Diastereomeric molecular associates

The interaction of a chiral analyte with a selector requires a temporary adsorption of the enantiomers on the surface of the stationary phase (see Fig. 4). Different models have been proposed to explain the reversible formation of energetically different quasi-diastereomeric molecular associates however, none of them can explain all of the observed retention mechanisms. Association and dissociation constants for enantiomers interacting with a CSP can be different, since the rates at which the diastereomeric associates are formed can differ. Analogously, the dissociation rates can be different... 

Fig. 4. The enantiomers form diastereomeric molecular associates with the CSP. Association constants k j and k 2 and dissociation constants fc", and k 2 not be identical.

An attractive method for the analysis of mixtures of enantiomers is chiral gas chromatography (GC). This sensitive method is unaffected by trace impurities, and is quick and simple to carry out. The premise upon which the method is based is that molecular association may lead to sufficient chiral recognition that enantiomer resolution results. The method uses a chiral stationary phase which contains an auxiliary resolving agent of high enantiomeric purity. The enantiomers to be analysed undergo rapid and reversible diastereomeric interactions with the stationary phase and hence may be eluted at different rates. There are certain limitations to the method, some of which are peculiar to the gas chromatographic method. The sample should be sufficiently volatile and thermally stable, and, of course, should be quantitatively resolved on the chiral GC phase. Occasionally this... 

Given the information above, the question remains as to the nature of the monolayer states responsible for the stereo-differentiation of surface properties in racemic and enantiomeric films. Although associations in the crystalline phases are clearly differentiated by stereochemical packing, and therefore reflected in the thermodynamic and physical properties of the crystals, there is no indication that the same differentiations occur in a highly ordered, two-dimensional array of molecules on a water surface. However, it will be seen below (pp. 107-127) that conformational forces that are readily apparent in X-ray and molecular models for several diastereomeric surfactants provide a solid basis for interpreting their monolayer behavior. 

Chirality is expressed on both the molecular and the supramolecular levels. Like a molecule, a supermolecule may exist in enantiomeric or diastereomeric forms. Supramolecular chirality results both from the properties of the components and from the way in which they associate. 

Studies aimed at clarifying the mechanism of enantioselection associated with 41, making use of a combination of molecular dynamics calculations and NMR techniques <2001J(P2)1685>, as well as X-ray diffraction data <20040BC3470>, have led to a better comprehension of the subtle interactions that characterize and differentiate the diastereomeric complexes of tetra-acid 41 with the D,L-amino acid pairs. 

In Figure 22-2, the diastereomeric associates between the selectand/selec-tor are formed through one, two, or three substituents of the asymmetric carbon. The chirality of the selector or the selectand can arise from an asymmetric carbon, the molecular asymmetry, or the helicity of a polymer. Also, the bonds between substituents of the selectand and the selector can involve a single bond, but could also involve multiple bonds or surfaces. Such bonds represent the leading interactions between selectand and selector. Only when the leading interactions take place and the asymmetry of the two bodies are... 

Chiral separation can be observed when there is a suitable difference between free energies (AG) of diastereomeric associates formed by R- and S-enantiomers of selectand and a chiral selector (SO). The energy differences can be directly attributed to the chiral recognition phenomena, involving complementarity of size, shape, and molecular interaction of SO and SA molecules. These factors are also controlled by experimental conditions such as temperature, mobile phase... 

Observations on the chirality of crystals made it possible for Pasteur and others to identify dissymmetry as the true origin of optical activity. It became quickly evident that the molecular chirality associated with a given compound could be directly evident in the bulk crystallography of that compound. This in turn led to observable differences in a variety of physical properties, such as the melting point and the solubility of such species. Many chiral molecules have been observed to resolve spontaneously upon crystallization, forming enantiomorphic crystals that can be physically separated. Others can only be resolved through the formation and separation of diastereomeric species. 

---