Cleavage Plane Positioning

The crystal structure of benzene itself is even more difficult, and it is only quite recently that accurate results have been obtained. The early work on naphthalene and anthracene was also inconclusive and did not at first lend any support to the idea of strictly planar molecules. The first really conclusive results were obtained for the molecule of hexamethylbenzene (Lonsdale, 1929). In the triclinic crystal structure the atoms occupy general positions, but a careful study of the intensities of the reflections, particularly those from the pronounced cleavage plane in which the molecule is found to lie, established that the molecule was planar to within narrow limits, and also that the benzene ring was a regular hexagon. Soon afterwards the more difficult structures of naphthalene and anthracene were fully analysed with the aid of absolute intensity measurements and the use of Fourier methods of analysis (Robertson, 1933a), and it was shown that the atoms were coplanar to within a few hundredths of an Angstrom unit. 

However, if a force causes one layer to move slightly, ions with the same charge will be positioned next to each other. The cations in one layer are now lined up with other cations in a nearby layer. In the same way, anions from one layer are lined up with other anions in a nearby layer. Because the anions are next to each other, the like charges will repel each other and the layers will split apart. This is why all salts shatter along a line extending through the crystal known as a cleavage plane. 

The astute observer wiU see that the fluidized bed crystaUization system shown in Fig. 11-21 has an unusual feature. Flow sonication units are in position to operate on pumped slurry from the seed beds. In other systems, the sonicators are located intemaUy in the bottom of the column. The sonifiers are power ultrasonic horns, which create sufficient cavitation and impact energy to break the crystals along cleavage planes. 

Further experimental support has been provided by the results of structure determinations of surfaces cleaned by ion bombardment and the annealing method (i5). They show that the surface atoms are rearranged from bulk positions especially for the diamond structures of the semiconductors Ge and Si. At a clean cleavage plane of silicon the Si-tetrahedra are distorted to such an extent that the interatomic distances between surface Si-atoms are 260 pm as compared to 384 pm within the crystal lattice (ij). 

In each half of the spindle, a single centrosome at the pole organizes three distinct sets of microtubules whose (—) ends all point toward the centrosome (Figure 20-3lb). One set, the astral microtubules, forms the aster they radiate outward from the centrosome toward the cortex of the cell, where they help position the mitotic apparatus and later help to determine the cleavage plane in cytokinesis. The other two sets of microtubules compose the spindle. The kinetochore microtubules attach to chromosomes at specialized attachment sites on the chromosomes called kinetochores. Polar microtubules do not interact with chromosomes but instead overlap with polar microtubules from the opposite pole. Two types of interactions hold the spindle halves together to form... 

Fig. 14.8 Transverse cleavage (shown with arrows) with a curvature of the transverse positive-sign cleavage planes, as detected in Nal(TI) single crystals 520 mm (a) and 450 mm (b) in diameter.

Figure 8.3 (a) The layered structure of aromatic acid salts. The cleavage plane exposes the apolar side of the molecules. The polar parts, with the acid moieties and M cations, are bnried. The cleavage plane properties (e.g., hydrophobicity) and unit-cell parameters can be modulated to some extent by the nature (f-butyl, chlorine, etc.) and also by the position (ortho, meta, or para) of the substituent X. Reproduced from Reference [11] with permission of Pergamon Press, (b) Combined diffraction pattern of poly(ethylene terephthalate) and the sodium salt of p-chlorobenzoic acid. The salt reflections are sharp spots the PET reflections are more arced. The contact plane of PET is (100) the chain axis is vertical. 

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