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

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Supramolecular Synthons and Pattern Recognition 73 Scheme 7. Outlier refcodes in the CSD study of C-H◊◊◊O hydrogen bonding in CHCl 3 and CH 2Cl 2 solvates frequent occurrence of the urea motif is a result of this marginally higher stability. A more likely reason is that the synthon 35 is not common in imides such as 31 because it can occur only when both the carbonyl groups are syn to each other. Examination of the crystal structures of the retrieved imides shows that a large number of these compounds (such as, say, 33) exist in the conformation wherein the two carbonyl groups are anti to one another. Thus, in ureas the stablest molecular conformation is in consonance with the requirements for the target tape motif, whereas in imides the conformation of the molecule is in conflict with that necessary for the extended, linear hydrogen bonded tape.Another possible reason for the formation of the infinite tape 34 in the ureas and the nonformation of the infinite tape 35 for the imides is the match of donor (2 H atoms) and acceptor (1 C=O group with 2 acceptor sites) groups in the former and mismatch (1 donor, 4 acceptor sites) in the latter. The examination of outlier structures in a CSD analysis is often instructive. It is well-known that C-H◊◊◊O bonds formed by CHCl 3 are in general shorter than those formed by CH 2Cl 2 because of carbon acidity effects [38]. However, histograms of C-H◊◊◊O distances of these two populations show some degree of overlap so that some of the shorter contacts in the CH 2Cl 2 sample are shorter than the longer contacts in the CHCl 3 sample. This corresponds to the outlier region. For example, a long and bent contact is formed by CHCl 3 in the crystal structure of BOLVAZ (Scheme 7). Here, the acceptor oxygen is electron-deficient because it is bonded to a Cu(II) atom and this reduces its propensity to form a strong C-H◊◊◊O hydrogen bond (d=2.88 Å, q =125.3). Conversely, a short and linear contact is formed by CH 2Cl 2 in the structure of PIDDOV. In this phosphonium salt, the acceptor phenoxide O – -atom is unusually basic and forms a strong hydrogen bond (d=1.96 Å, q =162.4). In both instances, the outlier status of the points is confirmed by examination of the individual structures and this provides additional insight about the chemical nature of the intermolecular contact. 6 Multipoint Recognition Patterns The presence of two or more complementary hydrogen bond donor and acceptor sites in a recognition event, that is multipoint recognition, confers stability
74 A. Nangia · G.R. Desiraju and robustness to the resulting supramolecular synthon. Multipoint recognition between the DNA base pairs with Watson-Crick hydrogen bonding schemes may be represented as DA:AD for adenine-thymine and DDA:AAD for guaninecytosine (D=hydrogen bond donor, A=hydrogen bond acceptor). The distinct arrays DDA:AAD, DAD:ADA and DDD:AAA in three-point recognition motifs have been investigated separately by Zimmerman and Murray [39] and Mingos et al. [40]. The association constant (K assoc) for the complex 36 of the DAD : ADA type is weak and in the 10 2 M –1 range (Scheme 8). The DDA:AAD type complex 37 is more stable with K assoc in the 10 4 M –1 range. The DDD:AAA complex 38 was the tightest examined and has K assoc>10 5 M –1 . Studies on triply hydrogen bonded complexes are of additional interest. Jorgensen et al. have interpreted the trends in the stabilities of these bonded arrays as arising from the different arrangement of D and A sites [41]. Since the primary hydrogen bonds are similar in the three systems, differences in association energies are held to arise from differences in secondary electrostatic interactions. In the DDA:AAD complex there are two attractive and two repulsive secondary interactions, in the DAD:ADA complex all four secondary interactions are repulsive, while in DDD:AAA all four secondary interactions are attractive. Thus, for similar hydrogen bonds between D and A, the DDD:AAA complex is expected to be the strongest, the DDA:AAD complex of intermediate stability, and the DAD:ADA complex the weakest. However, cooperativity of p-effects predicts the opposite trend [39, 40]. The delocalised p-system is able to enhance the hydrogen bonding interaction in DDA:AAD and in DAD:ADA but not in the DDD:AAA system. In addition to the function of ordered hydrogen bonded arrays in the biological cell [42], the design of new materials is closely connected with the organised self-assembly of supramolecular structures [43]. The physical and chemical properties of molecular aggregates depend on the nature of the constituent molecules as well as on the manner in which the molecules assemble in the solid state. This is because the properties and architectural features of the supramole- 36 37 38 Scheme 8. Donor-acceptor complexes with multi-point recognition motifs DAD:ADA 36, DDA:AAD 37 and DDD:AAA 38 (from Zimmerman and Murray [39])
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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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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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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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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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Springer and the environment At Spr
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