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

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202 M.R. Caira successful design of auxiliaries depends on a detailed knowledge of the packing arrangements, as well as the relationship between packing arrangements and crystal morphology for all species that might appear in the system. An early demonstration of this approach (which was subsequently successfully applied to several other systems) involved resolution of racemic histidine hydrochloride [165]. In the report of this resolution, it was mentioned that this approach was being tested for use in the precipitation of metastable polymorphic crystals by kinetic control. 5 gossypol 6 indomethacin 7 cholic acid Recent application of these principles to polymorphic control is shown schematically in Fig. 15 for a hypothetical dimorphic system in which one polymorph is centrosymmetric and the other crystallizes in a polar space group [36]. In the former crystal, the molecules are arranged in antiparallel orientation whereas in the latter they are aligned along a common direction. A tailor-made auxiliary which binds to both crystals would do so at the two (indistinguishable) ends of the centrosymmetric crystal but only at one end of the polar crystal. The latter would therefore grow at the expense of the former and polymorphic control will have been achieved. This concept was successfully applied to N-(2-acetamido-4-nitrophenyl)pyrrolidene (PAN) where crystallization of the metastable polymorph of PAN could be induced by addition of a remarkably small amount (0.03%) of the designed inhibitor [36]. Generalisation of this strategy is not straightforward, however, since polar crystals may belong to space groups of relatively high symmetry which require the constituent molecules to adopt a variety of orientations with respect to the polar axis [35]. Strategies which have been explored to engineer non-centrosymmetry into crystals for NLO properties have been listed in a recent paper describing the polymorphism of 1,3-bis(m-nitrophenyl)urea [166]. Research on the use of tailor-made surfaces for promotion of nucleation has been pursued in parallel with studies of crystal growth inhibition [35, 36]. The objective here is to gain a deeper understanding of the mechanism of molecular
Crystalline Polymorphism of Organic Compounds 203 a b Fig. 15 a Schematic illustration of the packing arrangement in a polar and a centrosymmetric crystal. b The effect of a designed inhibitor on crystal growth. (Adapted from [36] with permission clustering associated with the formation of pre-critical nuclei of crystalline phases. The relevance for organic crystal polymorphism is that direct observation of the dynamics of crystal nucleation could elucidate the effects of additives on polymorphic control. Much of this work has been based on the use of Langmuir films to induce nucleation at the air-water interface [36]. To illustrate the level of interpretation which is currently possible from such experiments, a recent study of the self-assembly of crystalline monolayers and multilayers of n-alkanes on a water surface is cited [167]. Surface observations were carried out using synchrotron grazing incidence X-ray diffraction and specular X-ray reflectivity. These allowed detailed characterisation of the crystalline films including determination of their space groups. It is noteworthy that these were found to be identical to those of the corresponding three-dimensional crystals, thus demonstrating that the full crystal symmetry already exists even for crystallites whose thickness corresponds to only a few molecular layers. Related to the above strategy is the concept of engineering solid surfaces for promoting nucleation and epitaxial growth of desired polymorphs. If it is assumed that the pre-nucleation aggregate for a polymorph resembles the mature crystal, it should be possible to design a surface which mimics a particular crystal plane of the desired species, and upon which heterogeneous nucleation will occur and epitaxial polymorphic growth will result. This has been realised in the technique of ledge-directed epitaxy [168] in which the intersection of a terrace plane and a step on a crystal surface (a “ledge”) functions as a nucleation site for organic crystals. For polymorphic control, the crucial requirement is that two well-defined, close-packed crystal planes of the polymorph have a dihedral
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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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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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60 A. Nangia · G.R. Desiraju elect
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62 A. Nangia · G.R. Desiraju 14 15
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64 A. Nangia · G.R. Desiraju [23]
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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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72 A. Nangia · G.R. Desiraju ingfu
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74 A. Nangia · G.R. Desiraju and r
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76 A. Nangia · G.R. Desiraju bonds
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78 A. Nangia · G.R. Desiraju 2.86-
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80 A. Nangia · G.R. Desiraju from
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82 A. Nangia · G.R. Desiraju ted a
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84 A. Nangia · G.R. Desiraju envir
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86 A. Nangia · G.R. Desiraju PYRAZ
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88 A. Nangia · G.R. Desiraju napht
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90 A. Nangia · G.R. Desiraju b c F
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92 A. Nangia · G.R. Desiraju all a
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94 A. Nangia · G.R. Desiraju 42. J
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Hydrogen-Bonded Ribbons, Tapes and
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132 Y. Aoyama 4 Catalysis . . . . .
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134 Y. Aoyama 2.2 Symmetry-Controll
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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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Page 165 and 166: 158 Y. Aoyama 5 Concluding Remarks
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198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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