COMPLEMENTARY TECHNIQUE TO X-RAY DIFFRACTION

Single crystal neutron diffraction is in many ways a complementary technique to X-ray diffraction. In neutron diffraction scattering by the atomic nuclei rather than the electron density gives rise to diffraction. However, neutrons have a spin and polarisation of the neutron beam can be used to undertake diffraction experiments to map the distribution of unpaired electrons (the spin density) in a crystal. 

X-rays to differing extents depending on the total number of electrons in the ion and, consequently, different types of ions can generally be distinguished from one another. Use of X-ray diffraction methods does have some hmitations. Firstly, the location of light atoms (e.g. H) in the presence of much heavier atoms is difficult and, sometimes, impossible. Neutron diffraction (in which neutrons are diffracted by nuclei) may be used as a complementary technique. Secondly, X-ray diffraction is seldom able to identify the state of ionization of the species present only for a few substances (e.g. NaCl) has the electron density distribution been determined with sufficient accuracy for this purpose. 

There is an enormous literature [55, 56] on even comparatively small aspects of the study of fats. What we seek to show here is how thermal methods, principally DSC, can be used to understand the complex behaviour of fats and indeed show where thermal methods are insufficient and require complementary techniques, mainly X-ray diffraction. However, we will start by considering one of the simplest DSC measurements on one of the most complex systems, namely the solid/liquid ratio of a complex mixture of triglycerides. 

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

Neutron scattering techniques are increasingly being used to study the structure and dynamics of molecules adsorbed in nanoporous materials. The most prominent example is neutron diffraction, which is complementary to X-ray diffraction to solve structural problems in zeolites and other microporous materials [1]. While the use of powder neutron diffraction is well established in the zeolite community, the spectroscopic applications of neutron scattering are less familiar. However, the constant amelioration of the neutron instrumentation and of the theoretical models provides unprecedented insights into the dynamics of the framework and of adsorbed molecules, at the atomic and... 

Some of the techniques included apply more broadly than just to surfaces, interfaces, or thin films for example X-Ray Diffraction and Infrared Spectroscopy, which have been used for half a century in bulk solid and liquid analysis, respectively. They are included here because they have by now been developed to also apply to surfaces. A few techniques that are applied almost entirely to bulk materials (e.g.. Neutron Diffraction) are included because they give complementary information to other methods or because they are referred to significantly in the 10 materials volumes in the Series. Some techniques were left out because they were considered to be too restricted to specific applications or materials. 

Table 1.1 summarises the characteristics. Electron microscopy and diffraction and X-ray topography and diffraction are complementary techniques in almost every respect. The neutron techniques have applications similar to X-rays but decisive advantages in some cases, such as the study of magnetic materials and of very thick samples. The theory is well rmderstood for all three. Two great... 

HRMAS Al and Si-NMR spectra frequently give useful information for the determination of Si-Al distribution in zeolites, but this technique should be considered as complementary to x-ray or neutron diffraction. Their combined use often provides a most complete and reliable description of the Si-Al distribution in zeolites. The limits of NMR spectra in zeolites with disordered Si-Al distributions and three or more symmetrically independent T sites must also be considered. 

Apart from the many advantages of X-ray diffraction and X-ray absorption spectroscopy, each method, as discussed in Sections II and IV, is also characterized by some important limitations in the investigation of catalysts. These limitations to a large extent are complementary, and they can therefore be overcome by using a combination of both techniques. The various approaches for performing experiments with the combined techniques are described in Section V, and some examples are given in Section VI. 

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