Other distortions

Tetbdal elongaion. This distortion is also known as tetrahedral thickening and implies an expansion for the tetrahedra in the direction perpendicular to the T sheet and a lateral compression (Radoslovich and Norrish 1962). The effect is measured by the angle t = Li= Ob-T-Oa)i /3 (xideai = 109.47°). It is particularly active in dioctahedral micas where it is related to the presence of the vacant octahedral site (Lee and Guggenheim 1981). 

OcUbdul flatning (orickening). This distortion is measured by the angle 

These distortions are not independent variables. In fact, besides specific aspects in part mentioned above, all of them are to some extent correlated with chemical substitutions and T/0 misfit. Several correlations have been proposed, particularly for the ditrigonal rotation (e.g., Lin and Guggenheim 1983 McCauley and Newnham 1971 Toraya 1981 Weiss et al. 

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Part of a 15-nm long, 10 A tube, is given in Fig. 1. Its surface atomic structure is displayed[14], A periodic lattice is clearly seen. The cross-sectional profile was also taken, showing the atomically resolved curved surface of the tube (inset in Fig. 1). Asymmetry variations in the unit cell and other distortions in the image are attributed to electronic or mechanical tip-surface interactions[15,16]. From the helical arrangement of the tube, we find that it has zigzag configuration. 

If a molecule having u center of symmetry undergoes a Juhn-TeHer distortion, the cemcr of symmetry must be maintained, and this is Ihe case when oct.ihedra undergo tetragonal distortions. Can you think of any other distortion of an octahedral complex that would be consistent with this principle 1... 

Another important factor in determining the strength of a repressor-operator interaction is the twist of the DNA or any other distortion of its regular helical structure. For example, in the center of the cro 434-operator complex the DNA is wound, while at the ends it is underwound.92 Conformational changes in either the repressor or in the DNA or in both may be needed to provide optimal binding. As is seen in Fig. 28-3 the lac repressor causes a distinct bend in the DNA. 

Some compounds in this series in fact retain this structure. Others distort, and it is easy to see why. Take GdPS. If we assign normal oxidation states of Gd3+ and S2, we come to a formal charge of P" on the dense-packed P net. From a Zintl viewpoint, P- is like S and so should form two bonds per P. This is exactly what it does. The GdPS structure72 is shown in 93, which is drawn after the beautiful representation of Hulliger et al.72 Note the P-P cis chains in this elegant structure. 

Cellulose powders can be created by cutting fibers into small particles, perhaps with a Wiley mill (Arthur H. Thomas Company, Swedesboro, New Jersey). On a laboratory x-ray system, powder diffraction patterns take 30 min. The positions of the peaks indicate the polymorphic form (I-IV) the powder diffraction pattern is often used as a fingerprint for comparison with the known pattern for a given crystalline form [207]. The breadth of the peaks is related to the extent of crystallinity (Figure 5.17, bottom). Using the Scherrer formula [245,246] and assuming no other distortions, the crystallite size can be calculated. Values for cotton perpendicular to the molecular axis are around 40 A. That corresponds to a 6x6 array of... 

Acoustic streaming, by definition, is the development of time independent large fluid velocities in a medium under the influence of an ultrasound wave. The primary causes of acoustic streaming are the reflections and other distortions of the wave propagation. Oscillations of cavitation bubbles may also contribute to acoustic streaming. The shear stresses developed by streaming velocities may affect the neighboring structures. ... 

It should be remarked here that the presence of a ion at a cubic site does not necessarily mean that the observed distortion must be related to the JT effect. In fact, we can not forget that this possibility is always in competition with other distortions driven by a PJT effect involving an odd mode. For instance all d impurities in... 

Developed by Englman and Halperin [4], the above steps represent the traditional way in the cooperative JT problem. It s conceptual advantage and important part is solving the respective one-cell JT problem (9) including the low-symmetry mean field (10) of all other distorted cells. In this way, both effects, the dynamic strengthening chemical bonds with low-symmetry lattice distortions and the intercell elastic coupling, are included. 

DXM contains chemicals that affect the way the brain functions. Abusing this medication can cause confusion, paranoia, and other distorted perceptions. 

Synthetic fibers are generally thermoplastic, and the careful use of heat—either wet or dry—has been used to set the twist or weave in fabrics. Heat setting for nylon, although it is not a chemical process, is necessary to prevent unwanted wrinkles or other distortions from appearing in subsequent processing and use. Glass fiber fabrics are subjected to the coronizing process where the fabric is heated to about 1100° F. 

The Zimm-plot technique, like q.l.s., also involves a difficult extrapolation to zero angle. Another difficulty is that its qiplication assumes that the particles are Rayleigh-Cans-Debye scatterers (namely, that there is no change of phase or other distortions of the incident radiation by the particle). For the larger mucins in particular, this may not be the case. 

The aim of the present communication is to discuss possible reasons for these and other distortions and to describe models with which one can predict the corresponding structures. The correlation of the ensuing structures with molecular properties is also discussed, and answers are sought to the question whether an accurate structural prediction might help to understand specific distortions. In addition, some fascinating properties and applications of bispidine complexes are presented and models are discussed, which assist their thorough understanding. 

A number of transition metals also show electronically driven asymmetries in their coordination environments. Typical of these are and U which form one or two strong bonds to O to give the vanadyl (VO) " and uranyl (U02) complexes. Cu typically shows an elongation of two trans bonds in its octahedral coordination sphere [46]. Cu and Hg ", though not strictly transition metals, tend to form two strong colinear bonds. Other distortions tend to be weaker and are only expressed when other factors favor a distorted crystal structure. The distortion of the Ti environment in BaTiOi is primarily a steric effect (see Section 10.6.2), but the polarizability of the Tr cation probably contributes. Similar effects are found for other cations with a d° configuration [47]. Most of these effects are not well understood in detail but their presence must be taken into account when using the bond valence model. 

Other distortions may arise and these are exemplified for Cu(II) complexes in Section 21.12. 

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