects ... 

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

Pasteur thus made the important deduction that the rotation of polarized light caused by different tartaric acid salt crystals was the property of chiral molecules. The (+)- and ( )-tartaric acids were thought to be related as an object to its mirror image in three dimensions. These tartaric acid salts were dissymmetric and enantiomorphous at the molecular level. It was this dissymmetry that provided the power to rotate the polarized light. 

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. 

Since all the molecules are asymmetric and have no plane of symmetry, all are optically active. Further structures I and II are enantiomorphs and so are structures III and IV. But structures III and I or IV and I are although stereoisomers but are not enantiomorphs. Such pairs of steroisomers which possess chirality but are not the mirror images of each other are called diastereomers. Thus III and IV are diastereomers of 1. So diastereomers will always be formed when the compound contains two dissimilar asymmetric carbon atoms and will exist in four stereoisomeric forms. 

The manual separation of the enantiomorphous crystals of sodium ammonium tartrate tetrahydrate (Figure 1) by Pasteur in 1848 (1) is historically significant, because it laid the foundations of modem stereochemistry. This experiment demonstrated for the first time that certain classes of molecules display enan-tiomorphism even when dissolved in solvent. These observations eventually paved the way for the inspired suggestion, made more than two decades later, by van t Hoff (2) and Le Bel (3), of a tetrahedral arrangement of bonds around the carbon atom. 

We illustrated in Section II why conventional X-ray diffraction cannot distinguish between enantiomorphous crystal structures. It has not been generally appreciated that, in contrast to the situation for chiral crystals, the orientations of the constituent molecules in centrosymmetric crystals may be unambiguously assigned with respect to the crystal axes. Thus, in principle, absolute configuration can be assigned to chiral molecules in centrosymmetric crystals. The problem, however, is how to use this information which is lost once the crystal is dissolved. 

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