Neutron diffraction with isotope substitution

The main problem for the X-ray method is that H atoms do not scatter X-rays sufficiently for clear scattering patterns to result. Since what is of interest is the interaction of ions with water, this is a clear drawback. However, modem second and third generation synchroton generated X-rays give a much more intense beam and this has partially compensated for the poor scattering capacity of H atoms. Neutron diffraction is not limited in this way. In modem work, isotopicaUy substituted ions are used, and here accessibility and cost can be a problem. Furthermore, if neutron diffraction with isotope substitution is used it is essential that the isotopes show significantly different scattering. For instance and 

Despite this, diffraction and computer simulations are easily the most searching tools for studying solvation. The main aim of the diffraction studies has been to determine the details of the atomic environment of the solvent and the solute particles. Other than NMR probes, the best way to determine such environments is through determination of the pair radial distribution function, gab r). This gives the probability of finding atom, b, at a distance, r, from atom a. In the present context of an electrolyte solution of X Yi, in HaOfl) there are lOpairs of types of atoms described by 10 pairwise radial distribution functions, gab(r)- They are  

The total radial distribution function g r) is a linear combination of these 10 individual pairwise radial distribution functions, and this can be found from the overall scattering in the diffraction experiment. This is a useful quantity to have, but the more informative pairwise functions are essential in determining the complete environment of the ions in solution. However, it is difficult to split up the observed overall g(r) determined from straightforward analysis of the scattering into its components. It would be much easier to generate g(r) from the individual components if they were available. 

Scattering patterns can be translated by standard diffraction techniques to give these pairwise distribution functions. This is difficult to do with X-ray scattering patterns, although modem techniques such as anomalous X-ray scattering have helped. It is relatively easy to determine these using the nuclear diffraction isotope substimtion technique. 

The electrolyte X Yj, is smdied in solution. What is wanted is to find out about the hydration around the cation and the anion, and to find out if the water stmcture is altered. If the cation is substimted by an isotope and if the scattering abilities of both are different, then a different scattering pattern will be found. If there is more than one isotope of the ion then different 

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A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

Mason, P.E., Neilson, G.W., Enderby, J.E., Saboungi, M.-L., and Brady, J.W., Structure of aqueous glucose solutions as determined by neutron diffraction with isotopic substitution experiments and molecular dynamics calculations, /. Phys. Chem. B, 109,13104-13111, 2005. 

In the present study, we have made X-ray diffraction, neutron diffraction with isotopic substitution, and quasi-elastic neutron scattering measurements on highly concentrated aqueous solutions of lithium halides in a wide temperature range from room temperature to below glass transition temperature, from which the microscopic behaviors of the static structure and dynamic properties of the solutions are revealed with lowering temperature. The results obtained are discussed in connection with ice nucleation, anisotropic motion of water, crystallization, and the partial recovery of hydrogen bonds. 

High-Resolution Electron Microscopy, Neutron Diffraction with Isotopic Substitution and X-Ray Absorption Fine Structure for the Characterisation of Active Sites in Oxide Catalysts... 

Figure 8 (left). Atom-atom radial distribution functions for subcritical water (T=573 K, p=0.72 g/cm ) from MC simulations with the TIP4P potential (solid lines). Thick dotted hne - X-ray diffraction (Gorbaty and Demianets 1983) thin dotted lines with error bars - neutron diffraction with isotope substitution, NDIS (Soper et al. 1997). [Reproduced with permission from JPhys Chem A 1997, 101, 9720-9727. 1997 American Chemical Society.]... 

CIP = contact ion pair LP = lone pair NDIS = neutron diffraction with isotope substitution RDF = radial distribution function SCW = supercritical water SCWO = supercritical water oxidation SPC = simple point charge SPC/E extended simple point charge SPCG = simple point charge gas phase dipole SShIP = solvent-shared ion pair SSIP = solvent-separated ion pair TST = transition state theory. 

Variation of b in three different experiments by varying the isotopic composition permits the three independent linear equations to be solved to yield the three partial structure factors. The Fourier transform of each of these yields the partial pan-correlation functions gu(r) for catimi-anion correlation, and gn(r) and gjj(r) for correlations of ions of the same sign. The Neutron diffraction with isotope substitution (NDIS) method permits this variation of b, because different isotopes of the elements have different b values, some of which may even be negative. 

