Planar-zigzag conformation

The conformation adopted by a molecule in the crystalline structure will also affect the density. Whereas polyethylene adopts a planar zigzag conformation, because of steric factors a polypropylene molecule adopts a helical conformation in the crystalline zone. This requires somewhat more space and isotactic polypropylene has a lower density than polyethylene. 

Like the polymethylene hydrocarbons, acyclic carbohydrates and alditols tend to adopt the planar, zigzag conformation wherein all of the carbon atoms lie in the same plane. " However, non-bonded interactions between parallel hydroxyl groups in the 1,3 positions (sy/i-axial interaction) are the most highly destabilizing, and, in xylitol and ribitol, this... 

Among the hexitols, n.m.r. and X-ray crystallographic data " showed that galactitol and D-mannitol exist as planar zigzag conformers. Solutions of the others contain appreciable proportions of sickle and other bent carbon-carbon chain conformers. The low sweetness of the hexitols could possibly be due to increased hydrogen-bonding due to increased chain-length. 

P2X Z = 2 Dx = 1.59 R = 0.11 for 881 intensities. The acyclic molecule has the planar, zigzag conformation. The terminal OH and N-OH groups are +syn and ap, respectively. There is an interesting system of O-H N hydrogen-bonds which forms a spiral along the screw-axis. Only seven of the hydrogen atoms were located. 

Various types of morphologies and crystalline modifications of s-PS have been reported [44], One of these modifications has an orthorhombic form (space group = P2i2i2i) with a planar zigzag conformation of back-bone chain. This form corresponds to the a-phase found by Kobayashi et al. [35] and to the P-phase named by Guerra et al. [36], The crystal structure of this form was analyzed and the plane of phenyl rings within this form is normal to the chain axis (c-axis) [37-39],... 

Porat et al. performed TEM (zero-loss bright field) studies of very thin Nation films that were cast from ethanol/water solutions, and some of the conclusions are as follows. It was suggested that the backbone had a planar zigzag conformation in large orthorhombic crystallites as in polyethylene, in contrast with the helical conformation found in poly(tetra-fluoroethylene). This is an interesting result, although there are no other studies that support this view. Sulfur imaging indicated the presence of sulfonate clusters that are 5 nm in size. 

Linear macromolecules having a constitutional repeating unit such as -CH2-CHX- (X H) show two further stereoisomerisms, i.e., optical isomerism and tacticity. The stereoisomerism named tacticity has its origin in the different spatial arrangements of the substituents X. When we arrange the carbon atoms of the polymer main chain in a planar zigzag conformation in the paper plane. 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

Fig. 3. —Distance Between 0-2 and 0-4 in the Planar, Zigzag Conformation of Ribitol.

In sharp contrast to the behavior observed when the three dithio-acetals aforementioned are treated with p-toluenesulfonyl chloride in pyridine, dialkyl dithioacetals of D-arabinose, treated under the same conditions, are converted into the corresponding 5-p-toluene-sulfonates, generally isolable crystalline in high yield.72 This remarkable difference has been interpreted72" on conformational grounds the D-arabinose dithioacetals are stable in the extended, planar zigzag conformation, whereas the other three examples experience some destabilization in the extended form, because of parallel 1,3-interactions.726 Furthermore, the transition state for closure of the 2,5-anhydro ring would be quite strained in the D-arabinose series, but not in the other three.72"... 

C20H26N4O8S2 (lR)-2,3,4,5-Tetra-0-acetyl-l-(l,6-dihydro-6-thioxopurin-9-yl)- 1-S -e thy 1- 1-thio-D-arab initol122 P2i Z = 4 Dx = 1.31 R = 0.063 for 3,955 intensities. The acyclic sugar residue attached to N-9 of the 6-thioinosine ring adopts the planar, zigzag conformation. 

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