Disorder screw

Figure 2.9 The B-Tphase diagram of MnP [13] with the magnetic field along the b-axis. Three different magnetically ordered phases - ferro, fan and screw - are separated by first-order phase transitions. The transitions to the disordered paramagnetic state are of second order and given by a dashed line.

Homopolymers such as poly[(V)-3,7-dimethyloctyl-2-methylpropylsilylene], 117, were initially studied, and the helix-helix transition was discussed in terms of an entropically driven phenomenon in which at temperatures below Tc the side chains of the helical polymer are in a very ordered state and enforce a particular screw sense, whereas above Tc, the side chains become disordered such that the main chain can relax into the opposite screw sense.314 This concept is expressed in Figure 47. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

Figure 4. Diffraction from a screw-disordered" arrangement of poly U poly A poly U molecules. The Bragg pattern from the fiber containing these triple-stranded polynucleotide molecules corresponds to a trigonal cell with a = b = 2.71 nm c = 3.65 nm. The molecules are 12, helices.

Figure 5. Composite diffraction diagrams of (a) the 11, form of poly(A) poly(U) (left,) and the 10, form of poly(C) pG (right,) and (b) the 11, form of 2 poly(U) A (left,) and the screw disordered 11, form of 2 poly(U) poly(A) (right,)...

The structure of C-DNA has hitherto been not refined since its X-ray diffraction (25) corresponded to a screw-disordered (26) fiber. We now have C-DNA in a polycrystalline system containing 9 helices of the synthetic DNA poly d(GGT)-poly d(CCA) (Figure 7a). Detailed X-ray analysis shows that C-DNA belongs to the same ttg tg+ genus as both B(IO ) and B-DNA (8 ). Figure 7b shows the negative tilt for the bases and the backward positioning of the base-pairs relative to the helix axis. 

Figure 7. (a) Composite diffraction pattern from C-DNA. Screw disordered, 28,... 

Solvent free LnL complexes (Type I) resulted from the silylamide route [184]. Crystals of Yb(trac) could be grown by sublimation (180 °C/10 2 Torr). The heptadentate ligand is wrapped around the ytterbium emerging in a disordered geometry. Both enantiomers, the right- (A) and left-handed (A) screw, are present in the unit cell. 

In order to determine whether a decrease in scattering at high angles is due to vibration effects or to disorder, the data should be measured at a series of temperatures. Only the vibration effects should show a strong temperature dependence. Displacements of atoms from their equilibrium positions can be anisotropic and are represented by anisotropic displacement parameters which, are refined by least-squares techniques together with the atomic coordinates (see Chapter 10). A further analysis of these anisotropic displacement parameters in terms of translation T, libration L, and screw S motions can give information on the nature of the molecular motion. 

OD point inhomogeneities, such as atomic disorder, vacancies, interstitials, emergence points of edge and screw dislocations,... 

The mechanism by which defects concentrate impurities is a subject of research that has important bearing on crystal growth, especially related to formation of crystalline materials for use in the electronics industry. Besides imperfections associated with isolated impurities (i.e., point defects), the other major types of structural defects are line defects (both edge and screw), planar defects, grain boundaries, and structural disorder (Wright 1989). The connection between defect formation and impurity uptake is evident in two of these defects in particular the edge defect and point defect. 

Sect. 1.2) has a less extended chain because of the more compact cii-configuration of the double bond. To arrive at an extended chain, the translational repeat has to be doubled, as illustrated in the sketch in Fig. 5.28. Chains A and B have the methyl groups substituted on carbon atoms 3 and 5, but have their chain axes reversed. Chains C and D have the methyl groups substituted on carbon atoms 2 and 6, and again, the action of the screw axis in the ab-plane turns the chain in going from C to D. These four possible arrangements of the same chain lead to disorder in the crystals, as in the vinyl helices of Fig. 5.22. The most frequent disorder occurs when pairs A-B and C-D do not alternate regularly. 

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