Asymmetry, molecular

There are several different ways in which quantum mechanics has been applied to the problem of relating the barrier to the frequency separation of the spectroscopic doublets. These are all approximation procedures and each is especially suitable under an appropriate set of circumstances. For example one may use perturbation theory, treating either the coupling of internal and external angular momenta, the molecular asymmetry, or the potential barrier as perturbations. Some of the different treatments have regions of overlap in which they give equivalent results choice is then usually made on the basis of convenience or familiarity. Extensive numerical tabless have been prepared which simplify considerably the calculations. 

Many of the mesogenic molecules are stericaUy asymmetric, which is determined by the fractures and bending of the molecular core as well as by the presence of the tail chains of different nature, including the branched, biforked or polyphilic moieties (Fig. 2c-f). In terms of the multipole model of molecular asymmetry introduced by Petrov and Derzhanski [34], we can speak about longitudinal or transverse steric dipoles or multipoles (Fig. 3). 

One of the fundamental concepts of structural chemistry is that of molecular asymmetry or chirality. The most typical example is that of a tetrahedral carbon atom with four different substituents, C(abcd), which can produce two different arrangements, which are nonsuperimposable mirror images of one another. Such a carbon atom is usually called asymmetric or chiral. In contrast, when two of the substituents are alike, as in C(abc2), the system is usually termed symmetrical or achiral, except for a special class of compounds... 

The computed results in Fig. 36 capture the observed phase behaviour of mikto arm-star copolymer melts. The effect of the molecular asymmetry seems to saturate for n > 3, and phase boundaries do not change significantly as a function of 0 or higher asymmetry. 

This compound is optically active because of molecular asymmetry. Of course, the presence of an asymmetric carbon atom lends asymmetry to the molecule and hence gives rise to optical activity but it is not always essential. The presence of an asymmetric carbon atom may or may not give rise to optical activity. The case of meso. tartaric acid is worth mentioning in this connectiar. Although the molecule has two asymmetric carbon atoms but it is optically inactive. 

The inactivity in the molecule is due to the fact that it is perfectly symmetric as shown by the dotted line, the upper half exactly coinciding with the lower half. Therefore, molecular asymmetry and not the presence of asymmetric carbon atoms is responsible for optical activity. Since the term asymmetric has been found to be inadequate, now the term chirality has been introduced. The word chiral (the Greek word cheir means hand pronounced kiral) signifies, the property of Handedness . An object that in not superimposable upon its mirror image is chiral and this mirror-image relationship is the same as left hand has with the right. If an object and its mirror image can be made to coincide in space, they are said to be achiral. 

The allenes are another example of compounds which show molecular asymmetry if both ends are dissymmetric. 

Bentley, R. Molecular asymmetry in biology, Vol. II. New York Academic Press 1970. 

Becker, P. Evolution in open systems Bistability and the origin of molecular asymmetry, Nature New Biology 241, 72—74 (1973). 

Bentley, R. Molecular Asymmetry in Biology Academic Press New York, 1970 Vol. 2. 

Garay, A. S. Hrasko, P. Neutral currents un weak interactions and molecular asymmetry in International symposium on the generation and amplification of asymmetry in chemical systems Thiemann, W Ed. Juelich, Germany, 1974, pp 449-469. 

The molecular asymmetry of 9 is due to the four axial pendants containing the chiral L-vahne group. The efficiency of the gas-phase exchange reaction 30 where A are representative amino acids and B is either (5)-(- -)- or R)- — )-2-butylamine is appreciably affected by the configuration of both A and B. The guest exchange kinetic results are reported in Table 17. The presence of more than one reacting [9-H-A]" structure is observed with A = DOPA. A similar behavior was observed with permethylated /8-CD as the host." " ... 

One consequence of the molecular asymmetry is that the core particle presents two distinct faces, arbitrarily labeled ventral and dorsal in our images. These two faces have subtle yet distinct dilferences in the electrostatic surface potentials they present. 

Molecular asymmetry, chirality and enantiomers The observation of Louis Pasteur (1848) that crystals of certain compounds exist in the form of mirror Images laid the foundation of modem stereochemistry. He demonstrated that aqueous solutions of both types of crystals showed optical rotation, equal in magnitude (for solution of equal concentration) but opposite in direction. He believed that this difference in... 

With the discovery of crystalline polypropylene in the early 1950 s, polymer stereochemical configuration was established as a property fundamental to formulating both polymer physical characteristics and mechanical behavior. Although molecular asymmetry was well understood, polymer asymmetry presented a new type of problem. Both a description and measurement of polymer asymmetry were essential for an understanding of the polymer structure. 

LCs are materials that undergo physical reorganization in which at least one of the rearranged structures involve molecular alignment along a preferred direction, causing the material to exhibit nonisotropic behavior and associated birefringent properties, i.e., molecular asymmetry. 

The molecular asymmetry typically occurs not because of intermolecular interaction, but because two molecules cannot occupy the same space at the same time. Molecular chains can... 

The question of the origin of life is usually linked to the question of the origin of molecular asymmetry, specifically why only L-aminoacids (with extremely few exceptions) are present in nature (see Figure 3.8). 

In Table 9 some racemates resolved on microcrystalline cellulose triacetate30 are shown. This method appears to be particularly useful for the separation of compounds with molecular asymmetry. 

Thomson, J. B. Molecular asymmetry in the ferrocene series. Tetrahedron Letters [London] 195916, 26. 

Optical Activity in the Series.—Another type of isomerism is possible in the series, for the as-diehloro-salts present a case of molecular asymmetry similar to that observed in 1-, 2-dinitro-diethylenediamino-cobalt salts. Two configurations are possible, the one being the mirror image of the other, thus ... 

See (a) Pastenr, L. Researches on the Molecular Asymmetry of Natural Organic Products, Alembic Clnb Reprints University of Chicago Press Chicago, 1902. (b) Pastern, L. Oevres de Pasteur, Masson et Cie. Paris, 1922 Vol. 1, pp 315, 329. 

FIGURE 1-19 Molecular asymmetry chiral and achiral molecules. 

Local structural features have been postulated for amorphous polymer systems, based on the asymmetry of chain-like molecules. Flory (56) has shown that molecular asymmetry in itself is no barrier to a dense random packing of the chains are sufficiently flexible. Robertson (57) suggests, however, that some degree of local alignment is required simply to accomodate linearly connected sequences in the rather limited space available. Unfortunately, Calculations of local cooperative effects are extremely difficult and sensitive to specific assumptions about available packing arrangements. 

These results make it clear that the forms of t]0 — rjs and Je° are completely independent of model details. Only the numerical coefficient of Je° contains information on the properties of the model, and even then the result depends on both molecular asymmetry and flexibility. Furthermore, polydispersity effects are the same in all such free-draining models. The forms from the Rouse theory cany over directly, so that t]0 - t]s, translated to macroscopic terms, is proportional to Mw and Je° is proportional to the factor A/2M2+, /A/w. Unfortunately, no such general analysis has been made for models with intramolecular hydrodynamic interaction, and of course these results apply in principle only to cases where intermolecular interactions are negligible. 

Bentley, R. (1970) Molecular Asymmetry in Biochemistry, Vol. 2, Academic Press, New York... 

As for low-molecular-weight aliphatic compounds, also for polymers made up by aliphatic macromolecules the existence must be excluded of optical activity in solution connected with the molecular asymmetry arising from hindrances to the free rotation around >C—C< single bonds. 

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