Type in CSPS

Most of the molecular modeling studies involving Type II CSPs, as illustrated above, do not directly involve computations of the analjrfe with the CSP to discern where and how chiral recognition takes place. The reason for this is, clearly, the lack of structural information about these polymeric CSPs. This is in contrast to modeling studies of Type I CSPs described in an earlier section of this chapter and to those computational studies of Type III CSPs discussed below. 

A slightly different approach by Roussel and Favrou [61] was taken toward understanding chiral separations by cyclodextrins and quantifying the effect of substituents. Using compounds 23-30 described above, the authors carried out chiral separations using p- and T cyclodextrins as a chiral mobile phase additive. The full factorial design methodology was applied to k o, (retention without cyclodextrin), k (+), k (-) and a for two different achiral stationary phases. In the presence of y-CD on a nonendcapped phase, equations 22-25 were derived. 

From eqn. 22 one finds that without y-CD, two structural features, X2 and X3, have a predominant influence on retention. A change in electrostatics by converting the carbonyl to thiocarbonyl does not much influence the capacity factor. In contrast, when y-CD is present, equations 23 and 24 show Xi, X2 and X3 to affect retention similarly for each atropisomer, and, these influences are weak. In the presence of the chiral modifier y-CD, then, the change from carbonyl to thiocarbonyl becomes important with the latter being more retained. Finally, from eqn. 25, it is seen Xi and X3 are most important. 

A theoretical model for this behavior, in terms of the asymmetry in the it-facial molecular electrostatic potentials of the phenyl ring, was proposed by Camilleri and Rzepa [62]. Using semiempirical molecular orbital theory the electrostatic potentials on the syn and anti faces of the aryl ring were determined. Those analytes with larger electrostatic differences between syn and anti faces appear to correlate with separations but precisely how this electrostatic asymmetry works was not explained. 

Most of the aforementioned studies represent quantitative structure-retention relationship studies where a series of analytes are used as probes of enantiodiscrimination. There are, however, a number of atomistic molecular modeling studies where the interactions of chiral guests (analytes) with chiral hosts (CSPs) are explicitly determined. Here guest and host are considered as transient diastereomeric complexes and both liquid and gas chromatographic separations have been modeled. 

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