Crystal polymorphism, methylene chain

Figure 2.18. Note in the form II spectrum (b) that two methylene carbon resonances appear at 49.1 and 38.1 ppm (vs. TMS). As half the methylene carbons in form II s-I are trans to both of their y-substituents(CH s), while the other half are gauche to both of theirs, it is not surprising that two methylene carbon resonances separated by 2 x (11 ppm) are observed. Identical behavior is observed [78] for s-PP, which also crystallizes in the. ..ttggttgg... helical conformation. All the methylene carbons in the all-rrans form I crystalline chains are trans to their y-substituents, consistent with the single CH2 resonance observed for this polymorph at 48.4 ppm, which is virtually coincident with that half of the methylene carbons in form 11 crystals (49.1 ppm) that are also not shielded by their y-substituents.

The two most extensively studied series of liquid crystal dimers are the a,characteristic behavior of dimers. The acronyms used to refer to these series are BCBOn and m.0n0.m, respectively, in which n refers to the number of methylene groups in the flexible spacer and m that in the terminal alkyl chains. The particular interest in these series arises, in part, because they may be considered to be the dimeric analogues of the 4- -alkyloxy-4 -cyanobiphenyls (nOCB) and the N-(4-n-alkyloxybenzylidene)-4 -n-alkylanilines, respectively, which are probably the most widely studied series of conventional low molar mass liquid crystals. In addition, members of the m.OnO.m series were the first dimers for which extensive smectic polymorphism was observed. 

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