Tetragonal packing

Fig. 2.12 (Continued) (b) Tetragonal packed oxide array in a-Li2.5Alo.5Si04 showing location of distorted channel sites.

Figure 1.32 Rod packings (a) hexagonal (honeycomb) packing (b) tetragonal packing (c) tetragonal layer packing and (d) primitive cubic rod packing. (After O Keeffe Andersson, 1977).

Figure 15.5 Ordered microdomain structures of 3-miktoarm star terpoiymers in three-dimensional space. A and B > 0.5, pg > 0.5) are shown in gray and black, respectively, (a) Honeycomb structure (b) tetragonally packed cylinders (c) parallel plane structure (d) cylinder-shell structure. Schematic of section perpendicular to the cylinders, J is the rich junctions region and I the region containing the mixtures of blocks A-C and junctions. He et al. [35]. Reproduced with permission of Elsevier.

Fig. 2.5 a Depiction of the tetragonal packing in fluorobenzene via C-H- F-C and interactions (ref code PACFAQ crystallizes in [64]. b Formation of a molecular sheet... 

Higher values of the ratio r/R, close to V2 -1 (similar to the situation stabilizing NaCl), are obtained in the cases of many vinyl polymers with chains in complex helical conformation s M N) with a fractional ratio MIN, ranging between 3 and 4, such as form I of isotactic poly(4-methyl-l-pentene) (7/2 helix) [38], form II of isotactic poly(l-butene) (11/3 helix) [46], isotactic poly(m-methylstyrene) (11/3 helix) [86], and syndiotactic poly(4-methyl-l-pentene) (12/7 helix) [72]. In these cases, a tetragonal packing of enantiomorphous chains. 

In other optically active polymers, as well as in some polymers from achiral monomers, chiral structures, characterized by all isomorphous helical chains in the unit cell, have been found. For instance, in isotactic poly(5-methyl-l-hexene) [94], isotactic poly(ubutylacrylate) [95], and optically active isotactic poly((S)-5-methyl-l-heptene) [94b,c], chiral pseudo-hexagonal or tetragonal packing with isomorphous 3/1 or 4/1 helical chains have been found. 

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