D. A. Neumann

Increasing the density of adsorbed hydrogen with coordinatively unsaturated metal centers in metal-organic frameworks.

Storing molecular hydrogen in porous media is one of the promising avenues for mobile hydrogen storage. In order to achieve technologically relevant levels of gravimetric density, the density of adsorbed H2 must be increased beyond levels attained for typical high surface area carbons. Here, we demonstrate a strong correlation between exposed and coordinatively unsaturated metal centers and enhanced hydrogen surface density in many framework structures. We show that the MOF-74 framework structure with open Zn(2+) sites displays the highest surface density for physisorbed hydrogen in framework structures. Isotherm and neutron scattering methods are used to elucidate the strength of the guest-host interactions and atomic-scale bonding of hydrogen in this material. As a metric with which to compare adsorption density with other materials, we define a surface packing density and model the strength of the H(2-)surface interaction required to decrease the H(2)-H(2) distance and to estimate the largest possible surface packing density based on surface physisorption methods.

The high-flux backscattering spectrometer at the NIST Center for Neutron Research

We describe the design and current performance of the high-flux backscattering spectrometer located at the NIST Center for Neutron Research. The design incorporates several state-of-the-art neutron optical devices to achieve the highest flux on sample possible while maintaining an energy resolution of less than 1 μeV. Foremost among these is a novel phase-space transformation chopper that significantly reduces the mismatch between the beam divergences of the primary and secondary parts of the instrument. This resolves a long-standing problem of backscattering spectrometers, and produces a relative gain in neutron flux of 4.2. A high-speed Doppler-driven monochromator system has been built that is capable of achieving energy transfers of up to ±50 μeV, thereby extending the dynamic range of this type of spectrometer by more than a factor of 2 over that of other reactor-based backscattering instruments.

Molecular dynamics of solid-state lysozyme as affected by glycerol and water: a neutron scattering study.

Glycerol has been shown to lower the heat denaturation temperature (T(m)) of dehydrated lysozyme while elevating the T(m) of hydrated lysozyme (. J. Pharm. Sci. 84:707-712). Here, we report an in situ elastic neutron scattering study of the effect of glycerol and hydration on the internal dynamics of lysozyme powder. Anharmonic motions associated with structural relaxation processes were not detected for dehydrated lysozyme in the temperature range of 40 to 450K. Dehydrated lysozyme was found to have the highest T(m) by. Upon the addition of glycerol or water, anharmonicity was recovered above a dynamic transition temperature (T(d)), which may contribute to the reduction of T(m) values for dehydrated lysozyme in the presence of glycerol. The greatest degree of anharmonicity, as well as the lowest T(d), was observed for lysozyme solvated with water. Hydrated lysozyme was also found to have the lowest T(m) by. In the regime above T(d), larger amounts of glycerol lead to a higher rate of change in anharmonic motions as a function of temperature, rendering the material more heat labile. Below T(d), where harmonic motions dominate, the addition of glycerol resulted in a lower amplitude of motions, correlating with a stabilizing effect of glycerol on the protein.

Photoionization and Absorption Spectrum of Formaldehyde in the Vacuum Ultraviolet

Absorption and photoionization coefficients have been measured for H2CO in the 600–2000‐A region. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 A. From the photoionization curve the first adiabatic ionization potential was found to be 10.87±0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single‐configuration self‐consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states. On this basis, weak absorption features between 1340 and 1430 A have been assigned to the 1B1(σ → π *) valence state. The 1A1(π → π *) valence state is deduced to be strongly autoionized just above the 2B2 ionization limit.

Quantum rotation of hydrogen in single-wall carbon nanotubes

We report inelastic neutron scattering results on hydrogen adsorbed onto samples containing single-wall carbon nanotubes. These materials have attracted considerable interest recently due to reports of high density hydrogen storage at room temperature. Inelastic neutron scattering clearly shows the ortho‐para conversion of physisorbed hydrogen in a nanotube containing soot loaded with hydrogen. From the rotational Ja 0! 1 transition, no indication of a significant barrier to quantum rotation is seen. ” 2000 Elsevier Science B.V. All rights reserved.

