Liquids, diffraction methods

Table 8.53 shows the main features of XAS. The advantages of EXAFS over diffraction methods are that the technique does not depend on long-range order, hence it can always be used to study local environments in amorphous (and crystalline) solids and liquids it is atom specific and can be sensitive to low concentrations of the target atom (about 100 ppm). XAS provides information on interatomic distances, coordination numbers, atom types and structural disorder and oxidation state by inference. Accuracy is 1-2% for interatomic distances, and 10-25 % for coordination numbers. 

The methods available for structure determination are surveyed. Those that are applicable to the gas phase, i.e. electron diffraction and rotational spectroscopy, are suitable mainly for small molecules. Data for the crystalline phase are usually relatively straightforward to obtain, but acquiring reliable structural data for silicon compounds as liquids or in solution by diffraction methods or liquid crystal NMR spectroscopy remains a challenge. 

Synthesis of the first mesoionic nematic and smectic A liquid crystals derived from sydnones has been described and their self-organization into liquid crystal phases has been studied by optical, calorimetric, and powder X-ray diffraction methods <2005CC1552>. 

Experimental equipment for X-ray diffraction methods has improved enormously in recent years. CCD detectors and focusing devices (Goepel mirror) have drastically reduced the data acquisition time. Cryogenic systems have been developed which allow structural studies to be extended down to the liquid helium temperature range. These developments have had important implications for SCO research. For example, fibre optics have been mounted in the cryostats for exploring structural changes effected by light-induced spin state conversion (LIESST effect). Chaps. 15 and 16 treat such studies. 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

The basic modem data describing the atomic stmcture of matter have been obtained by the using of diffraction methods - X-ray, neutron and electron diffraction. All three radiations are used not only for the stmcture analysis of various natural and synthetic crystals - inorganic, metallic, organic, biological crystals but also for the analysis of other condensed states of matter - quasicrystals, incommensurate phases, and partly disordered system, namely, for high-molecular polymers, liquid crystals, amorphous substances and liquids, and isolated molecules in vapours or gases. This tremendous... 

With these factors in mind, a new method to evaluate the conformation of an amphiphilic molecule at the site of interest was Introduced. The method is built on the fact that the determination of Interlayer spacings of a lamellar liquid crystal using low angle X-ray diffraction methods in combination with density measurements will provide sufficient information to calculate the cross-sectional areas occupied by each amphiphlle (19). 

Other crystalline inorganic polymers such as poly(dichlorophosphazene), poly(aryloxyphosphazenes), liquid crystalline polysiloxanes and poly(dichloro-silane) have also been studied by X-ray diffraction methods, enabling the conformations in the crystallites in the solid state to be established. 

X-ray diffraction methods have also been used in the study of the structures of liquids. The continual movements which occur in liquids do not affect the determination of the principal interatomic distances. For earlier work on the subject see Randall s book (1934). Among later papers, those of Harvey (1939) on ethanol and Bray and Gingrich (1943) on carbon tetrachloride are typical. 

Other diffraction methods include electron diffraction, which may be used to determine the structures of gases or of volatile liquid substances that cannot be obtained as crystals suitable for x-ray diffraction, and neutron diffraction, which has special application for crystals in which the exact location of hydrogens is desired. Hydrogen does not have sufficient scattering power for x rays to be located precisely by x-ray diffraction. 

It should be mentioned that in some cases it may happen that a questioned ink can be more positively identified through presence of fluorescent or other unique components in the formulation. When sufficient questioned ink is available and the proprietory formula composition has been furnished, further analysis can lead to the identification of a component which may provide additional proof of the identity of the ink. For example, there are a variety of fatty acids, resins, and viscosity adjusters added to inks which can be readily identified by TLC or gas liquid chromatography (GLC), when sufficient ink is available. As further examples, amorphous carbon and graphite, which are common dispersion ingredients in ballpoint inks, can be distinguished using electron diffraction methods. 

The solution X-ray diffraction method has been widely used for the structural analysis of liquids, solvated ions, and complexes in solution,... 

Several attempts have been made to determine the symmetry (and hence the conformation of the phosphazene ring) of halogenocyclo-phosphazenes in the solid, liquid, and solution states using infrared and Raman spectroscopy (2, 136, 249, 255, 255a, 422). With some exceptions, there is reasonable agreement between the structures determined by diffraction methods and those predicted by vibrational spectroscopy. The calculation of force constants in N3P3C16 and assignment of vibration frequencies have been discussed (118). 

Furukawa, if. The Radial Distribution Curves of Liquids by Diffraction Methods. Rept. Progr. Phys. 25, 395 (1962). 

This paper will discuss the state of the art in 3D structure refinement using empirical, semi-empirical and ab initio methods. We believe that the success story of liquid state NMR in protein structure elucidation is going to continue within the solid state (or membrane environment) if chemical shifts can be successfully exploited. Neutron and X-ray diffraction methods owe their success to a simple formula that connects the measured reflex intensities with the nuclear positions or the electron density, respectively. The better we understand how chemical shifts change with the three-dimensional arrangement of atoms, the more reliably we can construct molecular models from our NMR experiments. As we can in principle determine up to six numbers per nucleus if we perform a full chemical shift tensor analysis, we need to address the question whether whole CS tensor or at least its principal values can be used in structure calculations. 

An interesting feature of XANES for structure determination in condensed matter is that it can be used to study the local structure both in crystalline and disordered materials. Therefore once a local structure has been solved for a crystalline material also the similar structure in amorphous, liquid or complex materials, where diffraction methods cannot be applied, can be solved. 

E.A.V. Ebsworth, D.W.H. Rankin and S. Cradock (1991) Structural Methods in Inorganic Chemistry, 2nd edn, CRC Press, Boca Raton, FL - A chapter on diffraction methods includes electron diffraction by gases and liquids. 

In the case of a crystalline solid it is possible to determine, by diffraction methods, the equilibrium positions and vibrational amplitudes of all the atoms involved and this information specifies the structure of the crystal. A liquid, by its very nature, cannot have a structure in this sense. The environment of each atom or molecule is continually changing and we must usually be satisfied with some sort of time-averaged specification of the environment or, which is essentially the same thing in this case, a space average over the environments of many different molecules. 

Furukawa, K. (1962). The radial distribution curves of liquids by diffraction methods. Rep. Prog. Phys. 25, 395-440. [73]... 

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