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Attempts to understand the molecular structure of water were first made well over a century ago. Looking back at the various attempts, it is illuminating to see how these were conditioned by the state of knowledge of chemistry and physics at the time and the experimental and theoretical tools then available. Progress in the intervening years has been facilitated by not only conceptual and theoretical advances in physics and chemistry but also the development of experimental techniques and instrumentation. Exploitation of powerful computational methods in interpreting what at first sight may seem impenetrable experimental data has led us to the consistent and detailed picture we have today of not only the structure of liquid water itself and how it changes with temperature and pressure but also its interactions with other molecules, in particular those relevant to water's role in important chemical and biological processes. Much remains to be done in the latter areas, but the experimental and computational techniques that now enable us to do what might reasonably be termed "liquid state crystallography" have opened the door to make possible further advances. Consequently, we now have the tools to explore further the role of water in those processes that underpin life itself—the very prospect that inspired Bernal to develop his ideas on the structure of liquids in general and of water in particular. [ABSTRACT FROM AUTHOR]

Ice V is a structurally highly complex material with 28 water molecules in its monoclinic unit cell. It is classified as a hydrogen-disordered phase of ice. Yet, some of its hydrogen-bonded water molecules display significant orientational order. Upon cooling pure ice V, additional orientational ordering cannot be achieved on the experimental time scale. Doping with hydrochloric acid has been shown to be most effective in enabling the phase transition of ice V to its hydrogen-ordered counterpart ice XIII. Here, we present a detailed crystallographic study of this phase transition investigating the effects of hydrochloric and hydrofluoric acid as well as lithium and potassium hydroxide doping. The magnitudes of the stepwise changes in the lattice constants during the phase transition are found to be more sensitive indicators for the extent of hydrogen order in ice XIII than the appearance of new Bragg peaks. Hydrofluoric acid and lithium hydroxide doping enable similar ordering processes as hydrochloric acid but with slower kinetics. The various possible space groups and ordered configurations of ice XIII are examined systematically, and the previously determined P21/a structure is confirmed. Interestingly, the partial hydrogen order already present in ice V is found to perpetuate into ice XIII, and these ordering processes are found to be independent of pressure. Overall, the hydrogen ordering goes along with a small increase in volume, which appears to be the origin of the slower hydrogen-ordering kinetics under pressure. Heating pressure-quenched samples at ambient pressure revealed low-temperature "transient ordering" features in both diffraction and calorimetry. [ABSTRACT FROM AUTHOR]

A hydrogen/deuterium isotopic substitution neutron diffraction technique that was used to measure the structural correlation functions of liquid tetrahydrofuran at room temperature is presented. Analysis to the level of the orientational correlation functions shows that the liquid displays a preference for T-like configurations between the tetrahydrofuran molecules, a local structure that results in voidlike regions of approximately 1.25 A(degree) radius within the bulk liquid.

Hydrogen/deuterium isotopic substitution neutron diffraction techniques are used to measure the structural correlation functions in a 0.23 mole fraction solution of tetrahydrofuran in water at room temperature. Despite the small size of the solvent water molecules, the 0.23 mole fraction aqueous solution is still found to contain small voids akin to those in pure liquid tetrahydrofuran.

The anomalous negative excess of entropy of mixing characteristic of aqueous lower alcohols containing hydropholic groups is quantitatively consistent with a model exploiting only the experimentally observed molecular-scale segregation of the components across the entire concentration range. The simple model used obviates the need to invoke 'iceberg' or other water restructuring concepts, which, though frequently postulated in explaining the hydrophobic interaction, are unsupposed by experiments.

Neutron powder diffraction experiments show that, in the presence of ice and at helium-gas pressures exceeding 0.28 GPa, a gas hydrate with the composition He(6+x)H2O is formed. Its host lattice closely resembles that of ice II. However, the stability range of the helium hydrate is considerably more extended as compared to that of ice II: helium hydrate melts and recrystallizes from the liquid and is more stable than ice III/IX or ice V, at least up to 0.5 GPa. The helium content increases with increasing-pressure/decreasing-temperature, but does not appear to follow an ideal solution behavior, as do the guests in some heavier noble-gas clathrates. The lattice parameters of helium hydrate are significantly different from those of ice II, while the atomic arrangement of the host lattice is very similar, showing full orientational order of the water molecules. [ABSTRACT FROM AUTHOR]

A transition as a function of increasing temperature from harmonic to anharmonic dynamics has been observed in globular proteins by using spectroscopic, scattering, and computer simulation techniques. We present here results of a dynamic neutron scattering analysis of the solvent dependence of the picosecond-time scale dynamic transition behavior of solutions of a simple single-subunit enzyme, xylanase. The protein is examined in powder form, in [D.sub.2]O, and in four two-component perdeuterated single-phase cryosolvents in which it is active and stable. The scattering profiles of the mixed solvent systems in the absence of protein are also determined. The general features of the dynamic transition behavior of the protein solutions follow those of the solvents. The dynamic transition in all of the mixed cryosolvent-protein systems is much more gradual than in pure [D.sub.2]O, consistent with a distribution of energy barriers. The differences between the dynamic behaviors of the various cryosolvent protein solutions themselves are remarkably small. The results are consistent with a picture in which the picosecond-time scale atomic dynamics respond strongly to melting of pure water solvent but are relatively invariant in cryosolvents of differing compositions and melting points.

A study was conducted to investigate the recognition of a neutron incoherent spin-echo (NISE) signal on a hydrated powder of the enzyme glutamate dehydrogenase. The direct relationship between the scattering function and atomic trajectories was identified as the major biological responsibility of the NISE. The findings affirmed the importance of NISE in the analysis of nanosecond motions in biological specimens, thereby making it a complement of molecular dynamics.