Neutron-diffraction analysis

Since the recognition in 1936 of the wave nature of neutrons and the subsequent demonstration of the diffraction of neutrons by a crystalline material, the development of neutron diffraction as a useful analytical tool has been inevitable. The initial growth period of this field was slow due to the unavailability of neutron sources (nuclear reactors) and the low neutron flux available at existing reactors. Within the last decade, however, increases in the number and type of neutron sources, increased flux, and improved detection schemes have placed this technique firmly in the mainstream of materials analysis. 

Another major difference between the use of X rays and neutrons used as solid state probes is the difference in their penetration depths. This is illustrated by the thickness of materials required to reduce the intensity of a beam by 50%. For an aluminum absorber and wavelengths of about 1.5 A (a common laboratory X-ray wavelength), the figures are 0.02 mm for X rays and 55 mm for neutrons. An obvious consequence of the difference in absorbance is the depth of analysis of bulk materials. X-ray diffraction analysis of materials thicker than 20—50 pm will yield results that are severely surface weighted unless special conditions are employed, whereas internal characteristics of physically large pieces are routinely probed with neutrons. The greater penetration of neutrons also allows one to use thick ancillary devices, such as furnaces or pressure cells, without seriously affecting the quality of diffraction data. Thick-walled devices will absorb most of the X-ray flux, while neutron fluxes hardly will be affected. For this reason, neutron diffraction is better suited than X-ray diffraction for in-situ studies. 

Within nuclear reactors, neutrons are a primary product of nuclear fission. By controlling the rate of the nuclear reactions, one controls the flux of neutrons and provides a steady supply of neutrons. For a diffraction analysis, a narrow band if neutron wavelengths is selected (fixing X) and the angle 20 is varied to scan the range of values. 

Except for Ceo, lack of sufficient quantities of pure material has prevented more detailed structural characterization of the fullerenes by X-ray diffraction analysis, and even for Ceo problems of orientational disorder of the quasi-spherical molecules in the lattice have exacerbated the situation. At room temperature Cgo crystallizes in a face-centred cubic lattice (Fm3) but below 249 K the molecules become orientationally ordered and a simple cubic lattice (Po3) results. A neutron diffraction analysis of the ordered phase at 5K led to the structure shown in Fig. 8.7a this reveals that the ordering results from the fact that... 

X-ray and neutron diffraction analysis of the Ca -ATPase of sarcoplasmic reticulum... 

The high levels of, sy -diastereoselectivity suggest aldolization through a closed Zimmerman-Traxler-type transition structure via intermediacy of the Z-enolate. When the transformation is performed using PhSiDj, a single deuterium is incorporated at the /3-position of the product as an equimolar mixture of epimers, inferring rapid isomerization of the kinetically formed cobalt enolate prior to cyclization or reversible aldol addition. The stereochemistry of the deuterated product was established by single crystal neutron diffraction analysis (Scheme 44). 

A neutron-diffraction analysis of a single crystal of KHFj found the proton to be centred to within 100 pm (Peterson and Levy, 1952). An early nmr investigation of the H and spectra came to the same conclusion and narrowed the uncertainty to 60 pm (Waugh et al., 1953), and a later nmr analysis reduced it to 25 pm (Paratt and Smith, 1975). A neutron diffraction study of NaDFj showed this to have a centred deuterium atom with Rp. F = 226.5 pm (McGaw and Ibers, 1963). 

Similar to X-Ray and neutron diffraction analysis, electron dilFraction structure analysis consists of such main stages as the obtaining of appropriate diffraction patterns and their geometrical analysis, the precision evaluation of diffraction-reflection intensities, the use of the appropriate formulas for recalculation of the reflection intensities into the structure factors, finally the solution of the phase problem, Fourier-constructions. 

Figure 3. Schematic diagram of the hydrogen-bond structure of p-maltose monohydrate (MALTOS11). The anrows indicate infinite chains. Distances and angles are from the neutron diffraction analysis.

