Crystal gauche

In addition to existing as helices in crystals, there is evidence that certain vinyl polymers also show some degree of regular alternation between trans and gauche conformations in solution. In solution, the chain is free from the sort of environmental constraints that operate in a crystal, so the length of the helical sequence in a dissolved isotactic vinyl polymer may be relatively short. 

Unlike other synthetic polymers, PVDF has a wealth of polymorphs at least four chain conformations are known and a fifth has been suggested (119). The four known distinct forms or phases are alpha (II), beta (I), gamma (III), and delta (IV). The most common a-phase is the trans-gauche (tgtg ) chain conformation placing hydrogen and fluorine atoms alternately on each side of the chain (120,121). It forms during polymerization and crystallizes from the melt at all temperatures (122,123). The other forms have also been well characterized (124—128). The density of the a polymorph crystals is 1.92 g/cm and that of the P polymorph crystals 1.97 g/cm (129) the density of amorphous PVDF is 1.68 g/cm (130). 

Baumeister et al. [116] described the crystal structure of 4-(2-cyanoethyl)-cyclohexyl 4-n-pentylcyclohexanecarboxylate. The almost fully stretched molecule is only distorted by the gauche conformation of the cyanoethyl group. The crystal packing is characterised by a discrete layered arrangement with an antiparallel orientation of neighbouring molecules. With respect to intermolecular interactions, no remarkable contacts between the cyano groups can be observed. 

Examination of the backbone torsion angles in a number of crystal stractures of /9-alanine-containing peptides reveals that the conformation around the C(a)-C(/9) bond of /9-alanine residues is essentially gauche or trans (anti) with values close to 60° or 180°, respectively [158]. Populating the gauche conformation of /9-ami-... 

Z = 8 Dx = 3.09 R = 0.076 for 1,253 intensities. The crystal structure contains two symmetry-independent molecules. The two l-ascorbate ions differ significantly only in the orientations of the terminal hydroxyl groups of the side chains, with C-6 - 0-6 to C-5 - 0-5 gauche and trans, respectively. The unprotonated oxygen-atoms are 0-1 and 0-3, with C-O distances ofbetween 127 and 133 pm. One Tl+atom has four oxygen atoms at distances of 258 to 298 pm the other has three, at distances of 258,268, and 300 pm. The 0-3 atoms have two Tl+ contacts,... 

The conformation of the chains of isotactic polymers in the crystalline state is generally helical and corresponds to a succession of nearly trans and gauche torsion angles, the exact values depending on the bulkiness of the side groups. Molecular mechanics calculations have been extensively used for the prediction of the chain conformation of polymers in the crystal.29... 

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