A. H. Narten

Diffraction pattern and structure of amorphous solid water at 10 and 77 °K

X‐ray diffraction data show that there are at least two forms of amorphous solid water which differ in density and second nearest‐neighbor oxygen–oxygen distribution. (a) The lower density form, made at 77 °K, has a diffraction pattern consistent with a structure that has oxygen–oxygen nearest‐neighbor tetrahedral symmetry on average, and a nearest neighbor O–O separation of 2.76 A with small dispersion. The density of this material is estimated to be 0.94 g cm−3. While it is not possible to uniquely define the structure, the data available support the notion that its fundamental characteristic is the existence of a randomized network of hydrogen bonds with O–O–O angular distribution derived from (i.e., centered about) that of ice Ih. Comparison of neutron diffraction and x‐ray diffraction data suggests strongly that the first shell hydrogen bonds are nearly linear and that orientational correlations between water molecules are limited to nearest neighbors. (b) The higher density form, made at 10 °K, has ...

Atom Pair Distribution Functions of Liquid Water at 25°C from Neutron Diffraction

The structure of liquid water is described by three atom pair distribution functions gOO(r), gOH(r), and gHH(r). These functions have now been derived from neutron diffraction data on four mixtures of light and heavy water. They will provide a crucial and sensitive test for proposed models of liquid water.

Hydrogen bonding in liquid methanol and ethanol determined by x‐ray diffraction

The x‐ray diffraction patterns of liquid methanol and ethanol have been measured at 20 °C. The data are analyzed to yield the molecular structures, and the distinct structure functions Hd(k) are analyzed to obtain the hydrogen bonding in these alcohols. The data show clearly that hydrogen‐bonded hydroxyl groups occur in methanol and ethanol with an OH⋅⋅⋅OH distance of 2.8A, and that each hydroxyl group has 1.8±0.1 nearest neighbors at this distance.

Liquid Water: Atom Pair Correlation Functions from Neutron and X‐Ray Diffraction

Neutron diffraction data for heavy water at 25°C, together with results derived from x‐ray scattering, show that the orientational correlation between pairs of molecules is of a much shorter range (<5A) than the positional correlation between molecular centers (<8 A). The neutron diffraction pattern is completely described by a model which assumes preferred average orientation between neighboring molecules and no orientational correlation between second and higher neighbors. The structure of an average D2O molecule in the liquid is not significantly different from that of an isolated molecule in the dilute gas. Atom pair correlation functions derived from the data and the model are in good qualitative agreement with results derived from molecular dynamics studies.

Diffraction pattern and structure of aqueous lithium chloride solutions

We present the first extensive study of an ionic solution by both neutron and x‐ray diffraction. Pure water and aqueous LiCl at concentrations to saturation (mole ratio D2O:LiCl=3) have been examined at room temperature by both methods. This extensive study reduces greatly the nonuniqueness of interpretation usually associated with diffraction data from liquids. All data are shown to be consistent with a simple model involving only nearest neighbor interactions. In the pure liquid and in dilute solutions, the molecular structure is observed to be close to that found for the gas molecule. The tetrahedral coordination of oxygen atoms around each water is clearly shown, with deuterium atoms located near the O–O lines. This basic water structure gradually diminishes with increasing LiCl concentration, and is unobserved at mole ratios of 10 or lower. The coordination of oxygen atoms about Cl− appears to be octahedral on the average, with a deuterium atom near each connecting line. The coordination around Li+ a...

Diffraction Pattern and Structure of Liquid Benzene

The scattering of x rays from the free surface of liquid benzene has been analyzed at 25°C. The diffractometer used was specially designed for the study of liquid structure. Intensity and radial distribution functions have been computed for a model structure and compared to those derived from experiment. The model assumes that each benzene molecule “sees” 12 neighboring molecules in preferred orientations similar to those observed for the crystal, surrounded by a continuous distribution of distances. The intensity and radial distribution functions computed for the model are in essential agreement with those derived from experiment.

X‐ray diffraction pattern and models of liquid benzene

X‐ray diffraction data for liquid benzene at 25°C are analyzed using scattering factors for C–H groups rather than for C and H atoms. This new approach permits the unique extraction of structure and correlation functions for carbon–carbon interactions from a single experiment. The data are used to find parameters for a model (RISM) of liquid benzene which permits calculation of all intermolecular atom pair correlation functions. The predominant arrangement in the liquid is one in which pairs of molecules fit together like six‐toothed bevel gear wheels whose axes are almost at right angles.

Neutron diffraction study of light and heavy water mixtures at 25 °C

Neutron diffraction data of very high statistical accuracy for four mixtures of light and heavy water containing 0.01%, 35.79%, 67.89%, and 99.75% deuterium are presented. The dynamic corrections described in a preceding article yielded a self‐consistent static coherent structure function for heavy water. For the other mixtures an empirical correction method was used. The four total structure functions were analyzed to yield the three partial structure functions aOO(k), aOH(k), and aHH(k). The function aOO(k) is not significantly different from previously obtained x‐ray diffraction results. The partial structure functions for hydrogen interactions will provide a sensitive test for proposed models of liquid water.

Diffraction Pattern and Structure of Noncrystalline BeF2 and SiO2 at 25°C

The scattering of x rays from noncrystalline samples of BeF2 and SiO2 at 25°C has been measured and analyzed. The diffraction patterns, typical of the atomic arrangement found in liquids, show interference to values of the scattering variable s=16 and beyond. The glasses contain BeF4 (SiO4) tetrahedra joined at the corners so that one anion is common to two tetrahedra. The packing of these tetrahedra is very similar to that found in the crystalline modifications of BeF2 and SiO2, and a model based on the quartz structure describes the diffraction data quantitatively.

Liquid carbon tetrachloride: Atom pair correlation functions from neutron and x‐ray diffraction

Neutron and x‐ray diffraction data for liquid CCl4 at 20 °C are presented and analyzed to yield structure and correlation functions for C–Cl and Cl–Cl atom pairs in different molecules. In the liquid, a central molecule is interlocked with four of its nearest neighbors and this arrangement explains the strong orientational correlations that exist over short radial distances. The data are sufficiently accurate to provide a sensitive test for molecular theories of the liquid state and for intermolecular force models in computer experiments.