Nuclear shielding surfaces

Application of Nuclear Shielding Surfaces to the Fundamental Understanding of Adsorption and Diffusion in Microporous Solids... 

Wigglesworth et al. have used LAOs to calculate nuclear shielding surfaces for water and its isotopes at the MCSCF level. The correlated surface for the proton is found to be similar to that obtained in an earlier uncorrelated calculation, but the oxygen surface is considerably changed in the correlated MCSCF method. An accurate force field has been used to represent the nuclear motion and good agreement with experimental isotope shifts has been found. Similar calculations have been performed for acetylene. ... 

Jameson, C. J. de Dios, A. C. "The Nuclear Shielding Surface The Shielding as a Function of Molecular Geometry and Intermolecular Separation", Nuclear Magnetic Shieldings and Molecular Structure (NATO AST. Series C Mathematical and Physical Sciences, Vol. 386) Tossell, J. A., Ed., Kluwer the Netherlands, 1993, pp 95-116. 

Figure 3.40 Calculated nuclear shielding surface for the C-a carbon of an o-substituted Schiff base. The NH bond length is varied. (Reproduced from West-Nielsen et al. [23]. Copyright (2006), with permission of Elsevier.)...

Shielding Surfaces for Two Interacting Molecules. In the Maryland meeting Jameson and de Dios reported the first ab initio calculations of the rare gas pair intermolecular shielding surfaces for Ar2, ArNe, Ne2, and NeHe (55). With these shielding surfaces it was possible to calculate the second virial coefficient for nuclear shielding as a function of temperature, using the well established intermolecular potential functions V(R) for the pair. 

Most of the quantum chemical calculations of the nuclear shielding constants have involved two classes of solvation models, which belong to the second group of models (n), namely, the continuum group (i) the apparent surface charge technique (ASC) in formulation C-PCM and IEF-PCM, and (ii) models based on a multipolar expansion of the reaction filed (MPE). The PCM formalism with its representation of the solvent field through an ASC approach is more flexible as far as the cavity shape is concerned, which permits solvent effects to be taken into account in a more accurate manner. 

Very recently, an even more directly useful presentation of the results has been produced by a direct evaluation of the intermolecular nuclear shielding values for protons of purines. Figure 8 represents such values (in ppm) due to the ring current in the N(9)H and N(1)H tautomers of purine in a plane 3.4 A distant from the molecular surface of the purine (which is the mean intermolecular distance in complexes involving purines with other conjugated rings). The diagrams for the N(7)H and N(3)H tautomers are very similar to the ones for N(9)H and N(1)H. 

Wigglesworth, R. D., Raynes, W. T., Kirpekar, S., Oddershede, J., and Sauer, S. P. A. (2000a). Nuclear magnetic shielding in the acetylene isotopomers calculated from MCSCF shielding surfaces. J. Chem. Phys., 112, 736-746. 

Nuclear Applications. Powder metallurgy is used in the fabrication of fuel elements as well as control, shielding, moderator, and other components of nuclear-power reactors (63) (see Nuclearreactors). The materials for fuel, moderator, and control parts of a reactor are thermodynamically unstable if heated to melting temperatures. These same materials are stable under P/M process conditions. It is possible, for example, to incorporate uranium or ceramic compounds in a metallic matrix, or to produce parts that are similar in the size and shape desired without effecting drastic changes in either the stmcture or surface conditions. OnlyHttle post-sintering treatment is necessary. 

An expensive method is the use of nuclear radiation to obtain information on the level in an apparatus. The nuclear sensor is mounted at one side, and at the other side a scintillation counter is fixed near the surface of the apparatus. Both systems are sheathed with lead-screen shields to give protection from nuclear radiation. A continous level indicator using nuclear radiation is very complicated and therfeore seldom applied. 

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