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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-].

Neutron diffraction measurements of H/D isotopic substitution are made at room temperature for seven H/D substituted hexafluoro-iso-propanol (HFIP; 1,1,1,3,3,3-hexafluoro-2-propanol)–water mixtures at 0.1, 0.2, and 0.4 HFIP mole fraction (xHFIP). The eight partial structure factors except for the H(CH)–H(CH) pair obtained are subjected to an empirical potential structure refinement (EPSR) method to derive all site–site pair correlation functions. It is found that with increasing HFIP concentration the ice-like network of water disappears between xHFIP = 0.1 and 0.2, followed by the formation of a chain-like water structure embedded in an intrinsic structure of HFIP evolved at xHFIP = 0.4. The hydroxyl group of HFIP forms hydrogen bonds with the surrounding water molecules at all HFIP mole fractions investigated. There is no evidence that the water structure is well defined around the CF3 groups of HFIP, but water molecules surround tangentially the CF3 groups of HFIP. [ABSTRACT FROM AUTHOR]

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]

NIMROD is the Near and InterMediate Range Order Diffractometer of the ISIS second target station. Its design is optimized for structural studies of disordered materials and liquids on a continuous length scale that extends from the atomic, upward of 30 nm, while maintaining subatomic distance resolution. This capability is achieved by matching a low and wider angle array of high efficiency neutron scintillation detectors to the broad band-pass radiation delivered by a hybrid liquid water and liquid hydrogen neutron moderator assembly. The capabilities of the instrument bridge the gap between conventional small angle neutron scattering and wide angle diffraction through the use of a common calibration procedure for the entire length scale. This allows the instrument to obtain information on nanoscale systems and processes that are quantitatively linked to the local atomic and molecular order of the materials under investigation. [ABSTRACT FROM AUTHOR]

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 [IrCl6]2- with [Co(CN)5]3-. [ABSTRACT FROM AUTHOR]

The development of third-generation synchrotron radiation sources has created considerable opportunities for the investigation of systems under extreme sample conditions of pressure and temperature. This paper presents the application of a recently developed pressure cell and pressurization and circulation system to the investigation of the hydrophobic hydration of noble gases by X-ray absorption spectroscopy. This system operates in the pressure range from 1 bar to 1 kbar, and allows for mixing of gaseous and liquid components and the control of the ultimate gas in liquid concentration. First results demonstrate the remarkable resilience of the first hydration shell of a nonpolar solute (krypton) to the application of pressure in the 150 bar to 700 bar range. These results suggest questioning the conclusions of some recent computer simulations, in particular with regards to the potential impact of such studies on our understanding of pressure induced protein denaturation. [ABSTRACT FROM AUTHOR]

It is no accident that the start of the 21st century has seen significant growth in the field of nanotechnology across all the sciences. The nanoscale is the realm of the molecule, where physics, chemistry and biology meet, and where the potential for engineering control of structures containing tens to thousands of atoms is just beginning to be realised. Myriad examples of nanotechnology's existing and potential impact range from the creation of new designer drugs, through the development of hydrogen storage materials, to state-of-the-art silicon technology. This is the science that underpins the rationale for the construction of the near and intermediate range order diffractometer, Nimrod, on the ISIS second target station. [ABSTRACT FROM AUTHOR]

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the solute-solute, solute-solvent and solvent-solvent structural correlation functions in a 0.06 mole fraction solution of tertiary butanol in water. The measurements were made at two temperatures, 25 °C and 65 °C. Data analysis to the level of intermolecular orientational correlation functions was performed by the Empirical Potential Structure Refinement (EPSR) technique in combination with a spherical harmonic coefficient analysis. The results reveal in particular a subtle temperature dependence of the intermolecular structure of the solvent water. This structural perturbation is suggestive of a hydrophobic hydration induced, local density enhancement in the solvent water that could relate to the entropic driving force underlying the hydrophobic interaction mechanism. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents an erratum on the study concerning the structure of pure tertiary butanol. Reproduction of the corrected figures; Use of spherical harmonic formalism to extract intermolecular correlations between the two models; Accounts of hydrogen bond interactions in the fixed potential model system.

The phase behavior of sorbed CO2 in an ordered mesoporous silica sample (SBA-15) was studied by neutron diffraction. Surprisingly, upon cooling our sample below the bulk critical point, confined CO2 molecules neither freeze nor remain liquid as expected, but escape from the pores. The phenomenon has additionally been confirmed gravimetrically. The process is reversible and during heating CO2 refills the pores, albeit with hysteresis. This depletion was for the first time observed in an ordered mesoporous molecular sieve and provides new insight on the phase behavior of nanoconfined fluids. [ABSTRACT FROM AUTHOR]

