Crystalline state crystal structure determination

In 1986, Walz and Haase [148] presented the crystal structure of the mesogenic hydrocarbon compound l,2-bis-(4 -pentylcyclohexyl)ethane. The compound exhibits a smectic B phase over a remarkably broad range of temperature. To our knowledge, this is the only crystal structure determination of a mesogenic hydrocarbon compound up to now. Since this compound does not contain any polar groups, the arrangement in the crystalline state is... 

A similar reaction of i -Bu2Mg with 2,6-di-fert-butylphenol in the presence of 18-crown-6 affords i-BuMgOC6H3Bu-r-2,6(18-crown-6) (209) as a crystalline solid. An X-ray crystal-structure determination showed that this compound in the solid state also exists as a monomer with a a-bonded i-butyl group and a a-bonded phenoxy oxygen atom. Three adjacent oxygen atoms of the crown-ether are involved in coordination to magnesium, resulting in penta-coordination. 

During these studies, 71 was obtained as a crystalline material and its structure in the solid state was established by an X-ray crystal structure determination . In 71 the two benzoyloxymethyl groups are symmetry-related via an inversion centre at the zinc position... 

The crystal structure of [Ag P(0Me)3 2N03]2 has been determined.195 In the crystalline state, the structure consisted of a centrosymmetric dimer (21) containing a bridged nitrate. The bridging occurred via only one oxygen and this was equidistant from both silver atoms (Ag—O distances were 245.6 and 245.4 pm). The Ag—Ag separation was 409.5 pm, while the Ag—P bond lengths were 241.1 and 241.2 pm. 

In the series of the binary halides of selenium and tellurium, the crystal structure determinations of tellurium tetrafluoride (100) and of tellurium tetrachloride on twinned crystals (65, 66) were the key to understanding the various and partly contradictory spectroscopic and other macroscopic properties (e.g., 66,161,168,169,219,220, 412), as well as the synthetic potential of the compounds. In contrast to the monomeric molecular i//-tbp gas phase structures with C2v symmetry (417), the solid state structures of both are polynuclear. As the prototype of the chlorides and bromides of selenium and tellurium, crystalline tellurium(IV) chloride has a cubane-like tetrameric structure with approximate Td symmetry (Fig. 1). Within the distorted TeCla+a octa-hedra the bonds to the triply bridging chlorine ligands are much longer than to the terminal chlorines. The bonding system can be described either covalently as Te4Cli6 molecules, or, in an ionic approximation, as [(TeCl Cn4] with a certain degree of stereochemical activity of the lone pairs toward the center of the voluminous cubane center (65, 66). 

In order to form a crystal, molecules must aggregate in an orderly manner. This implies that intermolecular interactions have occurred in specific ways. It therefore follows that the crystal structure per se contains information on preferred modes of binding between the molecules in the crystalline state. In this Chapter we show how information on the most likely stereochemistries of interactions between functional groups in different molecules can be extracted from the three-dimensional coordinates of atoms listed in reports of crystal structure determinations. Three-dimensional structural data on binding stereochemistry may also be obtained from X-ray diffraction studies of the binding of small molecules to crystalline proteins and other macromolecules. These two types of information can be used, for example, to predict how drugs will interact with their biological receptors. 

Another experimental technique to study the self-association of phenols is to investigate how molecules of phenols pack together in the crystalline state. This type of analysis is made possible by the availability of the computer-based CSD. The CSD contains unit-cell dimensions of more than 230,000 (April 2001 release) three-dimensional crystal-structure determinations that have been studied by X-ray or neutron diffraction. Each crystal structure is identified by a unique six-letter code, called its REFCOD, with an additional two digits for duplicate structures and measurements. 

Valinomycin, a cyclic 12-depsipeptide with the sequence cyc/o(LVal-DHyv-DVal-LLac)3, selectively transports K ions across natural and synthetic membranes. The conformations of the K complex and the uncomplexed form of valinomycin are different, but not as markedly different as the com-plexed and uncomplexed forms of antamanide. The molecular formula of valinomycin exhibits a threefold symmetry, which is maintained in the crystalline state for the K complex. If the differences in the side chains are overlooked, the approximate symmetry is raised to An early crystal structure determination of the KAUCI4 complex (Pinkerton et al, 1969) established that the ion is octahedrally coordinated to the carbonyl oxygen atoms of the six ester groups and that the carbonyl oxygen atoms from the six amide groups form hydrogen bonds with the six NH moieties (4 1 type... 

We have indicated that a finite value of An may provide a means of detecting orientation but its magnitude and sign may vary considerably from system to system and will depend on the chemical nature and material composition (amorphous v. crystalline). The chemical structure determines whether the refractive index (polarisability) is higher along the chain (polyethylene) or perpendici lar to the chain (polystyrene-rubbery state). The composition in turn is of importance because the intrinsic optical character of a crystal may difier from that of an amorphous chain. Furthermore, polarisability interactions known as internal field and form effects may also influence the observations and hence must be considered in many instances. 

Multiple melting phenomena of fatty substances were observed during the nineteenth century, and Malkin (1930) was able to explain this behaviour as being due to polymorphism, i.e. the occurrence of alternative crystal structures in the solid state. Later the structural features of lipids in the crystalline state were seen in the first crystal structure determinations by Vand and co-workers (1949). Numerous crystal structure determinations can now be found in the literature, and some general features in the solid state packing of lipid molecules are summarized below. 

The first X-ray crystal structure determination of a tetraalkoxide was of the centrosym-metric tetranuclear ethoxo compound [Ti4(/i3-OEt)2(/i-OEt)4(OEt)io]. Each titanium atom is in a distorted octahedral coordination as indicated in Fig. 4.35. Since solution molecular weight measurements showed that the trimer [Ti3(OEt)i2] is present, the crystal structure determination clearly emphasized that it is unwise to attempt to predict crystal structures from solution data. Various attempts have been made to deduce the structure of the trimer and a recent X-ray absorption study (XANES and EXAFS) inclines to the view that five-coordinated (TBP) titanium is present in a symmetrical molecule [Ti3(/z-OEt)3(OEt)9]. The tetranuclear structure in the crystalline state was reinforced by the partial structure of the mixed alkoxide [Ti4(OMe)4(OEt)i2] which had the same Ti40i6 framework as in Fig. 4.35. The same structure was also found for the tetramethoxide [1i4(/i3-OMe)2(M-OMe)4(OMe)io]. ... 

BBN exists as the dimer [A] both in the vapor state and in a crystalline solid state. The chair-chair conformation of the dimer and the B-H bridge has been confirmed by spectral studies [5] and crystal structure determination [6]. 

A distinction between a solid and liquid is often made in terms of the presence of a crystalline or noncrystalline state. Crystals have definite lines of cleavage and an orderly geometric structure. Thus, diamond is crystalline and solid, while glass is not. The hardness of the substance does not determine the physical state. Soft crystals such as sodium metal, naphthalene, and ice are solid while supercooled glycerine or supercooled quartz are not crystalline and are better considered to be supercooled liquids. Intermediate between the solid and liquid are liquid crystals, which have orderly structures in one or two dimensions,4 but not all three. These demonstrate that science is never as simple as we try to make it through our classification schemes. We will see that thermodynamics handles such exceptions with ease. 

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