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Energy-dispersive X-ray absorption spectroscopy is an increasingly powerful tool for the investigation of kinetic processes in chemical systems as an element-specific local structure and electronic-state probe. Advances in synchrotron radiation sources and detector technology are pushing the time resolution of the method to ever shorter periods, currently milliseconds to microseconds, while also providing a concomitant improvement in data quality that now makes feasible the identification of structural and electronic motifs characteristic of intermediate states in chemical processes. To maximize the value of the newly available high-quality time-resolved data, techniques for consistent data normalization and structural component analysis have been developed and here are illustrated in a model study of the electron-transfer reaction between [[Ir[Cl.sub.6]].sup.2-] with [[Co[(CN).sub.5]].sup.3-].

The structure of a series strontium vanadate glasses is studied by x-ray and neutron diffraction experiments. A decrease of the total V-O coordination number from 4·3±0·2 (10 mol% SrO) to 4·0±0·2 (50 mol% SrO) is observed which is interpreted as a transition from networks formed of VO5 and VO4 units to structures formed of only VO4 tetrahedra. The total V-O coordination number seems to be constant up to 30 mol% SrO and it decreases beyond this stoichiometry. The V-O distance peak is highly asymmetric and needs approximation by two or three Gaussian functions. The total Sr-O coordination number is about 8 also with asymmetric Sr-O distance peaks composed by two Gaussian functions. The V-V distances decrease from 0·344 nm (10 mol% SrO) to 0·337 nm in Sr vanadate glasses of 50 mol% SrO. [ABSTRACT FROM AUTHOR]

The phase behavior and the properties of carbon dioxide confined in an ordered mesoporous carbon (CMK-3) during freezing near the bulk triple point have been investigated by in situ total neutron scattering. The CO2 condensed in the mesopores of CMK-3 is found to be in a much densified state comparable with the bulk dry ice. So far, CO2 densification effects have been observed only in microporous carbon materials. [ABSTRACT FROM AUTHOR]

Neutron diffraction with H/D isotopic substitution is used to investigate the structure of low density amorphous ice produced from (1) high density amorphous ice by isobaric warming and (2) very high density amorphous ice by isothermal decompression. Differences are found in the scattering patterns of the two low density amorphous ices that correlate with structural perturbations on intermediate length scales in the hydrogen bonded water network. Atomistic modeling suggests that the structural states of the two samples may relate to a competition between short range and intermediate range order and disorder. This structural difference in two low density amorphous (LDA) ices is also evident when comparing their compression behavior. In terms of the energy landscape formalism this finding implies that we have produced and characterized the structural difference of two different basins within the LDA-megabasin corresponding to identical macroscopic densities. [ABSTRACT FROM AUTHOR]

Using isotope substitution neutron scattering data, we present a detailed structural analysis of the short and intermediate range structures of the five known forms of amorphous ice. Two of the lower density forms—amorphous solid water and hyperquenched glassy water—have a structure very similar to each other and to low density amorphous ice, a structure which closely resembles a disordered, tetrahedrally coordinated, fully hydrogen bonded network. High density and very high density amorphous ices retain this tetrahedral organization at short range, but show significant differences beyond about 3.1 Å from a typical water oxygen. The first diffraction peak in all structures is seen to be solely a function of the intermolecular organization. The short range connectivity in the two higher density forms is more homogeneous, while the hydrogen site disorder in these forms is greater. The low Q behavior of the structure factors indicates no significant density or concentration fluctuations over the length scale probed. We conclude that these three latter forms of ice are structurally distinct. Finally, the x-ray structure factors for all five amorphous systems are calculated for comparison with other studies. [ABSTRACT FROM AUTHOR]

Salting out of amphiphiles from aqueous solution is a process of central importance in aqueous solution chemistry and of major application in the concentration of macromolecules and the crystallization of proteins. Despite its importance, our understanding of the mechanism is poor, there being no direct experimental evidence to support the several competing explanations in the literature. Using neutron diffraction with isotope substitution, we report a structural study of the effect of adding a simple salting out agent (NaCl) to a dilute solution of an amphiphile (t-butanol). The results show clearly that the anion is central to driving the changes in association of the amphiphile through the formation of an anion bridge between the polar ends of neighboring alcohol molecules. This further exposes the nonpolar surface of the amphiphile and suggests that further association may then occur through hydrophobic interaction. The mechanism uncovered is both different from those currently in the literature and relatively simple, and offers a possible route forward to understanding the variation in salting out efficacy of different ions as indicated by the Hofmeister or lyotropic series. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure of a 0.86 mole fraction solution of tertiary butanol in water at room temperature has been investigated by neutron diffraction with hydrogen/deuterium isotope substitution. The intermolecular correlations between the alcohol and water molecules have been extracted using the empirical potential structure refinement technique. The results highlight the creation of water pockets within the solution structure. These pockets mediate the interactions that occur between the alcohol hydroxyl groups, reducing the level of direct alcohol-alcohol hydrogen bonding. The results show that only a small amount of water is required to drive this solution in the direction of hydrophobic behavior more commonly associated with the water-rich solution compositions. [ABSTRACT FROM AUTHOR]