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

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Supramolecular Synthons and Pattern Recognition 83 portion of the antibiotic indicates that the receptor cavity has a well-defined geometry and that shape recognition occurs without much ligand-induced fit. 8 Polymorphism Polymorphism is defined as the phenomenon wherein the same chemical substance exists in different crystalline forms [68]. In the context of the present article, polymorphism corresponds to the existence of different crystalline patterns for the same molecule. With the current levels of intense activity in crystal engineering, there is much interest in this well-known though littleunderstood phenomenon [69]. There is as yet no clear consensus on the exact definition of polymorphism. McCrone defined it as the existence of at least two crystalline arrangements in the solid state of the same chemical substance [70] while Buerger and Bloom defined it on the basis that the two polymorphs must have distinctive properties [71]. Some of the criteria employed for assessing the existence of polymorphs are differences in unit cell parameters, crystal packing arrangements and physical properties. Among the techniques used for identifying polymorphs, single crystal and powder X-ray diffraction are the most powerful, while others like hot-stage microscopy, IR spectroscopy and differential scanning calorimetry provide supporting evidence. Dunitz has expressed a more contemporary view of polymorphism [3]. Since a crystal is a supermolecule, polymorphic forms are superisomers and polymorphism is a kind of superisomerism. Similar ideas are implicit in the molecule Æ supermolecule analogy and crystalline polymorphs are the supramolecular equivalents of molecular structural isomers [9]. Polymorphism in crystals is a very complex issue, and this complexity relates not only to causes for its occurrence but also to criteria that can judge as to whether it is present at all. Conformational polymorphism [72] and configurational polymorphism are two different types of polymorphism that arise because molecules adopt different conformations or crystallise as geometrical isomers, diastereomers, enantiomers or tautomers in the solid state. The suggestion of Byrn that polymorphism is very common in pharmaceutical compounds [73] is illustrated in the recent high-profile legal battle concerning the polymorphs of one of the best selling drugs, ranitidine [74]. Pseudopolymorphism, the inclusion of different solvents in the crystal structures of the same chemical substance, is not, strictly speaking, polymorphism at all because it involves different chemical species. This notwithstanding, the phenomenon is of immediate concern to the pharmaceutical industry while filing patent applications because different solvates of the same drug can exhibit widely differing solubilities and bioavailability properties. The unpredictable and capricious nature of polymorphism has been discussed in a recent article, humorously entitled “Disappearing Polymorphs” [75]. The existence of polymorphism in crystals can, at one stroke, demolish a wellplanned and logically devised strategy to obtain a target supramolecular structure. Instead of treating this phenomenon as a complication or nuisance in crystal engineering strategies, a more positive approach is to view it as an excellent opportunity to study the same chemical substance in different crystalline
84 A. Nangia · G.R. Desiraju environments. The existence of polymorphic forms may also render the molecule more susceptible to the formation of molecular complexes and this is a helpful feature. By calculating the lattice energies of polymorphs, the energies of molecules in different conformations can be estimated. Different supramolecular synthons in polymorphs provide good comparisons of distinct intermolecular interactions that can arise from the same functional group(s) and their effects on crystal packing. A number of factors have been suggested as possible causes for polymorphism. Polymorphism can occur when the free energy differences between alternative crystal structures are very slight. This may happen: (1) because the intermolecular forces are feeble; (2) when the entropic contribution to the free energy is high; and (3) because kinetic rather than thermodynamic factors control the events leading to crystallisation. The degree of difference between polymorphs may itself vary.While there are polymorphs that contain completely different supramolecular synthons, there are others which contain the same synthon occurring in slightly different ways. Three distinct situations are possible: (1) the same synthons are formed by the same functional groups but the differences in overall packing are caused by variations in the rest of the crystal; (2) the same synthons are formed by the same functional groups but there are multiple occurrences of these groups in distinctive molecular locations leading to different packing arrangements; (3) and different synthons are formed leading to radically different packings. The three situations are discussed with representative examples [69]. In many cases, the same synthons are formed by the same functional groups in the various polymorphs. Both forms of resorcinol, RESORC, contain synthon 57. However, the difference between the two forms is that while linear chains are formed in form B, tetrameric helices are present in form A (Fig. 6). 4-Hydroxya- RESORC 57 A B Fig. 6. Polymorphs A and B of resorcinol (RESORC)
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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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24 J.P. Glusker rine-containing com
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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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136 Y. Aoyama A trigonal centre is
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138 Y. Aoyama 18 19 Fig. 5. 2D net
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140 Y. Aoyama 24 ded (O-H◊◊◊O
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142 Y. Aoyama 2.4 Interaction-Suppo
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144 Y. Aoyama In the presence of a
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146 Y. Aoyama clusion compounds (se
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148 Y. Aoyama are intriguing guests
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150 Y. Aoyama The tuning or enginee
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152 Y. Aoyama Fig. 16. Binding isot
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154 Y. Aoyama The desorption of the
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156 Y. Aoyama Fig. 17. Suggested me
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158 Y. Aoyama 5 Concluding Remarks
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160 Y. Aoyama 4. Hölderich W, Hess
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Crystalline Polymorphism of Organic
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Author Index Volumes 151-198 Author
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Author Index Volumes 151 - 198 2 1
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Author Index Volumes 151 - 198 213
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Author Index Volumes 151 - 198 2 ]
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Springer and the environment At Spr
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