Molecular packing in the crystal

Solid-state photoreaction pathways are dependent on the structiue of 1,3-diene compounds as the reactant (i.e., molecular packing in the crystals) cyclodimer, 1,4-polymer, or EE isomer is produced depending on the structiue of the substituent X and the molecular packing in the crystals, as shown in Scheme 1 and Table 1. Here, the stereochemical structure of the products is highly regulated during the photoreactions in the crystalhne state. 

Figure 1. Stereoscopic view of the molecular packing in the crystal structure of D-glucitol. The molecules, indicated by circles linked by heavy lines, are viewed down the carbon chain axis. The thin lines are the intermodular H-bonds.

The nature and order of the water network appears to depend on the molecular packing in the crystal and the specific sequence present. In peptide T3-785, less ordered water was seen in the terminal Gly-Pro-Hyp regions that were not well packed, than in the central Gly-Ile-Thr-Gly-Ala-Arg-Gly-Leu-Ala region with close molecular packing (Kramer et al., 2001). 

Molecular packing in the crystal structure of e-oxygen the solid and open lines indicate covalent and weak bonds, respectively, (a) C-centered layer of (02)4 clusters viewed along the c axis (b) the same layer viewed along the b axis. Symmetry codes Ax, y, z B x, y, z Cx, y, z. 

The supramolecular formations and the molecular packing in the crystals show close resemblance, and the nature of the interactions involved is very much the same. There is great emphasis on weak interactions in both. According to Lehn, beyond molecular chemistry based on the covalent bond lies supramolecular chemistry based on molecular interactions—the associations of two or more chemical entities and the intermolecular bond [85], Dunitz expressed eloquently the relevance of supramolecular structures to molecular crystals and molecular packing [86] ... 

In the hydroxyl-rich environment of the molecular packing in the crystal, intramolecular bonding is sterically less favorable than the intermolecular hydrogen-bond formation that occurs in all the alditol crystal structures. 

The existence of a substance in two or more forms, which are significantly different in physical or chemical properties, is known as polymorphism. The difference between the forms arises firom different modes of molecular packing in the crystal structure of certain triglycerides. Certain pure or mixed fatty acid triglycerides may show as many as five different melting points. Each crystal system has a characteristic melting point, x-ray diffraction pattern, and infrared spectrum. For example, tristearin can exist in three polymorphic forms with melting points of 54.7, 63.2, and 73.5°C. 

FIGURE 15.18. The structure of cholesterol myristate (Ref. 144). Some unit-cell edges are indicated, (a) Chemical formula, and (b) and (c) two views of the molecular packing in the crystal, (b) View onto the steroid ring systems, and (c) view along them. The equilibria, with temperatures, are ... 

FIGURE 35. Molecular packing in the crystal of para-chloroaniline... 

The shape change of molecules upon photoirradiation is directly linked to the macroscale shape change of the crystals. The suitable arrangement of molecules in crystals induces a cooperative motion of the crystal lattice and results in the mechanical motion of the crystals. The specific molecular packing in the crystals is therefore considered to play an important role in macroscale motion. 

However, httle is known about the crystal stracture in the first few layers of a vacuum sublimed pentacene film, the most important for charge transport. It is also widely recognized that the transport properties of crystaUine organic films depend strongly on the intermolecular overlap of electronic wave functions within the semiconductor layer, which is very sensitive to the molecular packing in the crystal [33],... 

Returning to question (c), whether the difference in activity between Mo and W complexes is determined by molecular electronic "design" and/or by molecular packing in the crystal, it seems from the above data that the similarities between the crystal structures and 0m indicate that the explanation for the variations in SHG efficiency must lie in electronic factors. This is reasonable in view of the differences between Ef for the reduction of the respective complexes, the tungsten compounds having reduction potentials between 450 and 500 mV more cathodic than their Mo analogues. It would appear that on photo-excitation of these molecules, electron transfer from the ferrocenyl "donor" to the metal nitrosyl "acceptor" occurs, and the SHG efficiency mirrors the ease or otherwise of this process. 

Because of the molecular packing in the crystals of a-chymotrypsin, particuliurly in the active-site region, attempts to bind larger substrates and substrate analogues than Af-formyl-L-tryptophan previously reported have been imsuccessful. However, this is not the case for the crystal forms of the other serine proteases. 

Figure 20.12 Molecular packing in the crystal structures of (a) Pigment Blue 80 (b) Pigment Violet 57 (solid solution of4b+4c+4d, p-phase). View along the b-axis.

Table 1.7, for example, in the difference between the volume change of anthracene and phenanthrene. The more regular shape of anthracene is clearly more favorable for tight molecular packing in the crystal than the bent shape of its isomer phenanthrene, whose crystal also has a lower packing coefficient and a lower heat of sublimation. These observations and conclusions are however quahtative and sporadic, and can hardly be used in systematic theories of molecular packing. 

Figure 12.13 The temperature dependence of XpT" (the inset shows the molecular packing in the crystal) and the magnetization curves of P-CF3PNN. 

In BrNN, on the other hand, the interchain AFM coupling seems to dominate over the intrachain FM coupling. In this approximation, the sheet structure of BrNN is reduced to a dimer model of equation (15.1) and the susceptibility is well analyzed with K. and 2J2/ b 7.6 K. The crystal of 2-methyl nitronyl nitroxide (MeNN) belongs to the monoclinic system of P2i/n [3] (different from that of BrNN or INN), but the molecular packing in the crystal is similar to that of BrNN and INN. The susceptibility of MeNN is also analyzed by the dimer model with 2J Jk 0 K and U2jk -3.2 K. 

In contrast to the cyclic alkanes, comparison of the solid state and SiNMR spectra of cyclooctamethyltetrasiloxane (n = 4) with its solution CNMR spectra demonstrates that the conformational mobility of such polar oligomers is significantly determined by the molecular packing in the crystal, and that the s-s transition (265 K) is only observed in solid state experiments and not in solution. The DSC behavior of both cyclic (M = 3.7-24.4 x 10 ) and linear polydimethylsiloxanes (PDMS) (M = 0.16-25.5 x 10 ) has been studied. " In addition to melting and the s-s transition, a and a cold crystallization exotherm were observed in cyclic and linear oligomers with Mn>3130 and Mn>2460, respectively. Furthermore, the temperatures of these transitions were much low er than those of octamethyltetrasilqxane. 

Recently, the structure of the first N- and C-protected tryptophan derivative, i.e., N-acetyl-L-tryptophan methyl ester, was solved by Cotrait and Barrans (97). There is one hydrogen bond NH. .. O = C of 2.86 A between the NH indole group and the acetyl C = O of different molecules. Molecular packing in the crystal is established by van der Waals forces between the two hydrophobic methyl groups and the aromatic ring. The conformation of the tryptophanyl side chain is very close to that of the L form of DL-tryptophan formate. Fig. 5 shows bond lengths and angles of N-acetyl-L-tryptophan methyl ester. 

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