Molecular structure in the crystal

Structure determination has greatly advanced with the invention of new ways to use x-ray crystallography, mainly new mathematical methods that permit the interpretation of the observed patterns of diffraction of x-rays by a crystal, translating it into the molecular structures in the crystal. A Nobel Prize in 1985 to Herbert Hauptmann and Jerome Karle recognized such an advance. 

Fig. 61. Molecular structure in the crystal of zwitterionic silicate 105.88 Reproduced with permission from the American Chemical Society.

Compounds 3-6 were structurally characterized by single-crystal X-ray diffraction. The molecular structures in the crystal are shown in Figs. 1-4 selected bond distances and angles are given in the respective figure captions. 

The arrangement shown in Figure 1(a) presupposes an extended conformation of the diacetylene molecule. For monomers with bulky end groups, the flexibility of the monomer molecule can result in a compact molecular structure in the crystal. The sidegroups then impede the reaction, in contrast to the situation in reactive monomers where the sidegroup interactions bring the diacetylene moieties into the desired reactive contact. 

The NMR spectrum of Fc3(CO)i2 is a sharp singlet (62, 63) down to 123 K (33), demonstrating a very low barrier to equilibration of the five carbonyl environments of the molecular structure in the crystal (99), if that structure (Fig. 4) were maintained in solution. Even though that may not be the case (33, 72), rapid equilibration must still be concluded. Mixed metal derivatives of the same structural type, viz. Fe2Ru(CO)i2 (104, 75) and [MnFe2(CO)i2] (II), are also nonrigid (62). 

Although three proton environments would be anticipated for (CgHg)Fe2(CO)5 if the molecular structure in the crystal were maintained in solution (60) (Fig. 14), only one sharp proton NMR line is observed down to 193 K (the lowest temperature studied). Evidently this complex is also fluxional, and, indeed, ring rotation is rapid even in the solid state at 77 K (19). 

Fig. 6.16 Hydroxy-functionalized bicyclic imidazole and molecular structure in the crystal...

A characteristic feature of the structure of samples obtained under the conditions of molecular orientation is the presence of folded-chain crystals in addition to ECC. Kawai22 has emphasized that the process of crystallization from the melt under the conditions of molecular orientation can be regarded as a bicomponent crystallization in which, just as in the case of fibrous structures in the crystallization from solutions, the formation of crystals of the packet type (ECC) occurs in the initial stage followed by the crystallization with folding . 

Single crystal x-ray diffraction is, without doubt, one of the most important techniques for the determination of molecular structures in the solid state. The technique is also pivotal to the determination of accurate bond lengths in molecules. However, the analysis of x-ray... 

Closer inspection of the blocks in the crystal structure, however, reveals some interesting variation as far as the individual spatial arrangements are concerned. The most compact association is found in the 1 2 inclusion compound of the spiro host 22 with dimethylformamide48> (Fig. 23, type la). This aggregate preserves a perfect twofold (C2) molecular symmetry in the crystal lattice. The formamide moiety acts as... 

Fig. 8 Molecular arrangement in the crystal packing a and the single sheet structure b all Py carbon atoms are omitted for clarity in 4-9...

Fig. 10 Molecular arrangement in the crystal packing a and ID column structure b all uncoordinated atoms in axial ligands are omitted in 14...

Fig. 21 Molecular arrangements in the crystal packing a and the Pc overlapping structure of adjacent Pc rings in trimer b of 79-84...

The compound a-[Fe(Cp )2][Pd(dmit)2] is not isostructural with [Fe(Cp )2] [M(dmio)2], with M = Pd and Pt, but presents a similar molecular arrangement in the crystal structure [55, 56], Again, in the case of a-[Fe(Cp )2][Pd(dmit)2], a strong dimerization was observed for the anions, presenting also very short Pd Pd contacts of 3.485 A. Short intralayer contacts, similar to the observed in... 

The synthesis of [2.2]paracyclophane (2) and its identification by X-ray structural analysis were first reported in a short communication by Brown and Farthing 8> in 1949. A more detailed report on its molecular structure followed in 19534>. Further investigations by Lonsdale et al.5> at two different temperatures (93 and 291 °K) provided additional information about the thermal expansion and molecular vibrations in the crystal. A recent X-ray structural analysis > confirms and supplements Lonsdale s observations. 

The reaction of l,4-bis(trimethylsilyl)-2,5,7-cyclooctatriene (37) with two equivalents of n-butyhithium in a mixture of hexane and 1,2-dunethoxyethane (DME) gives the dilithi-ated species 38 in a yield of 78% (Scheme 14) . The compound could be isolated by Edelmann and coworkers as single crystals and thus the molecular structure in the solid state could be determined. It reveals an almost planar eight-membered ring which is capped by two lithium-DME units in a /j -manner from both sides of the ring plane. 

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. 

The isomerization from 10 to 11 in the crystalUne state requires not only the movements of atoms but also a change in the crystal symmetry and the reconstruction of the hydrogen bond network pattern. In the crystals of these primary ammonium carboxylates, ID ladder-type hydrogen bonds are observed. The isomerization from the ZZ to EE form is associated with the rotation of carbonyl groups and the change in the hydrogen bond structure in this case. The quantitative transformation of 10 to 11 in the crystalUne state suggests that the molecular motion in the crystals occurred cooperatively with the minimum movement of atoms in the crystals via a phase transition from the crystal of 10... 

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