Table 3.5 Molten salt structural data from neutron diffraction with isotope substitution...

In situ aerodynamic levitation and neutron diffraction with isotopic snbslitution was used to determine the atomic structure of liquid Invar (Fe6sNi35) at -1800 K. Although the introduction of Ni is substitutional, a small degree of chemical was apparent in the Bhatia-Thomton (1970) partial structure factors. Significant magnetic correlations were detected -1300 K above the Invar Curie temperature. 

Neutron reflectivity with isotopic substitution has been used [58] to determine the structure of triethylene glycol monododecyl ether adsorbed at the air/water interface. Besides, X-ray diffraction and reflection studies [59] have provided stmctural details on an angstrom level about molecular packing at the interface. Optical techniques such as Fourier transform-infrared [60], Brewster angle microscopy [61], fluorescence microscopy [62], SFG [63, 64, 65] and SHG [66, 67, 68] have provided... 

Since the mid-1970s, we have been concerned with the development of a variety of X-ray and neutron scattering methods aimed at the most detailed structural description experimentally possible of interatomic structure ofions in aqueous electrolyte solution. Foremost amongst these methods has been that of Neutron Diffraction and Isotopic Substitution (NDIS) — combinations of differences between the scattering patterns of isotopically labelled samples can be used to obtain information directly on all aspects of the pairwise inter-atomic structure of an aqueous electrolyte solution, including that of the water solvent itself" ... 

Total neutron diffraction stndies of ionic hydration in aqueous electrolytes have been carried ont/° however dne to the similarity of scattering lengths b) of all the atomic nnclei, the strnctural patterns are usually less informative than those obtained from X-ray diffraction studies. Therefore, as with X-ray diffraction, models are required to assist in the interpretation of data. By contrast, the difference methods of neutron diffraction and isotopic substitution (NDIS) can be used to determine directly and without modelling individnal jo ye( ) s or their linear combinations of the form Ga r) which is specific to the snbstituted species a. 

Structures of Aqua Complexes in Aqueous Solutions Derived from Neutron Diffraction Data in Combination with Isotopic Substitution ... 

For the chloride ion neutron diffraction in combination with isotopic substitution has been used for extensive investigations of the structure... 

Special techniques, isotope substitution in neutron diffraction with empirical potential structure refinement, permit partial pair correlation functions to be obtained. The three partial pair correlation functions g(Ow-Ow, r), g(Ow-Hw, r), and g(Hw-Hw, r), provide more information on the molecular structure of water as discussed below. Those for water at ambient conditions are shown in Fig. 1.2 (Soper 2000). 

Neutron diffraction scattering with isotope substitution (NDIS) is considered today the only means by which we can extract direct information about the site-site pair correlation functions of liquids and amorphous materials [143-146]. What makes this approach possible is the justifiable assumption that light (H2O) and deuterated D2O) water exhibit the same structural features [147], and allow the extraction of... 

The lithium acetate dihydrate (CH3C00Li-2H20 or Liac-h) crystal is a typical example to emphasize the need to utilize simultaneously the most advanced neutron diffraction and INS techniques, along with isotope substitution, to rationalize complex rotational dynamics beyond the isolated methyl group approximation. 

Neutron diffraction has been successfully used for structure determinations of aqua complexes of some metal ions, which have isotopes with sufficient differences in scattering lengths to be used for isotopic substitution methods. Not only bond lengths but also coordination numbers and the orientation of the water molecules in the first coordination sphere can then be determined. Several review articles have summarized these results (2-6). 

Structure factors and radial distribution functions (RDFs) were found to be nearly identical for ASW, HGW, and LDA by X-ray and neutron diffraction measurements [175]. A more recent isotope substitution neutron diffraction study on three sets of samples (D20, HDO, and H20) allowed determining the partial OO-, OH-, and HH-radial distribution functions [20]. As an example, the OO-RDF for ASW, HGW, and LDA is shown together with HDA and VHDA in... 

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