An on-line search of Chemical Abstracts will reveal more than 47000 entries under 'polymorphism'. Over 90% of these relate to genetic polymorphism, which at least in its origins can claim the true etymology of the word. Some selectivity between biological and chemical uses can be achieved, but there is no certain searching strategy.Searching under 'phase transition' and related concepts will generate a further 44000 entries, most of which refer to inorganic systems, and cannot be easily disentangled. Nevertheless, these represent only a proportion of the papers containing information on polymorphs and polymorphism. Hence it is not possible to state how many publications relate to those aspects of polymorphism described here.* Earlier literature can be accessed Ilia references I , 2 and 10. * In the case of phenothiazines'zl the point of interest is not that the ring system is bent, but that the heteroatoms are out of the plane of the aromatic rings and in the opposite sense to expectation.* According to Lumley and Walker"* '5000-1OOO0 candidate substances have to be synthesized and screened for every one new medicine that reaches the market'. * The fact that a homologue and a polymorph can produce similar degrees of difference was first noted by Jones as quoted by Rosenkrantz and Z a b l o ~. ~~~

Ostwald proposed that the solid first formed on crystallisation of a melt or a solution would be the least stable polymorph. It is shown that this can be rationalized on the basis of irreversible thermodynamics, structural relationships, or a combined consideration of statistical thermodynamics and structural variation with temperature. It is concluded that the rule cannot be a universal law but is only in the nature of a possible preferred tendency.

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One hundred 4,4'-disubstituted benzenesulfonamidobenzenes, X-C(6)H(5)-SO(2)-NH-C(6)H(5)-Y, where X, Y = NO(2), CN, CF(3), I, Br, Cl, F, H, Me, OMe, have been synthesized and their crystal structures determined. The resulting set of 133 structures, which includes polymorphic forms, is used to make a comparative study of the molecular packing and the nature of the intermolecular interactions, including the formation of hydrogen-bonding motifs and the influence of the two substituents X and Y on these features. Nine distinct supramolecular connectivity motifs of hydrogen bonding are encountered. There are 74% of all the structures investigated which exhibit one of two motifs based on N-H...O=S interactions, a dimer or a chain. There are three other, infrequent motifs, also employing N-H...O=S links, which exhibit more complexity. Four different chain motifs result from either N-H...O=N, N-H...C[triple-bond]N or N-H...OMe interactions, arising from the presence of a nitro (position Y), nitrile (X or Y) or methoxy (Y) substituent. The program XPac [Gelbrich & Hursthouse (2005). CrystEngComm, 7, 324-336] was used to systematically analyse the packing relationships between crystal structures. Similar discrete (zero-dimensional) and extended (one-dimensional and two-dimensional) structure components, as well as cases of isostructurality were identified. A hierarchy for the classification of the 56 distinct structure types of this set is presented. The most common type, a series of 22 isostructures containing the simple centrosymmetric N-H...O=S-bonded dimer, is discussed in detail.

or I are reported. Comparisons carried out with the program XPac show that all investigated structures display a common 3D arrangement of N-H … N bonded centrosymmetric dimers. This isostructural series is exceptional in its completeness and in the diversity of the substituents involved. Accordingly, the XPac dissimilarity index x, a measure of how far two structures deviate from perfect geometrical similarity, varies from 0.9 for the interchange R 1 = Br A Cl to 9.5 for the simultaneous interchange of R 1 = I A H and R 2 = CF 3 A I. XPac plots of individual dissimilarity parameters have been used to elucidate details of geometrical similarities and differences between structures. This indicated that the geometry is preserved most closely within bc-layers that are composed of 1D slipped stacks of N-H … N bonded dimers, with interlayer van der Waals interactions being dominated by R 1 … R 2 contacts. The main mode of geometrical adjustment for the size of R 1 is a parallel shift of neighbouring layers against one another in the direction perpendicular to the layer plane, whilst the relative orientations of molecules within each layer are altered to a much smaller extent, even for those structure pairs representing R 1 substituents that are very different in size. This study shows that the ability of two compounds to crystallise in fundamentally the same crystal structure depends not only on how much their molecules differ in shape but also, critically, on the specific tolerance characteristics of the crystal packing arrangement concerned. As a comparison with the present study, dissimilarity indices are reported for a set of 24 isostructural 4,49-substituted benzenesulfonamidobenzenes previously subjected to XPac analysis.