Diffraction based techniques

Two different types of guest-induced flexibilities exist in MOF host lattices. The first can be considered as essentially static in nature, involving bulk framework deformations that may be readily characterised using diffraction-based techniques and which are frequently observable at the macroscale through changes in crystal dimensions. The second are dynamic and arise due to molecular vibrations or local guest-induced framework deformations away from the parent structure. The latter are not so readily detectable by diffraction methods and... 

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy has emerged over the years as a powerful analytical method in solid-state chemistry, especially with the advancements in techniques that allow the acquisition of high-resolution spectra [47]. In the broadest sense, ssNMR is mostly applied in characterization of crystalline materials as a means to support PXRD structural analyses by providing information on the number of molecules in the asymmetric unit or the symmetry of the occupied positions within the unit cell. Another major field of application is the structural characterization of amorphous and disordered solids where standard X-ray diffraction-based techniques fail to give detailed structural information. When discussing ssNMR in the context of API polymorphism and synthesis of co-crystals,... 

Diffraction-based techniques are widely applicable to crystalline molecular sieves. For samples available as crystals ca. 10 pm or more in size. X-ray single... 

In this chapter I will show how the structural chemistry of framework molecular sieves has been elueidated by diffraction-based techniques and spectroscopies. Computer simulation of structures will be discussed separately in Chapter 4 as an additional technique. 

The techniques can be broadly classified into two groups those which directly identify the atomic species present and then provide structural information about the identified species from diffraction or scattering effects (EXAFS, SEXAFS, and XPD) and those which are purely diffraction-based and do not direcdy identify the atoms involved, but give long-range order information on atomic positions from... 

In this chapter, we provide an overview of our recent efforts to develop a fundamental science base for the design and preparation of optimal lipid-based carriers of DNA and siRNA for gene therapy and gene silencing. We employ synthesis of custom multivalent lipids, synchrotron X-ray diffraction (XRD) techniques, optical and cryo-electron microscopy, as well as biological assays in order to correlate the structures, chemical, and biophysical properties of cationic liposome (CL)-NA complexes to their biological activity and to clarify the interactions between CL-NA complexes and cellular components. Earlier work has been reviewed elsewhere [1-7] and will not be covered exhaustively here. 

In pulse-echo-based techniques, the time of flight in a sample cannot be determined simply from the observation of the time span between adjacent echoes in the echo pattern if plane parallel transducers operated at resonant frequencies are employed. Transducers introduce substantial errors if the velocity is derived from such measurements, especially if relatively short samples are used. Various correction approaches have so far been developed in order to consider the influence of resonant transducers and the effects of diffraction [31-33]. The need for corrections can be avoided and a broad operational bandwidth obtained by using short pulses of duration equal to or shorter than the transduction [34] this requires a time resolution better than the transit time in the transducer. This short-pulse excitation (e.g. the maximum for a 10-MHz transducer is 50 ns) requires a high-power wide-band ultra-linear amplifier to ensure the detection of US signals with sufficient resolution under non-resonant conditions. 

The scattering of the X-ray beam is directly proportional to the number of electrons an element has. In any X-ray-based technique, elements which are isoelectronic (have the same number of electrons) are indistinguishable to X-rays. This can cause problems in assigning sites in single crystal diffraction, and can lead to additional absences in the powder X-ray diffraction pattern. 

In the case of thin films or monolayers, two different techniques can be employed these are the total external-reflection Bragg diffraction (TERBD) technique introduced by Eisenberger and Marra and the previously mentioned technique based on crystal truncation rods, introduced by Robinson, which can provide in-plane structural information or information on interfacial roughness, respectively. 

Also, the result of any diffraction-based trial-and-error fitting is not necessarily unique it is always possible that there exists another untried structure that would give a better fit to experiment. Hence, a multi-technique approach that provides independent clues to the structure is very ffuitfiil and common in surface science such clues include chemical composition, vibrational analysis and position restrictions implied by other stmctural methods. This can greatly restrict the number of trial structures which must be investigated. 

With respect to other major literature on or related to XRE, are chapters in various analytical series and individual books. Two chapters are in the first edition of the famous Treatise on Analytical Chemistry. Comprehensive coverage of X-ray methods absorption, diffraction, and emission is provided by Liebhafsky et al. (1964) in a 90-page chapter in the section on Optical methods of analysis (E. J. Meehan, section advisor). This is immediately followed by the chapter by Wittry (1964) on X-ray microanalysis by means of electron probes. Chapters on relevant topics appearing in the other well known series on analytical chemistry. Comprehensive Analytical Chemistry, are by Beretka (1975) (Analytical applications of electron microscopy) with a brief mention of the XRF-based technique electron probe... 

Following the observations of Batterman, a quantitative explanation of the fluorescence yield has been based on the dynamical diffraction theory of von Laue and Ewald (Laue 1960). The formal theory for the Bragg diffraction XSW technique has been described by Afanas ev et al. (1978), Takahashi and Kikuta (1979), Hertel et al. (1985) and Bedzyk and Materlik (1985), as well as in a review by Zegenhagen (1993). The theory of dynamical X-ray diffraction has been reviewed by Laue (I960), Batterman and... 

Spectroscopic methods give struetural details complementary to those available from diffraction-based teehniques. The most generally applied methods continue to be NMR and vibrational spectroseopy (especially IR), but element specific techniques such as X-ray absorption spectroscopy are also of importance. As I will describe in Chapters 7 and 8, NMR and IR have also been used extensively for the study of the interactions of adsorbed moleeules with molecular sieves and to investigate the nature of acidity in these solids. 

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