Other Crystalline Systems

Beside the site line at 571.9 nm, terrylene in hexadecane presents a broad background in which single molecule lines can be observed and persistent spectral holes can be burned [69]. The histogramme of single molecule linewidths compares well with the average width of the holes, extrapolated to low burning fluences, which 

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Several other catalyst systems have been suggested, including boron fluoride and both crystalline and noncrystalline siUcas and alurninosihcates. Although no commercial faciUty exists, the concept of using a crystalline siUca or alurninosihcate catalyst in an integral reaction and distillation apparatus has been proposed (9). 

PAs have also been copolymerized with other polymer systems and, in particular", with polyesters and poly ethers. In the copoly esteramides the crystallinity is decreased by copolymerization, as the crystalline structure of the amide unit is very different from the ester unit. However, alternating polyesteramides behave as homopolymers with a glass ttansition temperature and a melting temperature intermediate to the polyester and the PA polymer (Figs. 3.10 and 3.11).23,24 Polyesters, such as PBT and PET, modified with a small amount of diamide are also copolymers that have a high order.24,73... 

Experiments at present are concentrated on sd-metals and Pt-group metals. The sp-metals, on which theories of the double layer have been based, are somewhat disregarded. In some cases the most recent results date back more than 10 years. It would be welcome if double-layer studies could be repeated for some sp-metals, with samples prepared using actual surface procedures. For instance, in the case of Pb, the existing data manifest a discrepancy between the crystalline system and the crystal face sequence of other cases (e.g., Sn and Zn) the determination of EgaQ is still doubtful. For most of sp-metals, there are no recent data on the electron work function. 

Other crystalline phases have been reported for the magnesium oxysulphate system. The 2 3 5 phase has been claimed to be formed from... 

Secondly, for some crystalline systems, the structure obtained by diffraction techniques may be incomplete. For example, in some cases the diffraction data may not reveal dynamic aspects of the solid-state structure (as in the case of fluxional organo-metallics) and in others it may not be possible to distinguish clearly between different atoms (as for example 27A1 and 29Si in zeolites) and a combination of the NMR and x-ray data will yield a more complete and meaningful description of the structure. 

Nemst asserted his postulate although the available data were inconclusive. In fact, Richards extrapolated some of his data to give a graph such as that shown in Figure 11.2, which suggests that Equation (11.1) is valid but that Equation (11.3) is not. Numerous subsequent experiments have conhrmed Nemst s postulate if it is limited to perfect crystalline systems. Apparent exceptions have been accounted for satisfactorily. The term perfect implies a single, pure substance. Other restrictions are implied by this term, but they will be discussed later. 

The amount of amorphous polymer, which is generally produced in small percentage (9-16%) contemporaneously with the non-atactic polymer, is independent of reaction time (see Table II). It is on the contrary closely connected with the nature of the catalytic system employed and changes, for instance, when the triethylaluminum is substituted by other metal alkyls (beryllium alkyls, propylaluminum, isobutylaluminum, etc.) 5,28). It also depends on the purity of the a-titanium trichloride, in particular increasing in the presence of other crystalline modifications of titanium trichloride [i.e. -TiCU (27)] and of titanium compounds obtained by reduction of titanium tetrachloride at low temperature with aluminum alkyls. 

The kinetic-acetonation procedure previously reported,107 employing alkyl isopropenyl ethers in N,N-dimethylformamide, and applied to the common pentoses and hexoses, has been extended to various hexuloses, oligosaccharides, and other sugar systems.108 Maltose gave the 4, 6 -monoacetal, isolated as the crystalline hexaacetate, on treatment with isopropenyl methyl ether inN,N-dimethylformamide in the presence of p-toluenesulfonic acid. 

It is interesting to discuss briefly the relative stabilities of these addition compounds in the crystalline phase. It is well known that the sharpness of m.p. maxima is an indication of the stability of the crystalline addition compounds. Rounded maxima indicate the dissociation of the compounds into components on melting. From that point of view the compound in the system sulphuric acid-nitromethane is the most stable as compared with the compounds in the other four systems. The greater stability of this compound in the crystalline... 

Although the chloroaluminates are the best known room-temperature molten salts, there are several other interesting systems. For example, if one mixes the crystalline solids trielhylamnionium chloride and copper[I) chloride, an endothermic reaction takes place Lo form a light green oil. The most reasonable reaction is the coordination of a second chloride ion to the copper(l) ion34... 

As already described, other crystalline forms of arsenic besides the rhombohedral are known or suspected to exist. The crystals of the yellow allotrope belong to the cubic system, while native arsenolamprite (p. 8) contains crystals belonging to the rhombic, or possibly to the monoclinic, system. Yellow arsenic is soluble in carbon disulphide. 

Since the state of a crystal in equilibrium is uniquely defined, the kind and number of its SE s are fully determined. It is therefore the aim of crystal thermodynamics, and particularly of point defect thermodynamics, to calculate the kind and number of all SE s as a function of the chosen independent thermodynamic variables. Several questions arise. Since SE s are not equivalent to the chemical components of a crystalline system, is it expedient to introduce virtual chemical potentials, and how are they related to the component potentials If immobile SE s exist (e.g., the oxygen ions in dense packed oxides), can their virtual chemical potentials be defined only on the basis of local equilibration of the other mobile SE s Since mobile SE s can move in a crystal, what are the internal forces that act upon them to make them drift if thermodynamic potential differences are applied externally Can one use the gradients of the virtual chemical potentials of the SE s for this purpose ... 

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