X-rays and neutrons

The structure of a fluid is characterized by the spatial and orientational correlations between atoms and molecules detemiiued through x-ray and neutron diffraction experiments. Examples are the atomic pair correlation fiinctions (g, g. . ) in liquid water. An important feature of these correlation functions is that... 

The equilibrium properties of a fluid are related to the correlation fimctions which can also be detemrined experimentally from x-ray and neutron scattering experiments. Exact solutions or approximations to these correlation fiinctions would complete the theory. Exact solutions, however, are usually confined to simple systems in one dimension. We discuss a few of the approximations currently used for 3D fluids. 

All scattering phenomena (light, x-rays and neutrons) can be interpreted in tenns of this equation (B1.9.5). 

The diffraction of low-energy electrons (and any other particles, like x-rays and neutrons) is governed by the translational syimnetry of the surface, i.e. the surface lattice. In particular, the directions of emergence of the diffracted beams are detemiined by conservation of the linear momentum parallel to the surface, bk,. Here k... 

The main problem with x-ray (and neutron) diffraction is that the infonnation it is made to yield is essentially... 

F H, O Kennard, D G Watson, L Brammer, A G Orpen and R Taylor 1987.1 ables of Bond Lengths determined by X-ray and Neutron Diffraction. 1. Bond Lengths in Organic Compounds. Journal of he Chemical Society Perkin Transactions 11 51-519. 

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

A detailed account is given in Reference 20. The techniques giving the most detailed 3-D stmctural information are x-ray and neutron diffraction, electron diffraction and microscopy (qv), and nuclear magnetic resonance spectroscopy (nmr) (see Analytical methods Magnetic spin resonance X-ray technology). 

Silver in the +3 oxidation state, including silver peroxide, ie, black oxide, marketed as AgO, is obtained by the action of the vigorous oxidising agent S20 g on Ag20 or other Ag compounds. X-ray and neutron diffraction analyses show the nominal AgO unit cell to be Ag20 Ag202- Both Ag" and Ag " are present. Another compound of potentially important commercial value is Ag O, which has a unit cell of two Ag and two Ag ions. Its preparation is as follows ... 

X-Ray and Neutron Scattering as Probes of the Dynamics of Biological Molecules... 

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

The X-ray and neutron scattering processes provide relatively direct spatial information on atomic motions via detennination of the wave vector transferred between the photon/neutron and the sample this is a Fourier transfonn relationship between wave vectors in reciprocal space and position vectors in real space. Neutrons, by virtue of the possibility of resolving their energy transfers, can also give infonnation on the time dependence of the motions involved. 

II. BASIC EQUATIONS RELATING ATOMIC POSITIONS TO X-RAY AND NEUTRON SCATTERING... 

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. 

H2 could be quantitatively removed at room temperature either by partial evacuation or by sparging the solution with argon. Definitive confirmation that the complexes did indeed contain 7J--H2 came from X-ray and neutron diffraction studies on the bisftri t-propylphosphine) analogue at —100°, which revealed the side-on coordination of H2 as shown in Fig. 3.2. During the past decade many other such compounds have been prepared and studied in great detail, and the field has been well reviewed. ... 

Figure 3.2 The geometry of iner-/ran.v-[W(CO)3-(ij--H2)(Pf 3)2l from X-ray and neutron diffraction data r(H-H) 84pm (compared with 74.14 pm for free H2), I(W-H) 175 pm. Infrared vibration spectroscopy gives v(H-H) 2690cm compared with 4159cm (Raman) for free Hj.

Intimate information about the nature of the H bond has come from vibrational spectro.scopy (infrared and Raman), proton nmr spectroscopy, and diffraction techniques (X-ray and neutron). In vibrational spectroscopy the presence of a hydrogen bond A-H B is manifest by the following effects ... 

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