Achirality molecular models

The thujanes (cis-, 44 trans-, 45) are the parent hydrocarbons of a significant series of terpenes, the chemistry of which has been reviewed The bicyclic system, as such, is achiral. Molecular modeling indicates that in this case the isopropyl group does not occupy all conformations equally, but resides mainly in that one in which it is... 

Wnte structural formulas or make molecular models for all the compounds that are tnchloro derivatives of cyclopropane (Don t forget to include stereoisomers ) Which are chiraL Which are achiral" ... 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

The dissection of a molecular model into those components that are deemed to be essential for the understanding of the stereochemistry of the whole may be termed factorization (9). The first and most important step toward this goal was taken by van t Hoff and Le Bel when they introduced the concept of the asymmetric carbon atom (10a, 1 la) and discussed the achiral stereoisomerism of the olefins (10b,lib). We need such factorization not only for the enumeration and description of possible stereoisomers, important as these objectives are, but also, as we have seen, for the understanding of stereoselective reactions. More subtle differences also giving rise to differences in reactivity with chiral reagents, but referable to products of a different factorization, will be taken up in Sect. IX. 

Given atomic coordinates for a particular conformation of a molecule and some property value assigned to each atom, one can easily calculate a chirality function that distinguishes enantiomers, is zero for an achiral molecule, and is a continuous function of the coordinates and properties. This is useful as a quantitative measure of chirality for molecular modeling and structure-activity relations. 

Now take one of the models you constructed in no. 7, and on one of the carbon centers exchange any two colored component groups. Does the new model have a plane of symmetry (8a) Is it chiral or achiral (8b) How many stereocenters are present (8c) Take this model and one of the models you constructed in no. 7 and see whether they are superimposable. Are the two models superimposable (8d) Are the two models identical or different (8e) Are the two models mirror images of each other (8f) Here we have a pair of molecular models, each with two stereocenters, that are not mirror images of each other. These two examples represent diastereomers, stereoisomers that are not related as mirror images. 

The problems here are best considered with the aid of a molecular model of both enantiomers of each compound. If the enantiomers are not superimposable, the molecule is chiral, (a) Chiral, exists as a pair of enantiomers (cf. trans-1,2-dimethylcyclohexane (b) chiral (c) achiral (plane of symmetry) (d) chiral (e) chiral (f) achiral (g) chiral (h) achiral (plane of symmetry). 

The method and accuracy of proving the presence of a helical structure varies depending on the type of study and the structure of the polymer. Structural questions can be addressed by (1) various methods based on computer calculations or observations of molecular models, (2) achiral spectroscopic evidence (NMR spectra, absorption spectra, X-ray diffraction),... 

These concepts are difficult for many people to fully grasp unless they use molecular models. This form of stereoisomerism is known as optical isomerism because chiral molecules have unique effects on polarized light. Molecules containing all carbon atoms with three or fewer different entities attached cannot be enantiomers and are called achiral. 

Figure 7). Osawa s recent force-field calculation study (21) of cage-shaped molecules with ethano bridges affords an excellent demonstration of this twist deformation at the molecular level. In six achiral cage-shaped molecules so far studied, his calculations showed that each molecule assumed a twisted, chiral conformation to minimize torsional and nonbonding strain. Tricyclo[4.2.2.2.2,s]dodecane (5) was shown to be 1.1 kcal/mol more stable in a twisted D2 than in the eclipsed D2h conformation, and his calculation also suggests that perhydrotriquinacene and C16-hexaquinane should assume C3 rather than conformations, contrary to naive pictures obtained by a casual observation of molecular models. 

The situation is more complicated in homoadamantane (7) where a casual study of the molecular model would suggest that the molecule ought to assume two strain-free enantiomeric C2 conformations (6 and 6 ). Schleyer s force-field calculation (22) has predicted, however, that the achiral C2v conformer (7) will be more stable than the twisted conformers, and this has been borne out by a dynamic NMR study (23), as well as by CD spectral analyses of its mono- and C2 diketone derivatives (24). 

Chirotopic The property of any atom, and, by extension, any point or segment of the molecular model, whether occupied by an atomic nucleus or not, that resides in a chiral environment [83]. Achirotopic is the property of any atom or point that does not reside in a chiral environment (see also [84]). Chirotopic atoms located in chiral molecules are enantiotopic by external comparison between enantiomers. Chirotopic atoms located in achiral molecules are enantiotopic by internal and therefore also by external comparison. All enantiotopic atoms are chirotopic [83]. 

Another report on ion-pair chromatography, where computed molecular descriptors from molecular modeling were nicely correlated with experimental separation factors, was published by Karlsson, Luthman, Pettersson and Hacksell [81]. They examined factors responsible for separation of aminotetralins on achiral stationary phases in the presence of the chiral additive N-benzyloxycarbonylglycyl-L-proline (L-ZGP), a protected peptide derivative. 

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