Polymorphic changes, classification

The relationships between the polymorphous forms of different substances are so varied that any systematic classification is difficult. Nevertheless, it is convenient to recognize certain fairly well defined types, and we shall classify polymorphic changes under the following heads ... 

Small changes in substituents may lead to different aurophilic aggregation patterns. Thus, different structures have been described for very similar species and polymorphs of different nuclearities have even been found for one complex. Thus, a classification based only on the nuclearity of the aggregates is not only difficult, but may also hide other interesting comparisons. For these reasons some chain structures have been mentioned above. The following tries to complete those commentaries. 

In light of the variety of behaviour exhibited by solvates, Byrn (1982) has suggested a classification scheme for crystal solvates based on that behaviour, rather than on stability. He proposed that the solvates for which the solvent can be removed from the crystal and added back to the crystal reversibly without greatly changing the X-ray powder diffraction pattern (Section 4.4) would be considered pseudopoly-morphic solvates. Those which undergo a change in structure, as evidenced by a different powder diffraction pattern, would be described as polymorphic solvates. The appellation does not seem to have been adopted by many other workers. 

The polymorphism of this genus has meant that its taxonomical classification has been constantly changing over the years. 

The classification of polymorphic substances into monotropic and enantiotropic classes from the standpoint of observed phenomena is not appropriate as a basis for the lattice theory of polymorphism. In the lattice theory, one begins by considering whether the lattices of the polymorphic forms are related in structure or not. Only in the case of lattices that are structurally related in structure or not. Only in the case of lattices that are structurally related will it be possible for mutual transformation to take place at a transition point fixed by the temperature and pressure of the system. If the lattices differ in such a way that atoms or molecules must be completely regrouped during the transformation (changing their state of bonding), no point of contact for mutual reversible transformation will exist. 

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