Heterochiral interactions

Vapor Pressure. Differences in vapor pressure (i.e., in the heat of sublimation [Ai/subiim]) have been observed for enantiomers and racemates. It has been suggested that heterochiral interactions between solid enantiomers (as in a racemic compound) can be either stronger or weaker than the homochiral interactions, i.e., between the crystals of the 1 1 mechanical mixture [32]. These interactions have not been widely investigated in pharmaceutical formulations containing racemates or enantiomers. The difference in A/fsubiim can also be used to separate enantiomers from a mixture, and it is thought to be a better method of separation, at least in some cases, than recrystallization. 

The simplest model taking into consideration heterochiral interactions was given by Franck (1953) ... 

The four possible stereomers of a chiral surfactant with two asymmetric centers within the polar head group have been synthesized and their absolute configuration determined by X-ray diffraction. One of the diastereomers exhibits a chiral discrimination when spread on water interface the monolayer racemic film undergoes a phase transition from a liquid-expanded towards a liquid-condensed phase upon compression, while the pure enantiomers only have a liquid-expanded phase, as revealed by the measured pressure-area isotherms. The transition pressure-composition diagram indicates that heterochiral interactions are favored. Our results are compared to predictions of Andelman and de Gennes based upon a statistical model. 

Racemic species contain equal amounts of two enantiomers and raise various issues concerning how to count the number of substances present in a sample. Most racemic species are distinct compounds (racemates) one solid compound forms, because heterochiral interactions dominate. Racemic molecules (or complexes ) may exist in the liquid state. Enantiomers may also form solid solutions or racemic conglomerates. The latter are eutectic mixtures of (-h) and (—) enantiomers two separate solid phases form, each crystal type comprises a single enantiomer (because homochiral interactions dominate). In practice, spectra of racemic and enantiomeric forms are indistinguishable, although one would be inclined to consider the racemate and the separate enantiomers as three different species. In the solid state, properties such as melting point, solubility behaviour, and density are different for the racemate and the respective enantiomers. Hence, some of their thermodynamic properties must be different. 

On the other hand, in the single crystals prepared from equivalent amounts of heterochiral 1 1 complexes, a pair of two heterochiral 1 1 complexes are incorporated in a unit cell to form a layered structure with alternate layer distances of 7.33 and 7.6 A. Two perchlorate ions stay in the narrower gap, and two additional acetone molecules as crystallization solvent occupy the wider gap. The perchlorate ions interact with two axial water ligands by hydrogen bonds (3.71 and 3.77 A) to construct a layered structure. The adjacent two molecules of heterochiral 1 1 com-... 

Since the walls between heterochiral domains are unacceptable defects in an LC display, enantiomericafly enriched dopants are added to the LC to favor one sign of twist over the other in actual devices, providing a monodomain in the TN cell. It should be noted, however, that the chirality of the structure derives from the interaction of the LC director with the surfaces the molecular chirality serving simply to break the degeneracy between mirror image domains to favor one over the other. 

The Dq value of the complex between Cr and 1-butanol (2.6 0.2 kcal mol ) well conforms to the approximate value of 2-3 kcal moP indirectly estimated for the dissociation energy of the complex between F/ = (l )-(- -)-2-naphthyl-1-ethanol and methanol. ° Concerning the diastereomeric complexes, the homochiral adducts are invariably more stable than the heterochiral ones. This trend extends to the corresponding Si excited complexes as well. This observation, coupled with the appreciable deviation from linearity of the corresponding Ap values (Fig. 9), corroborates the view that the interaction forces in these complexes are affected by steric congestion to a different extent. Their sensitivity to steric factors is demonstrated by the diverging observations that (Table 4) (i) in the diastereomeric... 

Figure 19. Intermolecular interactions in the 8-form crystal of racemic NPMe3 viewed (a) down the a axis and (b) along the a axis, and (c) the schematic representation of the heterochiral steplike sheet structure, in which the ellipsoid and circle indicate the ammonium ion and sulfonate ion, respectively. The dashed lines show the intermolecular hydrogen bonds and electrostatic interactions. (Reprinted with permission from ref 26. Copyright 2003 American Chemical Society.)...

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