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

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168 M.R. Caira approach is visual representation of phase transformations. Figure 2 illustrates MD-derived images of a crystalline cluster of 188 molecules of t-butyl chloride at various stages of freezing. MD simulations show that when sufficiently supercooled, the tetragonal phase spontaneously transforms to a lower temperature, ordered monoclinic phase. Diffraction patterns computed from the MD molecular packing were consistent with experimental neutron powder patterns for this phase. Other investigators [45] have recently designed a numerical model to describe nucleation and growth of polymorphs with the aim of calculating the temporal sequences of precipitation and phase transformation in metastable solutions of polymorphic substances. Another group has recently modelled the formation and aggregation of polymorphs in continuous precipitation [46]. Consideration was given to the simultaneous growth and agglomeration of two different polymorphs as well as the case of nucleation of a single polymorph which subsequently transforms into a second one. The results indicated that the ratio of the nucleation rates, the ratio of the growth rates, and the aggregation tendencies determined polymorphic product composition as well as particle size distributions. This study is important since simultaneous precipitation of different polymorphs is encountered frequently in industrial crystallizations. A mathematical phase-field model for the kinetics of isothermal polymorphic crystallization has recently been proposed [47], according to which crystallization involves rapid relaxation of the metastable state followed by nucleation and growth of the polycrystalline phase. Computer simulations were used to obtain results which could be tested experimentally using X-ray scattering experiments. Growth rates of different polymorphic polymers have also been investigated [48]. Simultaneous development of spherulites of different polymorphs occurs at different rates under isothermal conditions. From observation of interspherulitic boundaries between the a- and g-forms of polypivalolactone, a b c Fig. 2 a – c. Images of a crystalline cluster of t-butyl chloride molecules at various stages of cooling, looking down the threefold molecular axis: a orientationally disordered tetragonal phase at 130 K; b nucleus of monoclinic phase growing in tetragonal phase at 80 K; c ordered monoclinic phase at 50 K after transformation. Surface molecules tend to be disordered at all temperatures. (Reprinted with permission from [44], copyright 1995 American Chemical Society)
Crystalline Polymorphism of Organic Compounds 169 their relative growth rates could be determined. The thermodynamics and kinetics of crystallization of large molecules from solution have been discussed [49]. Modelling of the crystallization of protein molecules indicated that the concepts and numerics of colloid stability theory are appropriate. Recent studies of polymorphic transformations in organic crystals mediated by melt, solution and interface have been reviewed [4]. Interface-mediated transformation has only recently been recognized as a distinct mode of polymorphic transformation. It involves nucleation and growth of a new polymorph through mass transfer across the interface connecting single crystals of two different polymorphs and it differs from solid-solid transformations in that a microscopic solution layer is required as an interface. Transformations mediated by melt, solution and interface are usually more rapid than solid-solid transitions; for the latter, the activation energy is larger due to the fact that nucleation and growth of the new phase within a second phase involve diffusion and structural rearrangement at the reaction interface. The fundamental thermodynamic relationships governing polymorphic solid state transitions have been reviewed [6]. It has also been pointed out that the mechanisms of polymorphic transitions in molecular crystals are largely unknown, though order-disorder transitions are understood in reasonable detail [5]. An important technique for studying solid-solid transformations is thermal analysis and a review of the basic thermodynamic principles for interpreting thermal analysis data for both polymorphic and pseudopolymorphic systems has appeared [23]. Kinetic and thermodynamic aspects of the thermal decomposition of inclusion compounds (a special class of pseudopolymorphs) have also recently been discussed [50]. Theoretical and experimental studies of the role of solvent on polymorphic crystallization and phase transformations abound in the literature of the last few years and some pertinent examples are described here. For solvent-mediated transformations, the driving force is the difference in solubility between different polymorphs. An important earlier paper on the kinetics of such phase transformations [51] described a model featuring two kinetic processes in solid to solid phase changes via a solution phase, namely dissolution of the metastable phase and growth of the stable one. The effect of solvent on the crystallization of polymorphs has recently been investigated [43] using as a model compound the antibacterial sulphathiazole whose four known polymorphs are well characterised. The study, whose express intention was to test the Ostwald law, involved crystallization of the pure polymorphic forms of the drug, solubility measurements and crystallizations from various solvent systems. Systematic variation of supersaturation was employed in an attempt to crystallize each of the four forms of sulphathiazole, as predicted by the Ostwald law. Solubility studies showed Form I to be the most soluble form, followed in order by Forms II, IV and III. The supersaturation crystallizations using acetone, acetone-CHCl 3 (3:2), n-propanol and water, revealed that only the acetone-CHCl 3 system yielded results in accord with theory, Forms I, III and IV being isolated from it by varying the supersaturation. Crystallization from n-propanol, for example, yielded only Form I at all supersaturation levels. Thus, the finding that some solvents selectively favour the crystallization of a
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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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86 A. Nangia · G.R. Desiraju PYRAZ
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88 A. Nangia · G.R. Desiraju napht
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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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Springer and the environment At Spr
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198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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