Circular dichroism, enantiomeric crystal

The first method of enantiomeric separation by direct crystallization is the mechanical technique use by Pasteur, where he separated the enan-tiomorphic crystals that were simultaneously formed while the residual mother liquor remained racemic. Enantiomer separation by this particular method can be extremely time consuming, and not possible to perform unless the crystals form with recognizable chiral features (such as well-defined hemihedral faces). Nevertheless, this procedure can be a useful means to obtain the first seed crystals required for a scale-up of a direct crystallization resolution process. When a particular system has been shown to be a conglomerate, and the crystals are not sufficiently distinct so as to be separated, polarimetry or circular dichroism spectroscopy can often be used to establish the chirality of the enantiomeric solids. 

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

Combining the structural specificity of FT-IR spectroscopy with the slereosensitivity of circular dichroism, VCD allows the determination of optical purity without enantiomeric separation or de-rivatization, as well as of absolute configuration without crystallization. Simultaneous monitoring of the optical purity of multiple chiral species, such as reactant and product molecules in a reaction process is also possible, as is the determination of conformations of biological molecules such as proteins, peptides, and DNA [131], [132J. 

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