Neutron diffraction with isotope substitution NDIS

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... 

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.]... 

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. 

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. 

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. 

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... 

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