Shielding Surface Research Articles

ChemInform Abstract: The Nuclear Shielding Surface: The Shielding as a Function of Molecular Geometry and Intermolecular Separation

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Isotropic and anisotropic proton chemical shielding surface of water

Extended-basis-set ab initio calculations have been carried out to obtain the hydrogen chemical shielding tensor for water as a function of the internal geometrical coordinates. The results of the calculations show that the isotropic shielding and the anisotropy are most dependent on the oxygen-hydrogen bond length, and that the variation with the bond length is nearly linear. The anisotropy shows a significantly greater dependence on stretching the OH bond of the other hydrogen than does the isotropic shielding. Furthermore, we find that the deformation of water's structure that occurs in ice Ih is associated with a decrease in anisotropy instead of the expected increase.

Effect of Shielding Surfaces on the Irradiance Received by Tilted Collectors

A model is developed to predict the total solar irradiance received by a tilted collector in the presence of a large shielding tilted surface. The model is suggested to replace the commonly accepted isotropic model which does not account for a shielding surface. The present model is found to depend mainly on the configuration factors between the collector and the sky and the ground in the presence of a shielding surface. Expressions for these configuration factors are derived for a general geometry of the collector and a large shielding surface. These expressions are found to depend on the tilt angles of the collector and the surface, the orientation angle and the location of the collector relative to the surface, and the height and width of the shielding surface. The expressions are verified numerically and also, by comparing them with the results obtained from the literature for special geometries. The expressions are simplified to facilitate their use for the case of a vertical collector and a vertical shielding surface. This special case is needed to accurately estimate the solar irradiance received by glass windows of buildings.

Nuclear magnetic shielding of nitrogen in ammonia

The nitrogen shielding surface in ammonia is calculated using the localized orbital-local origin (LORG) method of Hansen and Bouman, in terms of the symmetry coordinates for the molecule. With respect to the inversion coordinate, the N shielding surface has a shape similar to the potential surface. Rovibrational averaging of the N shielding in NH3 and ND3 molecules is carried out using numerical wave functions which are solutions to the inversion potential which best fits the spectra of all isotopomers. The other coordinates are vibrationally averaged in the usual way, assuming small amplitude motions. The calculated temperature dependence of the N shielding due to inversion is in the opposite sense to that observed for a large number of molecules, and is nearly canceling the contributions from all the other coordinates. The temperature dependence of the nitrogen shielding in ammonia has been measured in the range 300–400 K in samples with densities in a hundredfold range (0.37–33 amagat). When the temperature-dependent intermolecular effects are separated out, the remaining temperature dependence is small and is consistent with the calculations. The inversion contribution to the deuterium-induced isotope shift is of opposite sign to the contributions from all other coordinates. The agreement with the experimental isotope shift in the liquid phase is satisfactory.

Theory of the carbon-13 proton spin-spin coupling and the proton shielding surfaces of the methane molecule

The form of the carbon-13 proton spin-spin coupling surface of the methane molecule is investigated. The three first-order and twelve second-order independent symmetry coordinate coefficients required to determine the four distinct coupling constants for small but otherwise arbitary displacements from equilibrium are identified. Valence coordinate coefficients for the surface are also determined. The results will be of help in understanding the effects of temperature and of primary and secondary isotopic substitution on the observed coupling and also in simplifying ab initio calculations of the surface for methane and for 1 J(X, Y) in other XY 4 compounds of Td symmetry. The results are also applicable to the proton shielding surface of methane and the Y nuclear shielding surfaces in other XY 4 compounds of Td symmetry.

Acoustics of attached and unattached flow for simplified OTW configurations with 5:1 slot nozzle

The acoustics of simplified engine over-the-wing configurations, with and without jet flow attachment to the shielding surface were studied at model scale. The nozzle used consisted of a 5:1 slot nozzle (equivalent diameter, 5.1 cm) operated at a jet Mach number of 0.6, with the flow directed at angles of 0° to 10° to the shielding surface (flat board). The shielding length of the surface was varied from 16.4 to 54.3 cm, and the nozzle height above the surface was varied from 0 to 7.6 cm. Also, the velocity field at the trailing edge of each nozzle/surface configuration was mapped. With attached flow, the jet flow field is stretched in the flow direction, resulting in locally higher velocities than those for unattached flow or nozzle only. This increased the noise levels for the attached flow cases compared to those with partially or completely unattached flow. With attached flow, the shielding benefits were substantially reduced (up to half) compared with fully detached flow. Increasing the impingement angle of the jet flow from 0° to 10° generally caused the noise levels to increase. In some cases, the noise increase was sufficiently broad-band that the jet noise shielding benefits were noticeably reduced.

Metal Work Design of High Intensity Radiation Cells

Special factors to consider in design, engineering, detailing, and building cells; information concerning pertinent aspects of radiation shielding; examination of heavy shielding concrete, surface finishes, and requirements of related appurtenances.