Diffraction analysis—whether employing x-rays, electrons, or neutrons—is the method of choice for obtaining structural information on crystalline substances. The application of the well understood principles and methods of diffraction analysis to single crystals of sufficient size and perfection can lead to a detailed determination of the crystal structure, without recourse to any auxiliary methodology. Hundreds of mono- and oligosaccharide molecules have been characterized by these means (1), yielding not only an increased understanding of their structures in the solid state, but also a data base useful for extrapolation to other states and molecular interactions. 

Chromium metal may be analyzed by various instrumental techniques including flame and furnace AA spectrophotometry (at 357.9 nm) ICP emission spectrometry (at 267.72 or 206.15 nm), x-ray fluorescence and x-ray diffraction techniques, neutron activation analysis, and colorimetry. 

The section Analysis starts with elemental composition of the compound. Thus the composition of any compound can be determined from its elemental analysis, particularly the metal content. For practically all metal salts, atomic absorption and emission spectrophotometric methods are favored in this text for measuring metal content. Also, some other instrumental techniques such as x-ray fluorescence, x-ray diffraction, and neutron activation analyses are suggested. Many refractory substances and also a number of salts can be characterized nondestructively by x-ray methods. Anions can be measured in aqueous solutions by ion chromatography, ion-selective electrodes, titration, and colorimetric reactions. Water of crystallization can be measured by simple gravimetry or thermogravimetric analysis. 

O Connor, B. H., and Dale, D. H. (1966). A neutron diffraction analysis of the crystal structure of tetragonal nickel sulfate hexadeuterate. Acta Crystallogr. 21, 705-709. 

Nerol, oxidation, 788-9, 790 Neuroprostanes, H2-isoprostane bicycUc endoperoxides, 612 Neutron diffraction analysis, hydrogen peroxide, 96... 

Hosoya T, Uekusa H, Ohashi Y, Ohhara T, Kimura H, Noda Y (2003) Deuterium migration mechanism in chiral thiolactam formation by neutron diffraction analysis. Chem Lett 32 ... 

The compound K2 [Pt(CN)4] Br0 3-3H20 (KCP(Br)) forms lustrous, coppery-colored tetragonal12 crystals [a = 9.907(3) and c = 5.780(2) A, space group P4mm]. The structure has been established by neutron diffraction analysis.12... 

A porous particle contains many interior voids known as open or closed pores. A pore is characterized as open when it is connected to the exterior surface of the particle, whereas a pore is closed (or blind) when it is inaccessible from the surface. So, a fluid flowing around a particle can see an open pore, but not a closed one. There are several densities used in the literature and therefore one has to know which density is being referred to (Table 3.15). True density may be defined as the mass of a powder or particle divided by its volume excluding all pores and voids. True density is also referred to as absolute density or crystalline density in the case of pure compounds. However, this density is very difficult to be determined and can be calculated only through X-ray or neutron diffraction analysis of single-crystal samples. Particle density is defined as the mass of a particle divided by its hydrodynamic volume. The hydrodynamic volume includes the volume of all the open and closed pores. Practically, the hydrodynamic volume is identified with the volume included by the outer surface of the particle. The particle density is also called apparent or envelope density. The term skeletal density is also used. The skeletal density of a porous particle is higher than the particle one, since it is the mass of the particle divided by the volume of solid material making up the particle. In this volume, the closed pores volume is included. The interrelationship between these two types of density is as follows (ASTM, 1994 BSI, 1991) ... 

Pepinsky R. Hydrogen-deuterium replacement in neutron diffraction analysis, Eleventh Annual Pittsburgh Diffraction Conference, Program... 

Tenzer L, Frazer B. C. Pepinsky li. A neutron diffraction analysis of the NH4HaP04 structure Amer Cryst. Assoc., French Lick, Ind. Program and. Abstracts., p. 20 (1956), Ada QrysL 1958 11 505. 

It might be thought that the electron density maps which arise from an X-ray analysis should provide detailed information about electron distribution and hence bonding. The electron densities obtained from routine X-ray studies are not sufficiently accurate for such purposes. However, the finer details of electron density distributions in crystals can be obtained in favourable cases by very careful and accurate X-ray diffraction studies. Neutron diffraction (see below) can also provide such information. 

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