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Correlations of the chemical structures of products from organic solid state reactions with the molecular packing in the reactant crystal structures define the extent of lattice restraints on molecular motion before and during reaction. The molecular behaviour after the chemical transition state can be inferred from comparisons of the molecular orientations in the initial and final phases. The formation of preferentially oriented product phases suggests that highly specific geometrical mechanisms are operative but additional criteria are needed to establish a direct correspondence between specific reactant and product molecules. Observable intermediate mesomorphic phases are particularly useful in defining the directions in which crystallographic order is not preserved. Several studies of hydrolysis, isomerization and polymerization reactions suggest surprisingly large molecular migrations and conformational changes may occur without randomizing the total distribution of product molecules. Competing and consecutive transformations often give rise to several simultaneously present and mutually oriented product phases and, in some cases, large molecular moieties of the initial structure do not appear in any of the final ordered phases. Novel twinning frequently is observed when degenerate transformation mechanisms give rise to several preferred orientations of product structures which cannot reorganize to form a single phase of long-range order. While the short-range molecular arrangement in the oriented product phase usually is identical with that produced during conventional crystallizations, exceptions may be found in the formation of chemically unstable molecular structures or metastable polymorphs of the product phase.
Correlations of the chemical structures of products from organic solid state reactions with the molecular packing in the reactant crystal structures define the extent of lattice restraints on molecular motion before and during reaction. The molecular behaviour after the chemical transition state can be inferred from comparisons of the molecular orientations in the initial and final phases. The formation of preferentially oriented product phases suggests that highly specific geometrical mechanisms are operative but additional criteria are needed to establish a direct correspondence between specific reactant and product molecules. Observable intermediate mesomorphic phases are particularly useful in defining the directions in which crystallographic order is not preserved. Several studies of hydrolysis, isomerization and polymerization reactions suggest surprisingly large molecular migrations and conformational changes may occur without randomizing the total distribution of product molecules. Competing and consecutive transformations often give rise to several simultaneously present and mutually oriented product phases and, in some cases, large molecular moieties of the initial structure do not appear in any of the final ordered phases. Novel twinning frequently is observed when degenerate transformation mechanisms give rise to several preferred orientations of product structures which cannot reorganize to form a single phase of long-range order. While the short-range molecular arrangement in the oriented product phase usually is identical with that produced during conventional crystallizations, exceptions may be found in the formation of chemically unstable molecular structures or metastable polymorphs of the product phase.
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