Parity, chiral nematics

Fig. 14, then it is found that the number of atoms (n) that the chiral center is removed from the rigid central core determines the handedness of the helical structure of the chiral nematic phase. As the atom count by which the chiral center is removed from the core (n) switches from odd (o) to even (e) (parity), so the handedness of the helix alternates from left to right or vice versa. Sim-... 

Subsequent experimentation by contact studies with other chiral systems (/ )- and (S)-, of differing parity revealed the following helical twist sense relationships, entries 1 to 4 in Table 1. These results form the basis of a set of empirical rules termed the Gray and McDonnell rules [32] and are particularly useful in predicting the properties of a chiral nematic phase of a given com-... 

We have already seen that the transitional behavior of dimers depends strongly on the length and parity of the spacer. Blatch et al. [33] studied two series of chiral dimers, one symmetric (5)2MB.OnO.(5)2MB, and the other nonsymmetric, CBO-nO.(5)2MB, (see Fig. 1) in order to establish whether the form chirality of the chiral phases would also depend critically on the parity of the spacer. For the CBO-nO.(5)2MB series with n=l and 9 a blue phase was observed but not for n = 6 and 8. This was rationalized in terms of the smaller pitch for the odd relative to the even mem-bered dimers which arises from the smaller twist elastic constant of odd dimers and is related to their lower orientational order. Surprisingly, the helical twisting powers of the dimers in a common monomeric nematic solvent appear to depend solely on the nature of the chiral group and not on its environment. Thus similar helical twisting powers are observed for both odd and even membered dimers. 

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