Despite its extensive use in, for example, the concentration and crystallization of proteins, the salting out phenomenon remains poorly understood, with many--sometimes contradictory--explanations found in the literature. Following our earlier work using isotope substitution neutron scattering on an aqueous tertiary butyl alcohol (TBA) with added NaCl that examined in detail the molecular-level interactions in the system, and suggested that attempts to understand salting out through ion perturbation of the non-polar hydration shell may not be appropriate, we report here on two further sets of high resolution structural experiments that detail the structural interactions between ion, solvent and amphiphile in a wider range of relevant systems. First, a set of X-ray absorption spectroscopy experiments probed the effects on the hydrophobic hydration shell of Kr of adding a range of different salts (specifically Na2SO4, NaClO4, NaCl, NaNO3 and Mg(ClO4)2). The bottom line from these experiments is that the hydration shell is essentially unaffected by the added ions, underlining further the stability of the shell to such perturbations. The second set of experiments was a further isotope substitution neutron scattering study of the molecular-level solution structures of aqueous TBA, this time for a range of different added ions (CsF, NaBr, and NaCl at three concentrations), for which we have in addition extracted equilibrium constants to quantify the relative strengths of the various interactions. From this second set of results, we can conclude again that the solvent structure is essentially unperturbed by both the various ions and the amphiphile, and also identify a number of ion-specific effects. Comparing our results with those obtained from simulations that are not constrained by the experimental data underlines how sensitive structural conclusions are to the assumed potential functions, and that drawing conclusions from simulations not constrained to fit experimental data can lead to seriously erroneous conclusions.

In this contribution, neutron scattering experiments (with isotopic substitution) of concentrated and supersaturated methanolic benzoic acid solutions combined with empirical potential structure refinement (EPSR) were used to investigate the time-averaged atomistic details of this system. Through the determination of radial distribution functions, quantitative details emerge of the solution coordination, its relationship to the nature of the crystalline phase, and the response of the solution to imposed supersaturation.

The liquid state structure of the ionic liquid, 1-ethyl-3-methylimidazolium acetate ([C(2)mim][OAc]), an excellent nonderivitizing solvent for cellulosic biomass, has been investigated at 323 K by molecular dynamics (MD) simulation and by neutron diffraction using the SANDALS diffractometer at ISIS to provide experimental differential neutron scattering cross sections from H/D isotopically substituted materials. Ion-ion radial distribution functions both calculated from MD and derived from the empirical potential structure refinement (EPSR) model to the experimental data show the alternating shell structure of anions around the cation, as anticipated. Spatial probability distributions reveal the main anion-to-cation features as in-plane interactions of anions with the three imidazolium ring hydrogens and cation-cation planar stacking above/below the imidazolium rings. Interestingly, the presence of the polarized hydrogen-bond acceptor (HBA) anion (acetate) leads to an increase in anion-anion tail-tail structuring within each anion shell, an indicator of the onset of hydrophobic regions within the anion regions of the liquid. MD simulations show the importance of scaling of the effective ionic charges in the basic simulation approach to accurately reproduce both the observed experimental neutron scattering cross sections and ion self-diffusion coefficients.

Hydrogen/deuterium isotopic neutron diffraction techniques have been used to investigate the structure of a 1 m aqueous solution of YCl3 at room temperature. Empirical potential structure refinement (EPSR) has been used to build a three-dimensional model of the solution structure that is consistent with the bulk solvent correlations strongly probed by the neutron scattering technique. Optimization of the local structural environment of the Y3+ ion sites within the model has been performed through calculations of the yttrium K-edge, extended X-ray absorption fine structure (EXAFS) spectrum of the solution, and detailed information has been extracted on the structure of the ion hydration shell and the extent of inner-sphere ion pairing within the solution. The results demonstrate the significant potential of this hybrid data analysis approach to circumvent the limitations of the individual experimental methods, to refine atomic potential models, and to produce accurate, quantitative structural models of the local environment of dilute atomic species within tightly constrained bulk network structures.

The hydrophobic interaction is often thought to increase with increasing temperature. Although there is good experimental evidence for decreased aqueous solubility and increased clustering of both nonpolar and amphiphilic molecules as temperature is increased, the detailed nature of the changes in intermolecular interactions with temperature remain unknown. By use of isotope substitution neutron scattering difference measurements on a 0.04 mole fraction solution of tert-butanol in water as the solute clustering passes through a temperature maximum, the changes in local intermolecular structures are examined. Although, as expected, the solute molecules cluster through increased contact between their nonpolar head groups with the exclusion of water, the detailed geometry of the mutual interactions changes as temperature increases. As the clustering breaks up with further temperature increase, the local structures formed do not mirror those that were found in the low-temperature dispersed system: the disassembly process is not the reverse of assembly. The clusters formed by the solute head groups are reminiscent of structures that are found in systems of spherical molecules, modulated by the additional constraint of near-maximal hydrogen bonding between the polar tails of the alcohol and the solvent water. Although the overall temperature behavior is qualitatively what would be expected of a hydrophobically driven system, the way the system resolves the competing interactions and their different temperature dependencies is complex, suggesting it could be misleading to think of the aggregation of aqueous amphiphiles solely in terms of a hydrophobic driving force.

