Molecular enantiomers

This forward-backward asymmetry of the photoelectron distribution, expected when a randomly oriented sample of molecular enantiomers is ionized by circularly polarized light, is central to our discussion. The photoelectron angular... 

Consider a threaded rod, representing a molecular enantiomer, that lies away from an observer. If the observer reaches out and spins a nut on the rod clockwise with his right hand, the nut will travel forward, away from the observer, and will shortly fly off the rod. Here, the angular momentum imparted to the nut (electron) by the observer s hand (photon) causes it to be ejected in a specific direction from the rod (molecular enantiomer) in the observer s reference frame. This is mediated by the interaction between the chiral thread of the rod and nut (the chiral molecular potential). If the rod is turned through 180° and the action repeated, the nut (electron) still departs in the same direction, away from the observer. Hence, the orientation of the rod (molecule) in the observer s frame does not alter the direction in which the nut (electron) is ejected. 

The well-resolved C=0 li peak in the fenchone XPS provides an excellent opportunity to examine PECO from a single, well-characterized initial orbital. As has been previously mentioned, it might be thought that such a localized, spherically symmetric initial orbital would not be sensitive to the molecular enantiomer s handedness, but as can be seen in Fig. 15 (a) the dichroism in the electron yield recorded at the magic angle is sufficiently large to be easily visible by eye as a difference in the intensity of the Icp and rep spectra. 

A related phenomenon can also occur when the crystal lattice packing is chiral. This intrinsic handedness can result in formation of a 1 1 mixture of enantiomeric crystals. In this case, although there has been self-resolution into (+)- and (—)-crystals, both molecular enantiomers remain unseparated in each crystal. The fundamental distinction is that a conglomerate single crystal contains only one molecular enantiomer and therefore would be optically active in solution, while, for the latter, a single crystal contains both molecular enantiomers and its solution would be optically inactive. 

As explained earlier (Section 3), a 1 1 mixture of (+)- and (—)-crystals may also be obtained where self-resolution of the molecular enantiomers has not taken place. This occurs because the crystallisation process generates a lattice that is itself handed. Two examples of this phenomenon are presented here. 

L. Ivanov, V. Letokhov, The effect of charge and atom position on energy-level splitting in molecular enantiomers OAving to parity nonconservation in a weak interaction, Dokl. Phys. 43 (1998) 523-526. 

One of the most important goals in the coherent control of complex molecules is the realization of the control of stereoselective processes. The difference in the absorption spectrum between molecular enantiomers is very small and becomes manifest only with circularly polarized light. For this reason, the demonstration of the coherent control of left/right molecules by Shapiro et al. (2000) seems important for future... 

Fig. rV-IS. A fluorescence micrograph showing the dural solid domains formed in a mixture of the two enantiomers of dipalmitoylpho hatidylcholine (DPPC) at a pressure of 9 dyn/cm and average molecular area of 70 A. (From Ref. 169.)... 

In chemoinformatics, chirality is taken into account by many structural representation schemes, in order that a specific enantiomer can be imambiguously specified. A challenging task is the automatic detection of chirality in a molecular structure, which was solved for the case of chiral atoms, but not for chirality arising from other stereogenic units. Beyond labeling, quantitative descriptors of molecular chirahty are required for the prediction of chiral properties such as biological activity or enantioselectivity in chemical reactions) from the molecular structure. These descriptors, and how chemoinformatics can be used to automatically detect, specify, and represent molecular chirality, are described in more detail in Chapter 8. 

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40]. 

Chirality codes are used to represent molecular chirality by a fixed number of de-.scriptors. Thc.se descriptors can then be correlated with molecular properties by way of statistical methods or artificial neural networks, for example. The importance of using descriptors that take different values for opposite enantiomers resides in the fact that observable properties are often different for opposite enantiomers. 

Make a molecular model of one of the enantiomers of 3 buten 2 ol and the 2 butanol formed from it... 

If they are stereoisomers are they enantiomers or diastereomers" (Molecular models may prove useful in this problem )... 

Excluding enantiomers there are three isomeric cyclopropanedicarboxyhc acids Two of them A and B are constitutional isomers of each other and each forms a cyclic anhydnde on being heated The third diacid C does not form a cyclic anhydride C is a constitutional isomer of A and a stereoisomer of B Identify A B and C Construct molecular models of the cyclic anhy dndes formed on heating A and B Why doesn t C form a cyclic anhydride" ... 

The molecular ellipticity is analogous to specific rotation in that two enantiomers have exactly opposite values of 0 at every wavelength. Two enantiomers will thus show CD spectra having opposite signs. A compound with several absorption bands may show both... 

The property of chirality is determined by overall molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. The examples in Scheme 2.2 include allenes (entries 1 and 2) and spiranes (entries 7 and 8). Entries 3 and 4 are examples of separable chiral atropisomers in which the barrier to rotation results from steric restriction of rotation of the bond between the aiyl rings. The chirality of -cyclooctene and Z, -cyclooctadiene is also dependent on restricted rotation. Manipulation of a molecular model will illustrate that each of these molecules can be converted into its enantiomer by a rotational process by which the ring is turned inside-out. ... 

The column was 25 cm long, 4.6 mm I.D. and packed with Partisil 10. It is seen that linear curves were obtained for three different solutes and two different moderators in n-heptane. Scott and Beesley [14] obtained retention data for the two enantiomers, (S) and (R) 4-benzyl-2-oxazolidinone. The column chosen was 25 cm long, 4.6 mm I.D. packed with 5 mm silica particles bonded with the stationary phase Vancomycin (Chirobiotic V provided by Advanced Separations Technology Inc., Whippany, New Jersey). This stationary phase is a macrocyclic glycopeptide Vancomycin that has a molecular weight of 1449.22, and an elemental composition of 54.69% carbon. 

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