Framework structure determination

Zeolitic materials have been prominent amongst those so far studied by high resolution powder diffraction using synchrotron X-rays [36]. High definition synchrotron PXD data has been helpful in a number of framework structure determinations and has facilitated studies of planar faulting (see below). Successful Rietveld refinements of the framework structures of zeolite ZSM-11 [37, 38] and silica-ZSM-12 [39], and of the complete structures of zeolite Y containing cadmium sulfide [40] and cadmium selenide [41] clusters have been described. 

The secondary structure elements, formed in this way and held together by the hydrophobic core, provide a rigid and stable framework. They exhibit relatively little flexibility with respect to each other, and they are the best-defined parts of protein structures determined by both x-ray and NMR techniques. Functional groups of the protein are attached to this framework, either directly by their side chains or, more frequently, in loop regions that connect sequentially adjacent secondary structure elements. We will now have a closer look at these structural elements. 

Azinomycins A and B (64 and 65 Scheme 11.5) [115, 116] were isolated in 1986 from Streptomyces griseofuscus at the SS pharmaceutical company in Japan and were found to have potent in vitro cytotoxicities [117] and to display significant in vivo antitumor activity [118]. Since their discovery [117] and structure determination [119], their unusual molecular framework has attracted the attention of many chemists interested in their mode of action and in developing synthetic approaches to them [115,116]. The first total synthesis of azinomycin A was achieved in 2000 by Coleman et al. [120]. 

The diazaphosphane or aminoiminophosphane ligands with a NPN framework are another subclass of cyclophosphazenes. These compounds with both phosphorus in oxidation state (111) [104-110] and (V) [111-112] have been employed in the synthesis of four membered heterocycles and coordination chemistry with group 13 derivatives. Several complexes of trivalent phosphorus derivatives with both aluminum halide and alkyls are known as illustrated for 48 in Scheme 21 [113-119]. The structure determination of 48 confirms the formation of a four membered metallacycle [116, 117],... 

Malrieu predicted qualitatively that the l-silaallene framework would be nonlinear. but how much the moiety would deviate from 180 was unclear. The first quantitative values were seen with the isolation and structural determination of the two 1-silaallenes 56 and 59a, whieh have very similar Si=C=C angles of 173.5 and 172.0, respectively (Table III). This is an average deviation of 7.3 from linearity—significant, but relatively small compared to the deviations shown by the germanium and tin substituted allenes. 

For example, clusters identified by IR spectra and extraction as Ir4(CO)i2 on y-Al203 were found by EXAFS spectroscopy to have an Ir-Ir coordination number of nearly 3, consistent with the tetrahedral structure of the metal frame EXAFS spectroscopy produces the equivalent result for sohd Ir4(CO)i2 [27]. EXAFS spectroscopy is the most appropriate method for determination of framework structures of supported clusters, but it is limited by the errors to clusters with at most about six metal atoms. Thus, it has been used to determine frameworks that are triangular (EXAFS first-shell metal-metal coordination number of 2), tetrahedral (EXAFS first-shell metal-metal coordination number of 3), and octahedral (EXAFS first-shell metal-metal... 

The information on carbon chemical shifts and multiplicities is invaluable for structure determination. It would be ideal if we also had a method for obtaining information directly on carbon-carbon bonding in the compound under study, since this would allow us to draw on paper at least parts of the carbon framework of the molecule. 

The discovery of the base-paired, double-helical structure of deoxyribonucleic acid (DNA) provides the theoretic framework for determining how the information coded into DNA sequences is replicated and how these sequences direct the synthesis of ribonucleic acid (RNA) and proteins. Already clinical medicine has taken advantage of many of these discoveries, and the future promises much more. For example, the biochemistry of the nucleic acids is central to an understanding of virus-induced diseases, the immune re-sponse, the mechanism of action of drugs and antibiotics, and the spectrum of inherited diseases. 

Electron crystallography offers an alternative approach in such cases, and here we describe a complete structure determination of the structure of polymorph B of zeolite beta [3] using this technique. The clear advantage of electron microscopy over X-ray powder diffraction for elucidating zeolite structures when they only occur in small domains is demonstrated. In order to test the limit of the structural complexity that can be addressed by electron crystallography, we decided to re-determine the structure of IM-5 using electron crystallography alone. IM-5 was selected for this purpose, because it has one of the most complex framework structures known. Its crystal structure was solved only recently after nine years of unsuccessful attempts [4],... 

It has been several decades since oxo-centered triruthenium-carboxylate complexes with triangular cluster frameworks of Ru3(p3-0)(p-00CR)6 (R = alkyl or aryl) were first isolated [1,2]. In the early 1970s, the first oxo-centered triruthenium complex was structurally characterized by Cotton through X-ray crystal structural determination [3]. Since then, oxo-centered trinuclear ruthenium-carboxylate cluster complexes with general formula [Ru30(00CR)6(L)2L ]n+ (R = aryl or alkyl, L and... 

With respect to the naked metal atoms, this is the largest metalloid duster that has ever been structurally determined by diffraction methods. The Ga2 unit in the center of the 64 naked Ga atoms is remarkable and unique in this entire field of chemistry [Figure 2.3-28(c)]. The Ga2 unit, which contains a bond that is almost as short (2.35 A) as the above-mentioned Ga-Ga triple bond (2.32 A) and resembles the Ga2 unit of a-Ga (2.45 A), is surrounded by a Ga32 shell in the form of a football with icosahedral caps [see d-Ga (Figure 2.3-17)]. The apex and base atoms of this Ga32 unit are naked and are oriented towards each other in the crystal in an unusual fashion (see below). The Ga2Ga32 unit is surrounded by a belt of 30 Ga atoms that are also naked . Finally the entire Ga framework is protected by 20 GaR groups. 

The core of the current version of Candid is implemented in standard Fortran-77 and has been built upon the data structures and into the framework of the user interface of the program Dyana. The standard schedule and parameters for a complete automated structure determination with Candid and Dyana are specified in a script written in the interpreted command language Inclan that gives the user high flexibility in the way automated structure determination is performed without the need to modify the compiled core part of Candid [26]. 

Figure 2. X-Ray crystal structures of KI complexing with N7(2-methoxyethyl)-monoaza-15-crown-5 and (2-methoxyethy1)-monoaza-18-crown-6. Complete structures are illustrated at the left and "framework structures showing only KI and the ligand s donor groups are illustrated at the right. Structures were determined in collaboration with F. Fronczek and R. Candour.

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

Among the over thirty framework compounds, at least 6 have been structurally determined by means of single crystal X-ray diffraction. These include GaP04-C3, -C4, -C7, AIASO4-I, -2 and GaAs04-2. The details of the cell parameters and the primary building units (PBU) of their structures are given in Table 4. 

Fig. 11 Crystal structure of a metal-organic framework material Zn2(fina)2(bipy) prepared by mechanochemical synthesis, with the structure determined directly from powder XRD data. The structure is viewed (a) along the c-axis and (b) along the b-axis. The two (identical) interpenetrated frameworks are indicated by yellow and purple shading. For comparison, (c) and (d) show the corresponding views of the stmcture of a DMF solvate material Zn2(fma)2(bipy)(DMF)o.5 prepared by a solvothermal route. Although there is some similarity between these structures, there are nevertheless important stmctural differences between them...

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