Crystal structure effect

Muller et al. (206) have been concerned with the Lii ni white lines for Ca, Ti, Cr, Co, and Cu and LHi white lines for Sr, Zr, Nb, Ru, Rh, and Pd. In the latter series the shape of the white lines with increasing atomic number is determined by (i) the narrowing and increase of the 4d density of states and (ii) progressive filling of the 4d band. Crystal structure effects are much less conspicuous in the L-edge spectra than in the K-edge spectra. The shapes of the individual L, white lines of the 3d elements Ca, Ti, Cr, Co, Ni, and Cu are similar to those encountered in the 4d series, but the 2p1/2-2p3/2 spin-orbit splitting here is so small (from 6 eV for Ca to 20 eV for Cu) that the Lji and LfH spectra are superimposed. The relative size of the Lm and L white lines follows approximately the multiplicity ratio 2 to 1 of the 2p3/2 and 2pi/2 core states. 

Information on those features of the three-dimensional structure of a molecule (small or large) that are important with respect to their chemical or biochemical reactivities can often be obtained by comparisons of analogous molecular geometries in various different crystal structures. Effective techniques for comparing three-dimensional structures are therefore crucial to these types of analyses. If the structure of, for example, the citrate ion is compared as it occurs in several different crystal structures, some measure will be obtained of the possible conformational variability of this ion, as shown in Figure 16.1. Each such conformation determined from a crystal structure analysis represents a low-energy form of the ion. 

To enable an atomic interpretation of the AFM experiments, we have developed a molecular dynamics technique to simulate these experiments [49], Prom such force simulations rupture models at atomic resolution were derived and checked by comparisons of the computed rupture forces with the experimental ones. In order to facilitate such checks, the simulations have been set up to resemble the AFM experiment in as many details as possible (Fig. 4, bottom) the protein-ligand complex was simulated in atomic detail starting from the crystal structure, water solvent was included within the simulation system to account for solvation effects, the protein was held in place by keeping its center of mass fixed (so that internal motions were not hindered), the cantilever was simulated by use of a harmonic spring potential and, finally, the simulated cantilever was connected to the particular atom of the ligand, to which in the AFM experiment the linker molecule was connected. 

Calculations of this type are carried out for fee, bcc, rock salt, and hep crystal structures and applied to precursor decay in single-crystal copper, tungsten, NaCl, and LiF [17]. The calculations show that the initial mobile dislocation densities necessary to obtain the measured rapid precursor decay in all cases are two or three orders of magnitude greater than initially present in the crystals. Herrmann et al. [18] show how dislocation multiplication combined with nonlinear elastic response can give some explanation for this effect. 

Raman spectroscopy is sensitive to ordering arrangements of crystal structures, the effect depending on the type of order. Ordering atoms onto specific lattice sites in... 

In addition to the nucleating agents discussed in Section 18.4, many other materials have been found to be effective. Whilst the nylons may be self-nucleating, partieularly if there is some unmelted crystal structure, seeding with higher melting point polymers can be effective. Thus nylon 66 and poly(ethylene terephthalate) are reported to be especially attractive for nylon 6. 

The characterisation of materials is a central necessity of modern materials science. Effectively, it signifies making precise distinctions between different specimens of what is nominally the same material. The concept covers qualitative and quantitative analysis of chemical composition and its variation between phases the examination of the spatial distribution of grains, phases and of minor constituents the crystal structures present and the extent, nature and distribution of structural imperfections (including the stereological analysis outlined in Chapter 5). 

Only the bisected conformation aligns the cyclopropyl C—C orbitals for effective overlap. Crystal structure determinations on two cyclopropylmethyl cabons with additional stabilizing substituents, C and D, have confirmed file preference for the bisected geometry. The crystal structures of C and D are shown in Fig. 5.8. 

J. Holuigue, O. Bertrand, E. Arquis. Solutal convection in crystal growth effect of interface curvature on flow structuration in a three-dimensional cylindrical configuration. J Cryst Growth 180 591, 1997. 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

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