Lateral substituents cyano groups

Common lateral substituents are the halogens and methyl, ethyl, cyano and other small groups. There are some uncommon liquid crystals that contain large lateral substituents, even aromatic groups these will be discussed later. 

For many terminal substituents, and especially for cyano compounds, the magnitude of ye is much less than expected. This, and other effects, are probably due to antiparallel ordering of the polar molecules, which is reduced by a lateral fluoro substituent adjacent to the cyano group (Table 2.5). The extent of antiparallel ordering (for example, in a mixture) cannot be accurately predicted, and the dielectric anisotropies of mixtures are, as a result, also difficult to predict precisely. 

Riichardt originally made the assumption that substituents do not exert a noticeable electronic effect on the ground state of the radical precursor. He attributed the full electronic substituent power to the radical. On the basis of this presumption, the BDEs in [24] and [25] were interpreted in terms of radical stability. The original value for the BDE in [24] (Zamkanei et al., 1983) was later slightly modified (Birkhofer et al., 1987). The BDEs were compared with those where only one cyano- or one methoxyl group was incorporated. From Table 9 (Birkhofer et al., 1989) it can be derived that phenyl, cyano-, and methoxy-groups exert an additive substituent effect on... 

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