Trans conformation crystalline structures

In the uniaxially oriented sheets of PET, it has been concluded that the Young s modulus in the draw direction does not correlate with the amorphous orientation fa or with xa "VP2(0)> 1r as might have been expected on the Prevorsek model37). There is, however, an excellent correlation between the modulus and x,rans,rans as shown in Fig. 15. It has therefore been concluded 29) that the modulus in drawn PET depends primarily on the molecular chains which are in the extended trans conformation, irrespective of whether these chains are in a crystalline or amorphous environment. It appears that in the glassy state such trans sequences could act to reinforce the structure much as fibres in a fibre composite. 

With regard to the molecular structure of CCHn, both cyclohexyl rings having a chair conformation are substituted in the equatorial positions and the alkyl chain is completely extended in the all-trans conformation. The cyclohexyl rings are nearly coplanar. The crystal structures of the investigated CCHn show that various types of molecular overlapping are present in the crystal. The molecular packing in the crystalline state is quite different in all three compounds. 

In the case of crystals, both intramolecular (conformational) and packing energies should be taken into account simultaneously. Such a total energy minimization method, with suitable crystallographic constraints, has been applied in different steps of the analysis of crystalline structures of three different synthetic polymers. Structures of these molecules, namely, isotactic trans-1,4-poly-penta-1,3-diene (ITPP), poly-pivalolactone (PPVL), and isotactic cis-1,4-poly(2-methyl-penta-1,3-diene)(PMPD), do not have troublesome features such as charged groups, counterions, and solvent molecules. 

Poly(vinylidene fluoride) with the sequence (CF2-CH2)n is a well known ferroelectric polymer in the crystalline state where the chains adopt an all-trans conformation [424]. This structure could provide interesting properties if used as building block of LC. 

Conformations based on the few available X-ray structures of phospholipids in the crystalline state, generated from energy minimization or from conformational libraries of phospholipid alkyl chains, have been used as starting structures for the simulation. It has been argued that the latter is preferable in order to save time necessary for equilibration when starting from all-trans conformations of the alkyl chains observed in X-ray structures. To build up the configuration of the system, e.g. the lateral positions of the phospholipids, either crystal structure data or programs... 

The crystalline structure of the stereocomplex has not yet been elucidated. Liquali et al.301) proposed a model for the stereocomplex (see Eq. (13)) involving two flat syndiotactic chains with the same number of trans- and gauche-conformations which may lie in the grooves of one isotactic 5rhelix. More recently, Tadokoro et al.302) found that iso-PMMA forms a lOj-helix. Further-... 

Lewis acids prefer to lie syn to the smaller substituent of the carbonyl, e.g. syn to H for aldehydes, anti to —OR for simple alkyl esters. In a, 3-unsaturated systems, Lewis acid coo ination syn to the double bond favors the s-trans conformation, but in two crystal structures, where coordination anti to the alkene occurs, s-cis complexes are observed. - " Finally, chelation with titanium and tin occurs readily and yields stable, crystalline complexes. 

The true conformation in the crystalline structure of a polymer is more complicated, the backbone chain often taking a helical shape in which alternate chain bonds take trans and gauche positions, such that the carbon chain is not in a plane. For example. 

A number of structural investigations of polyorganophosphazenes has been carried out. Poly(dipropylphosphazene) can be characterized by one ordered crystalline phase and a thermotropic mesophase. The crystalline phase has a monoclinic symmetry with polymer chains with a nearly planar cis-trans conformation of the polymer backbone, in which the PNP angle is about 10° larger than in the cyclic trimer. ... 

Monodisperse diblock oligo (ethylene oxide) mono-n-alkyl ethers have also been prepared and studied.143 Oligomers with hydroxy-ended E-blocks formed bilayer crystals (cf. Section II.G and Figure 22), and the methoxy-ended oligomers formed monolayer crystals. The helical oxyethylene blocks were perpendicular to the layer plane, while the alkyl blocks were generally tilted at 30° to the layer normal. Monodisperse triblock oligomers with a central methylene block and outer oxyethylene blocks were also studied subsequently144 and both fully crystalline and partly crystalline structures were found, with chains in wholly trans-planar and mixed trans-planar/helical conformations. 

The crystalline disorder is due to the presence of the two possible (cis) and (trans) conformers (Figure 6.6). The enantiopure compounds (P)-(TTF-oxa)2AsF6 (P)-(6.13) and (5)-(TTF-oxa)2AsF6 (5)-(6.13) crystallize in the P-1 space group with one anion and two independent donor molecules with a (cis) and (trans) conformation. Overall, the stracture of the enantiopure compounds is less disordered, even if the structure of the ft type is fairly close to the racemic compound, with a head-to-tail arrangement of the cations. 

The crystal structure of l-(3 -methylphenylazo)-2-naphthylamine was determined by the X-ray diffraction method. No dynamic disorder was observed in the crystalline state, and the azo group was found to adopt trans-conformation with six-membered ring formed by the intramolecular hydrogen bond. In the solid state, CP MAS NMR spectrum of its acetamide derivative the doublet signals were found. The DFT computations were employed for the conformational analysis of 2-naphthylamines. 

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