The structure of the trimolecular liquid mixture of 2:6:1 cyclohexene, tert-butyl alcohol, and water has been investigated using hydrogen/deuterium substitution neutron scattering techniques, and a three-dimensional structural model refined to be consistent with the experimental data has been built using the technique of Empirical Potential Structure Refinement. The model shows a well-mixed solution of the three molecular components where the competing interactions between the nonpolar cyclohexene and polar water molecules are balanced in the solution leading to largely pure-alcohol-like interactions between the tert-butyl alcohol molecules. Cyclohexene molecules favor direct solvation by alcohol methyl groups while water molecules are accommodated, dispersed throughout the solution, via hydrogen bonding interactions with the alcohol molecule hydroxyl groups. Rare occurrences of direct cyclohexene-water interactions are of the classic hydrophobic hydration type and no evidence is found for microscopic heterogeneity in the trimolecular mixture in contrast to the general findings for binary alcohol-water solutions.

Neutron scattering with hydrogen/deuterium isotopic substitution techniques has been used to investigate the full range of structural interactions in a dilute 0.02 mol fraction solution of tertiary butanol in water, both in the absence and in the presence of a small amount of sodium chloride. Emphasis is given to the detailed pictures of the intermolecular interactions that have been derived using the empirical potential structure refinement technique. Analysis has been performed to the level of the spatial density distribution functions that illustrate the orientational dependence of the intermolecular interactions between all combinations of molecular and ionic components. The results show the key structural motifs involved in the interactions between the various components in a complex aqueous system. They underline the structural versatility of the water molecule in accommodating a range of different kinds of interactions while retaining its characteristic first-neighbour interaction geometry. Within this framework, the results highlight the complex interplay between the polar, non-polar and charged molecular interactions that exist in the system.

Since Kauzmann's seminal 1959 paper, the hydrophobic interaction has dominated thinking on the forces that control protein folding and stability. Despite its wide importance in chemistry and biology, our understanding of this interaction at the molecular level remains poor, with little experimental evidence to support the idea of water ordering close to a non-polar group that is at the centre of the standard model for the source of the entropic driving force. Developments over recent years in neutron techniques now enable us to see directly how a non-polar group actually affects the molecular structure of the water in its immediate neighbourhood. On the basis of such work on aqueous solutions of small alcohols, the generally accepted standard model is found to be wanting, and alternative sources of the entropic driving force are suggested. Moreover, the fact that we can now follow changes in hydrogen bonding as the alcohol concentration is varied gives us the possibility of explaining the concentration dependence of the enthalpy of mixing. Complementary studies of solute association on the mesoscopic scale show a rich concentration and temperature behaviour, which reflects a complex balance of polar and non-polar interactions. Unravelling the detailed nature of this balance in simple aqueous amphiphiles may lead to a better understanding of the forces that control biomolecular structural stability and interactions.

Neutron diffraction with isotope substitution has been used to determine the structural changes that occur on the addition of a simple salting-out agent to a dilute aqueous alcohol solution. The striking results obtained demonstrate a relatively simple process occurs in which interamphiphile anionic salt bridges are formed between the polar groups of the alcohol molecules. These ion bridges drive an increase in the exposure of the alcohol molecule nonpolar surface to the solvent water and hence point the way to their eventual salting out by the hydrophobic effect.

The detailed structure of a new dense amorphous ice, VHDA, is determined by isotope substitution neutron diffraction. Its structure is characterized by a doubled occupancy of the stabilizing interstitial location that was found in high density amorphous ice, HDA. As would be expected for a thermally activated unlocking of the stabilizing "interstitial," the transition from VHDA to LDA (low-density amorphous ice) is very sharp. Although its higher density makes VHDA a better candidate than HDA for a physical manifestation of the second putative liquid phase of water, as for the HDA case, the VHDA to LDA transition also appears to be kinetically controlled.

Neutron diffraction with isotope substitution is used to determine the structures of high (HDA) and low (LDA) density amorphous ice. Both "phases" are fully hydrogen bonded, tetrahedral networks, with local order similarities between LDA and ice Ih, and HDA and liquid water. Moving from HDA, through liquid water and LDA to ice Ih, the second shell radial order increases at the expense of spatial order. This is linked to a fifth first neighbor "interstitial" that restricts the orientations of first shell waters. This "lynch pin" molecule which keeps the HDA structure intact has implications for the nature of the HDA-LDA transition that bear on the current metastable water debate.

XAFS spectroscopy has increasingly been utilised to elucidate the nearest-neighbour structure in the condensed phases. In this paper, the XAFS spectra of NdCl3 and DyCl3 in both the solid and the liquid phases measured at the Nd and Dy L(III) absorption edges on beam line BM29 of the European Synchrotron Radiation Facility (ESRF) are presented. The Fourier transformed radial structure functions, phi(r) show that the prominent peaks corresponding to M-Cl (M: Nd or Dy) first shell contribution are shifted to shorter distances in the liquid melts as compared to those found in the corresponding solids. Similar behaviour has also been observed from other diffraction techniques in typical ionic melts such as NaCl. From the temperature dependence of the radial structure functions it is clear that the change in the M-Cl distance on melting is much larger in NdCl3 than that in DyCl3.