Subject silver halides

The use of sensitizing dyes in photography has been the subject of many studies and constitutes. still now. one of the most studied areas in specialized periodic publications (125, 126) or in textbooks (88. 127). It can be ascertained that one hundred years after Vogel s discovery of spectral sensitization, the basic mechanisms of action of dyes on their silver halide support still remain not fully understood. However, the theoretical reasons explaining why among many other dye families practically only cyanine methine dyes appear to be spectral sensitizers (128) are better known. 

It has been necessary to understand the relationship between molecular fine structure of cyanine dyes and important properties such as colour, dye aggregation, adsorption on silver halide and electrochemical potentials in order to design and prepare sensitizers with optimum performance. For general discussion of these topics and the mechanism of spectral sensitization, the reader is referred to recent surveys on the subject (B-77MI11401, 77HC(30)441). 

Rosenwasser ei alu have found that NaN, when irradia ed by y-rays at room temperature, develops a band ai 360 m/. This band may also be excited thermally although to a lesser extent. On the other hand, neutron irradiation gives bands at 660 mp and 760 v. However, near the decomposition point, the spectra of NaN3 subjected to any of the irradiation processes are the same. Apparently, many of the bands observed in NaN3 spectra may be connected with the decomposition of the compound. Cohen and Smith36 could excite a band at 275 mu by x-irradiation of Ge-doped synthetic quartz. This band was absent in pure quartz. The colour centre absorption is supposed to be caused by an electric dipole transition. Exhaustive investigations have been carried out on the colour centres in silver halides in connection with photography32. 

Ostwald ripening is of considerable importance in several technological situations. For example, in the preparation of photographic emulsions the silver halide particles are subjected to carefully controlled ripening. On the other hand, in agrochemical preparations it is important that ripening should not occur... 

Radiation-induced decomposition of insulating solids has been the subject of extensive research for many years. Because of their structural simplicity, the alkali and silver halides have perhaps received the widest attention. Studies of radiation-induced decomposition in azides could represent the next logical step in structural complexity. The azides in many respects are similar to the halides. Like the alkali halides, the alkali azides are primarily ionically bonded with band gaps of the order of 8 eV. Like the halides, there are azides with smaller band gaps (less than 4 eV). Important differences between the halides and azides are the presence of the triatomic azide anion and the lattice symmetry differences, which are perhaps a result of the nonspherical charge distribution on the azide ion. The salient questions which arise for the purpose of this chapter when one compares the azides to the hahdes are How does the the presence of the molecular anion influence radiation-induced decomposition are new and/or different kinds of defects produced how does the azide molecular anion influence the defect production process ... 

The typical and widely used procedure for the activation of the anomeric centre involves the initial preparation of acylated glycosyl halides, which are then subjected to SN2-like displacement by a phosphate anion in the presence of a silver-containing promoter and a base. 

The net result of all these competitive reactions is a possible mixture of normal and /3-glycosides and a possible mixture of two diastereo-isomeric orthoesters. In the course of the Konigs-Knorr procedure, water is usually formed from the reaction between hydrogen halide and silver oxide. The water formed, or deliberately employed, may take the part of the solvent alcohol, thus giving rise to products such as acidic orthoesters and normal tetra- and heptaacetates (for a disaccharide) of the sugars, and these compounds are subject to further changes, such as acyl migration. 

When the substrate is a triflate, or when a halide is subjected to appropriate reaction conditions in the presence of a halide scavenger (such as silver or thallium salts), the reaction proceeds via the cationic manifold. After oxidative addition, dissociation of X yields cationic intermediate 67. Alkene coordination provides 68 and migratory insertion delivers 69. /S-Hydride elimination then yields the desired product 66. Of importance in the pathway 62- 7— 68— 69— 66 is that both phosphines maintain contact with the metal throughout the process. This is ostensibly the factor responsible for the high enantioselectivities observed for reactions that are thought to proceed along this pathway. This contrasts with the course of events in the neutral pathway, where phosphine dissociation is thought to be responsible for low enantioselectivities. 

The possibilities for the formation of carbon—carbon bonds involving aromatic compounds have been enormously enhanced by the use of transition metal catalysts, and this area has been the subject of several reviews. Some of these concentrate on the applications of specific metals, and there have been surveys of the use of compounds of silver, copper and nickel,mthenium, and palladium in catalysis. The metalation of carbon-hydrogen bonds, preceding functionalization, may be aided by carboxylate ions, and this subject has also been reviewed. There is evidence here for concerted base-assisted deprotonation as shown in (10). In the carboxylate-assisted reaction of aryl ketimines with alkyl halides, a ruthenium-bonded intermediate (11) has been proposed, which subsequently adds the alkyl halide. " ... 

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