Planar zig-zag conformation

Linear guest molecules are included along these canals in an extended planar zig-zag conformation. Branched molecules are generally excluded unless branching occurs near the end of a long linear chain, but aromatic derivatives can be included if they have a long alkyl chain 38). The review article by Takemoto and Sonoda 21) contains an excellent survey of the types of molecules known to form urea inclusion compounds and of the means used to study their detailed conformations and thermal motion. 

The molecular arrangement within the crystal units cells of nylon is governed by the need to maximize hydrogen bonding between adjacent chains. Hydrogen bonding within crystallites is facilitated by the fact that nylon chains adopt planar zig-zag conformations with dipoles perpendicular to the chain axis to thin the plane of the molecule. Examples of nylon crystallite structures are shown in Figs. 23.8 and 23.9 for nylon 6 and nylon 66, respectively. In the... 

If the spin-spin information was being transmitted by the normal through-bond mechanism the upfield three proton signal would be expected to occur as a doublet because these protons are the only ones which can assume the required planar zig-zag conformation 77>78h Preliminary results, using the change in chemical shift method 79>, indicates that the energy barrier to rotation is of the order of 20 k.cal.mole O. As expected the silicon compound (39) shows a nine proton doublet... 

Figure 15 (A) Polyethylene chain in planar zig-zag conformation. (B) Energy diagram of...

Polyethylene has been studied spectroscopically in greater detail than any other polymer. This is primarily a result of its (supposedly) simple structure and the hope that its simple spectrum could be understood in detail. Yet as simple as this structure and spectrum are, a satisfactory analysis had not been made until relatively recently, and even then significant problems of interpretation still remained. The main reason for this is that this polymer in fact generally contains structures other than the simple planar zig-zag implied by (CH2CH2) there are not only impurities of various kinds that differ chemically from the above, but the polymer always contains some amorphous material. In the latter portion of the material the chain no longer assumes an extended planar zig-zag conformation, and as we have noted earlier, such ro-tationally isomeric forms of a molecule usually have different spectra. Furthermore, the molecule has a center of symmetry, which as we have seen implies that some modes will be infrared inactive but Raman active, so that until Raman spectra became available recently it was difficult to be certain of the interpretation of some aspects of the spectrum. As a result of this work, and of detailed studies on the spectra of n-paraffins, it now seems possible to present a quite detailed assignment of bands in the vibrational spectrum of polyethylene. 

Very early examples in this area are the predictions that polyethylene should crystallize in a planar zig-zag conformation, poly(oxymethylene) in a helix having nine repeat units per live turns, some isotactic poly(a-olefins) in helices having three repeat units per single turn, and a number of polypeptides and proteins in the now-famous a-helices. All these predictions, and many others, have been confirmed experimentally.39... 

There has been a great deal of interest in the UV-visible spectroscopy of the polygermanes, particularly in comparison with the analogous chains that have silicon or tin backbones.41,42 Both conventional and Raman spectroscopy have been employed. One interesting observation is that the symmetrically disubstituted polyfdi- n - a I k y I g e n nanes) exhibit thermochromic transitions at temperatures below those of their polysilane analogues. Another is the conclusion that in poly(di-n-hexylgermane) the side chains adopt trans-planar conformations as in the polysilane counterpart. The two chains are also similar in that both backbones can, under certain circumstances, also adopt planar zig-zag conformations. 

Now let s consider the entropy term. For a given crystal structure all the chains in the lattice are in identical conformations, the all trans, planar zig-zag conformation for polyethylene, for example. Upon melting, the chains escape the cage of the crystalline lattice and are allowed to take on a multitude of different shapes or conformations, providing there is sufficient bond rotational freedom. Some chains are far more flexible than others (Figure 10-46), so that the entropy change on going from the crystal to the melt will vary considerably with chain stiffness. 

The results in Table VII depict how the band structure of carbyne [-CaC] (corresponding to Y - in Table VII) is affected by periodic inclusion of selected atoms or groups which, by introducing a kink into the otherwise rectilinear chain of cart e, would provide greater conformational versatility and therefore possibly improve processability. The chains were considered in their trans planar zig-zag conformation. It is seen that any of the modifica-tions to carb e indicated in Table VII produces an increased E value and a decreased BW value. That electrical conductivity would be adversely affected is reasonable given the disruption of the conjugated system caused 1 such modifications. As yet, no clear relationship is apparent between the specific molecular nature of Y and the effect of its inclusion on E and BW the relatively minor effect for the case Y - CH2 is notewSrthy and under continued study. 

The C—C backbone of SPS takes a planar zig-zag conformation in the crystal whereas IPS takes a 3/1 helical conformation in crystal. This difference in chain conformation in the crystal affects the crystallization rate of these polystyrenes. Table 18.1 [32] compares the crystallization parameters between SPS and IPS. The work of chain folding, Q, of SPS is smaller than that of IPS, which suggests that the chain folding of SPS can take place more easily than that of IPS. Also, SPS has a smaller surface free energy. These differences are related to the crystallization rate difference between SPS and IPS, which have different stereoregularities. 

Barker, Bourne and Whiffen concluded that the empirical rules were soundly based by examining the consequences of a planar, zig-zag conformation of the carbon chain in glycitols. The writer feels that a study of the end-products of reaction is the safer approach to the problem of acetal formation, and of other reversible reactions, because deductions based on the conformations of the reactants will only be sound if these conformations, and the mechanism of the reactions, are well established. If all factors concerned could be accurately assessed, the two approaches would give identical answers. Formation of cyclic acetals seems to be the only instance in which both approaches to the problem of preferred ring structure are possible. 

For the purpose of this work, processing includes three steps PVTFA sample preparation, solvolysis, and hydration. PVTFA is a semicrystalline polymer that has not been studied extensively because of its poorly defined crystallinity. Wide-angle X-ray diffraction of unoriented polymer tends to give very diffuse scattering maxima. The polymer exhibited a distinct melting endotherm with a small AH of around 13 cal/g. In the most detailed structural study published, Bohn et al. (14) suggested that the X-ray data indicated a large-pitch helical structure for the chain. PVA, on the other hand, is believed to crystallize in a planar zig-zag conformation (15) with a much more typical AH of fusion of 30 cal/g (16). 

The chiroptical properties of optically active thiazoUdines derived from aldoses and natural mercapto aminoacids was studied [92]. PMR parameters for thiazoHdine-4(R)-carboxylate derivatives were obtained by computer-assisted analysis of their spectra. The polyacetoxy-alkyl side chains have planar zig-zag conformations. The configurations at C-2 in the di-astereoisomers were ascertained on the basis of the Jnh.ch coupling constants [93]. The conformation and stereochemistry of diastereomeric sulfoxides of methyl 3-acetyl-5,5-dimethyl-2-(D-galactopentaacetoxypentyl)-1,3-thiazolidine-4-carboxylate 1-oxides were performed by H- and C-NMR spectral analysis [94]. 

Table 2 shows that, apart from diamond and graphite, the highest moduli are encountered with the planar zig-zag conformation. The examples are polyethylene and polyvinyl alcohol. There is a marked drop in going to the very lazy helix (15/7) of polytetrafluoroethylene, which departs only slightly from the planar zig-zag. 

Fig. 2.16 A sketch of a 3D view through a polyethylene single crystal showing the planar zig-zag conformation of molecules referred to the orthorhombic lattice of PE (from Lin and Argon (1994b) courtesy of Springer-Verlag).

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