Ciystal impurities, effects

Performance information for the purification of p-xylene indicates that nearly 100 percent of the ciystals in the feed stream are removed as produc t. This suggests that the liquid which is refluxed from the melting section is effectively refrozen oy the countercurrent stream of subcooled crystals. A high-meltingproduct of 99.0 to 99.8 weight percent p-xylene has been obtained from a 65 weight percent p-xyfene feed. The major impurity was m-xylene. Figure 22-12 illustrates the column-cross-section-area-capacity relationship for various product purities. 

The usefulness of quadrupolar effects on the nuclear magnetic resonance c I 7 yi nuclei in the defect solid state arises from the fact that point defects, dislocations, etc., give rise to electric field gradients, which in cubic ciystals produce a large effect on the nuclear resonance line. In noncubic crystals defects of course produce an effect, but it may be masked by the already present quadrupole interaction. Considerable experimental data have been obtained by Reif (96,97) on the NMR of nuclei in doped, cubic, polycrystalline solids. The effect of defect-producing impurities is quite... 

Phosphonitrilechloride trimer with an impurity of tetramer is a ciystal-line product (the melting point is. 108-114 °C) with a light odour. It has a slight irritating effect on mucuous membranes. Trimer and tetramer can be separated by fractional vacuum distillation, fractional crystallisation or vacuum sublimation. 

All of these changes in ciystal habit caused by kinetic factors are drastically effected by the presence of impurities that adsorb specifically to one or another face of a growing ciystal. The first example of crystal habit modification was described in 1783 by Rome de Lisle [77], in which urine was added to a saturated solution d NaCl changing the crystal habit from cubes to octahedra. A similar discovery was made by Leblanc [78] in 1788 when alum cubes were changed to octahedra by the addition of urine. Buckley [65] studied the effect of organic impurities on the growth of inoiganic crystals from aqueous solution, and in Mullin s book [66] he discusses the industrial importance of this practice. 

Another possible approach for stereospecific synthesis is the use of zeolites with appropriate transition metal fi amework ions to effect asymmetric chemistry. One basis of the prerequisite asymmetry may be the disruption of the local symmetry introduced by the size and the electronic structure of the chosen transition-metal ion impurity contained in the ciystal fi amework. additional chemical manipulations, such as the replacement of the oxide ligand or the addition of activating ligands, may result in desired catalytic activity and selectivity and the maintenance of uniformity among active sites. 

The optical studies performed on most samples of table 1 were aimed at different aspects of the f-electron properties. A considerable amount of the work was coneemed with the energy level shifts under pressure. From these shifts, variations of free-ion parameters, ciystal-field parameters or crystal-field strengths with pressure have been deduced. Other studies concentrated on changes in lifetimes or intensities, the efficiency of energy transfer between rare earths or rare earths and other impurities or on electron-phonon coupling effects under pres-strre. The various aspects investigated under high pressttre will be presented within the next sectiorrs. 

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