The separation of enantiomers on modified cyclodextrin columns : measurements and molecular modeling

The enantiomers of 2-chloropropionic acid methyl ester, cis-pinane, 2-bromoethylbenzene, 2-bromobutane, 2-hydroxybutane trifluoroacetyl ester, and styrene oxide have been resolved on an octakis-(3-O-butyryl-2,6-di-O-pentyl)--cyclo-dextrin capillary column, and the separation of the styrene oxide enantiomers has also been studied on columns coated with octakis-(3-O-trifluoroacetyl-2,6-di-O-pentyl)-cyclodextrin, octakis-(2,3,6-tri-O-pentyl)--cyclodextrin, heptakis-(3-O-trifluoroacetyl-2,6-di-O-pentyl)- -cyclodextrin, and heptakis-(2,3,6-tri-O-methyl)--cyclodextrin. Thermodynamic parameters (G, H, and S) were determined from variable temperature measurements. The inclusion complexes containing styrene oxide were also studied by molecular modeling techniques. It has been found that a combined molecular mechanics-molecular dynamics approach may be a valuable tool for rationalizing the qualitative trends observed in the experimental separation factors. For the inclusion complexes considered here it is shown that the orientation of the guest relative to the cyclodextrin host is determined by the size and polarity of the cyclodextrin.