Chiral host-guest orientation

Another example of such difficulties is given in reference 36. One of the reasons for this lack of predictability is that in every host-guest system a new crystal must be made. It is easy to imagine that it would be difficult to transfer one stereo-orientation from one crystal to the next. It may prove in the future that the use of chiral porous organic solids as hosts will obviate this latter concern. 

Modem 2D techniques are applied, for example, in NMR on chiral solutes in chiral liquid crystalline solvents. NMR spectroscopy on such chiral host/chiral guest systems can be used for the spectral discrimination of enantiomeric solutes because the enantiomers show differential orientational ordering in the chiral environment. Based on this differential ordering effect (DOE), enantiomeric purity or excess can be quantitatively measured [92]. The DOE may also help to understand the intermolecular interactions involved in the chiral recogni-... 

As chiral molecules, CD hosts are one of the best chiral selectors [23]. They can also induce circular dichroism signal in an achiral guest. As shown by Zhang and Nau [24] for the complexes of p-CDwith bicyclic azaalkanes 290 or 291, this effect may allow one to determine the orientation of the latter molecule in the CD cavity. An interesting example of the influence of 11 on the guest conformation was reported by Brett and coworkers [25]. They have shown that p-amino-/ -nitrobiphenyl 292, which is planar in the solid state, becomes nonplanar in the solid state complex with 3-CD. 

Attachment of bulky chiral barriers to achieve enantiomeric selection is exemplified by ligands (61),204 (63)69 and (64).19 Ligands of type (61) are especially efficient chiral compounds with binap-thyl hinges . The orientation of the binaphthyl rings for (61) is shown more explicitly in (62).234 With optically pure (61), complete enantiomeric separation of ammonium salt racemates such as (67) is obtained.235-237 The structure has been determined crystallographically for the less stable d form of the PF6- salt of (67) with the (S,S) host (61). The results indicate that the receptor, substrate, counterion and solvent have undergone several steric concessions in order to relieve the strain imposed on the system by the visit of an unwanted guest.238... 

The chirality interaction tensor Wy is responsible for the interaction of the chiral guest, the chirality of which is described by the chirality tensor Cy, with the anisotropic host Wy = ikLkj- Ly covers the anisotropic host properties. W = Tr JT), is different from zero. The gyki are the orientational distribution coefficients of the guest in the molecular ensemble of the guest-host phase. From the two possible representations to avoid non-diagonal elements in (3.19), the representation in the system of principal axes of the order tensor is chosen instead of the principal axes of the chirality interaction tensor Wy. With Saupe s order parameters the HTP is also a sum of three terms... 

JT/3 is the contribution of the chiral guest to the HTP in the case of an isotropic orientational distribution of the guest in the anisotropic guest-host phase. Contributions to the coordinates of W introduced by other traceless tensors are canceled in the trace of Wy. From (3.20) with IF = 0, the equation of Nordio et al. [31] can be obtained, whereas with W W = 0 and (l/v )(lf22 ii) = 0 ihe approximation of de Gennes follows. In (3.19) and (3.20) the dependence on the variation of the host order is only indirectly taken into account as in the presentation of Nordio et al. The temperature dependence of the host order—especially for the isotropic W/3—is partially neglected. Experimental experience shows that there is no drastic effect. 

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