System, crystal isolated

Figure 7.1 The energy levels distribution in molecular and supramolecular systems (a) isolated molecule, (b) cluster, (c) nanocrystal, and (d) crystal...

N2O4. Such formulations may, in some cases, be correct, with molecules of N2O4 present, analogous to water molecules of crystallization in crystals isolated from an aqueous system. However, the results of X-ray diffraction... 

In our discussion we shall first look at the behaviour of this simple system in isolation and then see what changes are necessary when it is recognized to be part of a large crystal involving many identical units. The treatment cannot be definitive, since no comprehensive solutions have yet been derived, but most of the factors involved can be reasonably well appreciated. 

From the results given above, three cases can be considered (1) percolated systems where the crystallizable block is not in isolated MDs as most of the lamellar forming block copolymers (Avrami indices >2) (2) block copolymers that form cylinders within an amorphous matrix, which can be considered an intermediate case since it could contain a fraction of percolated cylinders and a fraction of isolated cylinders therefore, its fractionated crystallization process will be a reflection of the mixture of these two crystal populations (Avrami indices between 1 and 2) and (3) systems with isolated MDs that can be exemplified by spheres within an amorphous matrix. In this case the overall crystallization kinetics will be dominated by primary nucleation since the growth within such nano-droplets can be considered instantaneous (Avrami indices around 1 or lower). Table 12.3 shows a compilation on the Avrami index values obtained for several systems, and the data on this table are in agreement with the three cases we have just explained. 

The dichlorobenzene isomers have very similar vapor pressures making separation by distillation difficult. Crystallization is generally used in combination with distillation to obtain the pure 1,2 and 1,4-dichlorobenzene isomers. The small quantity of 1,3-dichlorobenzene isomer produced is not generally isolated as a pure product. Environmental concerns have led to the use of improved crystalliza tion systems that contain the products with minimal losses to the environment. 

As mentioned above, the unit operation of crystallization rarely exists in isolation but is normally part of a wider particulate processing system as illustrated schematically, in a very simplified form, in Figure 9.2. A particular feature of such processes is the variety of unit operations and the range of equipment types that may be employed at any stage. 

The tautomeric structure leads to ambiguities in the nomenclature of compounds in this series. Thus, 5-methyl- and 6-methylbenzo-furoxan denote two different molecules which, because of their interconversion, cannot be isolated separately at normal temperatures. Throughout this review, when we intend to refer to the ambiguous mixture, we shall use the system employing the lowest numbers. The above methyl derivative, for example, will be described as 6-methyl-benzofuroxan regardless of the form adopted in the crystal. When a... 

In the course of studying the bromination reactions of the bicyclic systems we noticed that the reaction temperature has a dramatic influence on the product distribution. Increasing of the temperature gives non-rearranged reaction products (refs. 1,2). For this reason, we submitted 1 to high temperature bromination. To a solution of 1 in decalin at 150 C was added a hot solution of bromine in decalin in one portion. The colour of bromine disappeared immediately. After silica gel chromatography followed by fractional crystallization we isolated four products 2-6 in yields 8, 35, 37, and 9 % respectively. The structure of these compounds has been elucidated on the basis of spectral data by iH NMR and NMR experiments and by comparison with those reported in the literature. Symmetrical endo-c/5-isomer 6 has been observed for the first time. Studies concerning the mechanism of syn-addition show that the syn-adduct can arise either from direct... 

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