Transitions linking group effects

FIGURE 7.6 Effect of the linking group on the odd-even character of the PM-type spacer. The transition entropy, 45ni/R, oscillates with the numher of methylene units (n) of the spacer. The upper three curves represent the dimer, and the lower two are for the monomer LCs. The filled symbols indicate the carbonate-type dimer, a ,ester-type dimer, a,< -bis[(4,4 -cyanobiphenyl)carbonyloxy]alkane (CB-n triangles), and the ether-type monomer LC, 4 -n-aUcoxy-4-cyanobiphenyl (nOCB squares). 

Increasing the length of the molecule is a major contributor to increasing transition temperatures, and to aid this effect a linking group is often placed between the rings ... 

Metal hydroxides of first- and second-group elements can enhance ortho substitution, the degree of which depends on the strength of metal-chelating effects linking the phenolic oxygen with the formaldehyde as it approaches the ortho position. Transition metal ions of elements such as Fe, Cu, Cr, Ni, Co, Mn, and Zn as well as boric acid also direct ortho substitutions via chelating effects (Fig. 7.9). 

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