Neutron scattering studies of C60 and its compounds

Abstract We describe neutron scattering studies of the structure and dynamics of C60-fullerite. Above the order-disorder phase transition temperature, Tc, diffraction and quasielastic scattering measurements indicate that the molecules are constantly reorienting while their centers remain on a face-centered cubic lattice. Below Tc the molecules are orientationally ordered with four molecules per unit cell. Recent diffraction results suggest that any given molecule occupies one of two possible orientations with unequal probability, and inelastic scattering measurements show that the molecules librate about their equilibrium orientations. The librational amplitude becomes large as the phase transition is approached. Pulsed source measurements yield direct information regarding the structure of the C60 molecule and also support the low temperature two-orientation model. High energy inelastic studies of undoped and alkali metal-doped buckminsterfullerene provide detailed information about intramolecular vibrational frequencies and point to the important role of specific modes in promoting superconductivity at low temperatures.

Energy curves of CO−2

Abstract Ab initio energy curves of the 2 A 1 and 2 B 1 states of CO − 2 are reported as a function of the bending angle. The 2 A 1 curve is found to be bound relative to the neutral curve at angles greater than 130° from the linear geometry. Both the 2 A 1 and 2 B 1 states are found to be valence in character for the geometries considered and the excitation energy for 2 A 1 → 2 B 1 is calculated to be 3.2 eV in the neighborhood of the CO − 2 equilibrium geometry while the experimental value for the maximum in the lowest absorption curve is almost 3.5 eV. From the bending energy curve it is deduced that gaseous CO − 2 in its ground vibrational level can have a relatively long life since the ion energy in the neighborhood of the ion equilibrium geometry is about 1 eV below the neutral molecule energy for that geometry.

The Inverse Relationship between Protein Dynamics and Thermal Stability

Protein powders that are dehydrated or mixed with a glassy compound are known to have improved thermal stability. We present elastic and quasielastic neutron scattering measurements of the global dynamics of lysozyme and ribonuclease A powders. In the absence of solvation water, both protein powders exhibit largely harmonic motions on the timescale of the measurements. Upon partial hydration, quasielastic scattering indicative of relaxational processes appears at sufficiently high temperature. When the scattering spectrum are analyzed with the Kohlrausch-Williams-Watts formalism, the exponent beta decreases with increasing temperature, suggesting that multiple relaxation modes are emerging. When lysozyme was mixed with glycerol, its beta values were higher than the hydrated sample at comparable temperatures, reflecting the viscosity and stabilizing effects of glycerol.

X-ray scattering studies of surface roughness of GaAs/A1As multilayers

We discuss the theory of X-ray scattering from multilayers with conformal roughness of the interfaces, and illustrate with an analysis of specular, diffuse and wide-angle scattering from a GaAs/A1As multilayer. This is a highly coherent multilayer structure deposited on a stepped, but otherwise smooth surface. The roughness due to the steps propagates through the layers and a distinct anisotropy is observed in the diffuse scattering. We discuss a method to treat diffuse scattering from such surfaces with slightly irregular steps.

In situ quasi-elastic scattering characterization of particle size effects on the hydration of tricalcium silicate

The effects of different particle size distributions on the real-time hydration of tricalcium silicate cement paste were studied in situ by quasi-elastic neutron scattering. The changing state of water in the cement system was followed as a function both of cement hydration time and of temperature for different initial particle size distributions. It was found that the length of the initial, dormant, induction period, together with the kinetics of hydration product nucleation and growth, depends on the hydration temperature but not on the particle size distribution. However, initial particle size does affect the total amount of cement hydrated, with finer particle size producing more hydrated cement. Furthermore, the diffusion-limited rate of hydration at later hydration time is largely determined by the initial tricalcium silicate particle size distribution.