198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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176 M.R. Caira with 2, 13 with 3, and 3 with 4 partners. Differences in molecular properties (P) (e.g. density, molecular volume, packing coefficient, AAP-calculated packing energies and other thermodynamic properties) were computed between cluster members (i, j) as DP=P j–P i or DP=100(P j –P i)/P j (j > i) for a total of 204 data points. Both monovariate and bivariate statistical analyses were performed on the data, yielding several revealing trends and correlations. Histograms of differences in properties between polymorphic pairs,shown in Fig. 6a–c,indicate respectively that differences in crystal packing energies, densities and lattice vibrational entropies for polymorphs are rather small while Fig. 6d reveals that an appreciable proportion (actually 18%) of polymorphs have Z¢ >1 (where Z¢ is the number of molecules in the crystal asymmetric unit).Some further important conclusions drawn from this study are as follows: both calculated and experimental values for the relative stability of crystal polymorphs are currently subject to large uncertainties; polymorphs with Z¢ >1 are as stable, or even more stable than those with Z¢ = 1; higher crystal density is, as expected, found to N(DE) a DE % DD % b N(DS) N(DD) N(DZ¢) c DS, IK d –1mole –1 DZ¢ Fig. 6 a – d. Histograms of differences in properties between polymorphs: a DE (packing energy, %); b DD (density, %); c DS (lattice-vibrational entropy, J K –1 mol –1 ); d DZ¢ (no. of molecules in the asymmetric unit). (Reprinted with permission from [79], copyright 1995, American Chemical Society)
Crystalline Polymorphism of Organic Compounds 177 correlate with higher packing energy. Finally, from the observation that 24 % of the polymorphic pairs analysed comprised both a centrosymmetric and a noncentrosymmetric partner, it was concluded that the link between molecular properties and centrosymmetry of crystals is evidently weak. The reader is referred to this seminal study [79] for the full exposition of the above conclusions and their justification, as well as further perceptive observations on the phenomenon of polymorphism. 3 Methodology for the Study of Crystal Polymorphism 3.1 Review of Preparative Methods Research on the polymorphism of a new molecular entity normally commences with experimental screening which can indicate the occurrence of more than one crystalline form of the substance. An inexpensive method of such testing is hot stage microscopy (HSM), which has been used very extensively and effectively by a leading proponent [80] for many years to provide preliminary indications of the presence of crystalline polymorphic and pseudopolymorphic (solvated), as well as glassy (amorphous) forms, all of which may have practical utility. Pseudopolymorphic forms are molecular adducts containing solvent of crystallization and have been classified [37] as stoichiometric solvates and nonstoichiometric inclusion compounds possessing channel, layer or cage (clathrate) structures. Procedures for detecting the existence of multiple forms by HSM have been outlined [37, 81]. These include, for example, the observation of solidsolid transformation upon heating the substance,observation of transformation (spontaneous or mechanically induced) following the freezing of a melt, and detection of gas evolution as bubbles from pseudopolymorphs immersed in silicone oil during heating (indicative of a solidÆ gas +solid transformation). Once the existence of multiple forms is established, practical methods for the preparation of specific forms on a larger scale may be explored. Frequently, recrystallization of the compound from solvents or solvent mixtures spanning a wide polarity range is effective in producing several of the different forms in sufficient quantity for complete characterisation by the analytical methods to be discussed.Most pseudopolymorphs are prepared by crystallization of the parent organic compound from the respective solvent, whereupon the latter becomes incorporated in the new crystal. Recrystallization from a mixed solvent system may yield a pseudopolymorph containing either or both solvents. Exposure of the parent organic compound to vapours may also result in the formation of pseudopolymorphs (as occurs e.g. when anhydrous drugs react with atmospheric water to form hydrates). This is exemplified by the drug indomethacin, for which four polymorphic forms (I–IV) have been identified [82]. Crystallization of the commercially available Form I from over fifty solvents yielded Forms I, II, IV and mixtures of these, as well as a number of pseudopolymorphs [83]. To ensure subsequent reproducibility in the preparation of specific forms by crystallization, careful attention to the details of solvent purity, degree of solu-
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198 Topics in Current Chemistry Edi
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Design of Organic Solids Volume Edi
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Volume Editors Prof. Dr. Edwin Webe
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VIII preface tion at an amazing lev
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Contents of Volume 196 Carbon Rich
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2 J.P. Glusker 5 Interactions of Pl
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4 J.P. Glusker Perturbations to the
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6 J.P. Glusker 2.1 Sources of Cryst
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8 J.P. Glusker other induced, will
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10 J.P. Glusker The forces here hav
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12 J.P. Glusker lographic structure
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14 J.P. Glusker a c Fig. 6 a - c. D
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16 J.P. Glusker hydrogen bond H◊
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18 J.P. Glusker Fig. 9. Citric acid
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20 J.P. Glusker peptide plane and a
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22 J.P. Glusker 4.5 F◊◊◊H Int
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26 J.P. Glusker Fig. 17. The packin
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28 J.P. Glusker bind to a metal ion
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30 J.P. Glusker As for carboxylate
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32 J.P. Glusker Fig. 24. The magnes
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34 J.P. Glusker structures of magne
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36 J.P. Glusker b a Fig. 28 a, b. A
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38 J.P. Glusker Fig. 30. Complexati
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40 J.P. Glusker 7.1 Descriptions of
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42 J.P. Glusker Fig. 34. Glycine cr
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44 J.P. Glusker bonding capabilitie
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46 J.P. Glusker 2. Non-intercalativ
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48 J.P. Glusker d Fig. 37 d. The ch
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50 J.P. Glusker Fig. 39. Hydrogen-b
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52 J.P. Glusker 72, 104], but other
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54 J.P. Glusker 23. Cody V, Murray-
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56 J.P. Glusker: Directional Aspect
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58 A. Nangia · G.R. Desiraju 7 Sup
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66 A. Nangia · G.R. Desiraju struc
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68 A. Nangia · G.R. Desiraju Fig.
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70 A. Nangia · G.R. Desiraju -NH 2
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198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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