Phase transitions in molecular crystals from a chemical viewpoint

Abstract  -  Why do molecules pack  in crystals as they do? The trivial answer: because  they thereby achieve periodic arrangements corresponding to minimum potential energy. In general, for any  given molecule,  there must be many  such  arrangements of  closely  similar energy; experience accumulates to show that polymorphism is of widespread Occurrence  in the world of molecular crystals.  It is often accompanied by radical changes in conformation  and hydrogen bonding patterns. Indeed, as far as molecular crystals are concerned, there seems to be no clear cut boundary between polymorphic transitions and solid state chemical reactions. Of particular interest are those cases where crystal orientation is preserved during the structural change, for there,  in principle,  the atomic positions  can be mapped  quite precisely  on both  sides of  the transition and mechanisms can be proposed for the molecular motions that are involved. Not all such processes Occur under topochemical  control,  i.e., with a minimum  amount of atomic or molecular movement from the known  starting structure. Many seem to occur, or at least to be initiated, at crystal defects where the regular arrangement of molecules is interrupted. Examples of both  types will be discussed, and the attempt will be made  to describe  these  and related phenomena from a chemical point of view. 



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