Solids from crystallographic information

A final concentration-like unit that is sometimes needed is the site fraction, which is essentially a unitless occupancy factor that can typically be obtained from crystallographic information and defect models of a solid. A site fraction gives the fraction of sites (e.g., crystalline lattice sites) of a particular type in a material which are occupied by a particular species i. Thus, it is a ratio of the number of sites of a particular type (/ ) that are occupied by a certain species i divided by the total number of sites of that particular type in the material ... 

In the case of DuP747 [24], XRD, DSC, and thermomicroscopic studies determined the polymorphic system to be monotropic. Distinct diffuse reflectance IR, Raman, and solid state 13C NMR spectra existed for each physical form. The complementary nature of IR and Raman gave evidence that the polymorphic pair were roughly equivalent in conformation. It was concluded that the polymorphic character of DuP 747 resulted from different modes of packing. Further crystallographic information is required in order to determine the crystal packing and molecular confirmation of this polymorphic system. 

Analogous to the DuP 747 study, complete crystallographic information was not possible on the fosinopril sodium polymorphic system [25], Two known polymorphs (A and B) were studied via a multidisciplinary approach (XRD, IR, NMR, and thermal analysis). Complementary spectral data from IR and solid state 13C NMR revealed that the environment of the acetal sidechain of fosinopril sodium differed in the two forms. In addition, possible cis-trans isomerization about the CgN peptide bond may exist. These conformational differences are postulated as the origin of the observed polymorphism in fosinopril sodium in the absence of the crystallographic data for form B (single crystals not available). 

The ordered mesoporous materials (or crystalline mesoporous materials) such as MCM-41 (MCM stands for Mobil composite of matter), MCM-48 and SBA-15 (SBA stands for University of California, Santa Barbara) are a new generation of materials that are different from nonordered (amorphous) mesoporous materials. They are amorphous and not ordered at the atomic level from a classical crystallographic view point, but their regular channels or pores are ordered at the nanometer level. Because of this, these materials have certain characteristics of crystalline solids. Their structural information can be obtained by diffraction methods and other structural analysis techniques. The discovery of periodic mesoporous structures is a major advance in composite organic-inorganic materials synthesis. 

Figure 1.6 is a schematic diagram of some of the important interactions between a solid specimen and an incident X-ray beam. These will result in diffracted X-rays, which have satisfied particular angular relations with lattice planes in the solid and will, therefore, contain crystallographic information. The interactions will also result in the ejection of photoelectrons, which will have energies equal to the difference between that of the incident photon and the binding energy of the electron in the state from which it is ejected. 

Solid-state C NMR spectroscopy (SS-NMR) is a powerful and sensitive technique for polymorph characterization and differentiation. Since different polymorphs have different crystal structures, the chemical environment of at least a few of the atoms will differ from one structure to another. This technique also provides valuable crystallographic information, such as the number of crystallo-graphically independent molecules in the crystal structure due to doubled peaks for the same C atom. In addition to C NMR, N CP-MAS NMR (cross-polarized magic-angle spinning) is useful for characterizing polymorphic systems, for example, polymorphs of sulfathiazole, those containing N-heterocycles, and neutral co-crystal versus salt formation. ... 

Representative examples of each coordination number follow. Coordination numbers below six are so far found only in ct-bonded organometalUcs, amides, and the like, which employ bulky ligands. Note that most precise information on lanthanide coordination comes from crystallographic studies, so that it is necessary to state the caveat that what is best known concerns the solid state, even though much is also known of solution behavior. 

Chapter 2 has presented summaries of mechanisms of formation of carbons, the origins of microporosity, the structure of carbons, and the structure and composition of surfaces which make up microporosity. From these information, over recent years, many attempts have been made to create models, ranging from hand-drawn cartoons, to computer graphics and to computer simulations, all of which attempt to describe essential properties of these carbons. This is far from being an easy task because not only must the model meet the requirements of, say, the crystallographer but it must also meet the requirements of all the other disciplines (Table 3.1), in particular the surface chemist whose requirements are detailed and lengthy. In this respect, activated carbon is one of the most complex and unique of materials existing in the solid phase (there is no doubt about this). 

NOESY NMR spectroscopy is a homonuclear two-dimensional experiment that identifies proton nuclei that are close to each other in space. If one has already identified proton resonances in one-dimensional NMR spectroscopy or by other methods, it is then possible to determine three dimensional structure through NOESY. For instance, it is possible to determine how large molecules such as proteins fold themselves in three-dimensional space using the NOESY technique. The solution structures thus determined can be compared with solid-state information on the same protein obtained from X-ray crystallographic studies. The pulse sequence for a simple NOESY experiment is shown in Figure 3.23 as adapted from Figure 8.12 of reference 19. 

ID-ROESY studies performed on the complex between (+)-BrPh and /3-CD in solution did not allow one to explain the NOE effect observed on the protons of the maleate counteranion (70). X-ray crystallographic studies performed on the monocrystals obtained from a 1 1 aqueous solution of (+)-BrPh maleate and /3-CD (Fig. 8) provide a plausible explanation for the contradiction maintained in Ref. 70. In particular, as shown in Fig. 8, (+)-BrPh forms with /3-CD, at least in the solid state, not a 1 1 complex but a complex with 1 2 stoichiometry. In this complex the (+)-BrPh molecule is sandwiched between two molecules of /3-CD. The 4-bromophenyl moiety of (+)-BrPh enters the cavity of one of the /3-CD molecules, whereas the cavity of another /3-CD molecule is occupied by the maleate counteranion. Thus, X-ray crystallography may provide useful information on the supra-molecular structure of the selector-selectand complexes and in this way complement well ID-ROESY data. However, the aforementioned possible differences between the structure of the complexes in solution and in the solid phase must be considered. 

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