Molecular structure absolute configuration

Kowalik, J. Sawka-Dobrowolska, W., and Glowiak, T., Synthesis, molecular structure, and absolute configuration of an optically active (l-amino-2-phe-nylethyl)phosphonic acid monohydrate, J. Chem. Soc., Chem. Commun., 446, 1984. 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

The wavelength dependence of specific rotation and/or molecular ellipticity is called the Cotton effect. The Cotton effect can provide a wealth of information on relative or absolute configurations. The sign of the Cotton effect reflects the stereochemistry of the environment of the chromophore. By comparing the Cotton effect of a compound of known absolute configuration with that of a structurally similar compound, it is possible to deduce the absolute configuration or conformation of the latter. 

The fundamental difficulty is that polymeric substances cannot be obtained in a structurally and molecularly uniform state, unlike low-molecular-weight compounds. Thus, macromolecular materials of the same analytical composition may differ not only in their structure and configuration (see Sect. 1.2) but also in molecular size and molecular weight distribution they are polydisperse, i.e., they consist of mixtures of molecules of different size. Hence, it is understandable that the expression identical is not, in practice, applicable to macromolecules. Up to the present time, there is no possibility of preparing macromolecules of absolutely uniform structure and size. It follows, therefore, that physical measurements on polymers can only yield average values. The afore-... 

Among the modem procedures utilized to establish the chemical structure of a molecule, nuclear magnetic resonance (nmr) is the most widely used technique. Mass spectrometry is distinguished by its ability to determine molecular formulas on minute amounts, but provides no information on stereochemistry. The third most important technique is x-ray diffraction crystallography, used to establish the relative and absolute configuration of any molecule that forms suitable crystals. Other physical techniques, although useful, provide less information on structural problems. 

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