Henri A. Levy

ANGLE CALCULATIONS FOR 3- AND 4-CIRCLE X-RAY AND NEUTRON DIFFRACTOMETERS.

Methods are derived for calculations useful in the operation of 3and 4-circle X-ray or neutron singlecrystal diffractometers. These include: (1) establishing the sample orientation from the cell parameters and the observed angles for two reflections, or from the observed angles for three reflections only, (2) calculating the angles for observing a given reflection either in a special setting or at a specified azimuthal angle, (3) obtaining the vectors needed for calculating absorption corrections, and (4) u sing observations of several reflections to refine cell and orientation parameters by the method of le ast squares.

NEUTRON DIFFRACTION STUDY OF CALCIUM HYDROXIDE

Single crystal neutron diffraction measurements on Ca(OH)2 at 20°C and —140°C confirm the x‐ray structure and the hydrogen positions previously postulated. Refinement of the neutron data by Fourier and least squares procedures yields a detailed description of the thermal motions. The amplitudes of the motion of the hydrogen atoms are compared with those deduced from infrared and Raman studies. The O–H distance, after allowance for asymmetric thermal motion, is 0.984 A.

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...

SUCROSE: PRECISE DETERMINATION OF CRYSTAL AND MOLECULAR STRUCTURE BY NEUTRON DIFFRACTION

This analysis provides the first precise molecular parameters for sucrose. All hydrogen atoms are included. Carbon-carbon distances are 1.51 1.53 �; carbon-oxygen 1.40 to 1.44 �; carbon-hydrogen 1.08 to 1.11 �; oxygen-hydrogen 0.94 to 0.99 �. The furanose ring conformation differs from that in sucrose sodium bromide dihydrate. Hydrogen bonds (two of them intramolecular) utilize every hydroxyl group except one.

Structure and Intermolecular Potential of Liquid Carbon Tetrachloride Derived from X‐Ray Diffraction Data

The scattering of x rays from the free surface of liquid carbon tetrachloride has been analyzed at 25°C. The diffractometer used was specially designed for the study of liquid structure. The radial distribution function derived from the experiments is in agreement with previously published work on CCl4 showing, however, much higher resolution. Intensity and radial distribution functions have been computed for a model structure and compared to those derived from experiment. The model assumes a perturbed close packing of chlorine atoms, surrounded by a continuous distribution of distances, as an adequate description of the short‐range order in liquid carbon tetrachloride. The model intensity and radical distribution functions are in quantitative agreement with those derived from experiment. An intermolecular potential function, derived from the model structure, predicts the heat of vaporization and second virial coefficient in agreement with experiment.

Crystal and molecular structure of hydrogen peroxide

The coordinates and thermal parameters of the O and H atoms in solid H2O2 have been determined by a single‐crystal neutron‐diffraction study. Final values were obtained by least‐squares refinement based on 91 observed intensities of the hk0, hhl, and h0l reflections. The molecular parameters (uncorrected for the effect of thermal motion) are as follows: O–O distance, 1.453±0.007 A; O–H distance, 0.988±0.005 A; O–O–H angle, 102.7±0.3°; dihedral angle between the two O–O–H planes, 90.2±0.6°. Hydrogen bonding occurs with an O–H···O distance of 2.799±0.008 A. The O–H bond distance corrected for thermal motion is 1.008±0.005 A.Comparison of these results with those reported by others from studies of H2O2 vapor and of various crystals containing H2O2 indicates that the dihedral angle is quite sensitive to the environment of the molecule.

. A neutron- diffraction study

Xenon reacts with fluorine to form XeF2 which can be isolated before it reacts with fluorine to form XeF4. The linear configuration of XeF2 with the 2.00-A bond length and the vibrational force constants support the assignment of 10 electrons to the valence shell of xenon. Similar arguments support the assignment of 12 and 14 valence electrons respectively to xenon in XeF4 and XeF6.

Xenon Difluoride and the Nature of the Xenon-Fluorine Bond

Eight molten alkali halide salts were studied by x-raydiffraction, and 3 of them also by neutron diffraction. Radial distribution functions clearly show the first 2 coordination shells, although not completely resolved. The first shell is a band of about 0.5 A half-width, with the most frequent interatomic distance being smaller than that in the solid at room temperature. It corresponds to an average coordination of about 4 to 5 neighboring ions. Neutron patterns confirm that the first coordination shell involves unlike ions. The second shell, which corresponds to 7 or more ion neighbors, is less well defined. This information, together with the density decrease on melting, indicates that the melts have quite open structures with a wide variety of individual ion coordinations. (auth)

X‐RAY AND NEUTRON DIFFRACTION STUDIES OF MOLTEN ALKALI HALIDES

The preliminary results of a neutron diffraction study are presented which confirm the existence in potassium hydrogen maleate of a short, strong, hydrogen bond and show the ion to be at least statistically symmetrical. The hydrogen is strongly linked to both neighboring oxygen atoms, and there is an existing mode of correlated motion of considerable amplitude in which the oxygen atoms are displaced but hydrogen is not. (J.R.D.)

Structure of Potassium Hydrogen Maleate by Neutron Diffraction

Neutron diffraction measurements on KHF2 single crystals show that the hydrogen atom occupies the central position, within 0.1A, in the linear F–H–F ion. The data also indicate asymmetry in thermal motion, which suggests that the bifluoride ion undergoes rotatory oscillation with appreciable amplitude. The study demonstrates the usefulness of single crystal neutron diffraction data for crystal structure determination.