Hypersensitivity, crystal structures

After PINS confirmation, explosive shells are best detonated on-site as even unfused old ordnance can have hypersensitive crystals growing in the exudate that forms in the bottom of the shell as the explosives age. For single UXO on a range, the author believes in the as is where is philosophy for detonating explosive ordnance. Simply build a containment structure and sandbag it over the shell and detonate it where it is. However, chemical shells with Lewisite or other arsenicals will leave behind arsenic, which must be cleaned up after detonation. If not detonated where found for fear of touching off adjacent munitions, or because of the proximity of buildings or people, the UXO should be robotically moved as short a distance as possible for detonation. 

Nevertheless, crystal fields cannot be completely ignored. The intensities of a number of bands ( hypersensitive bands) show a distinct dependence on the actual ligands which are coordinated. Also, in the same way that crystal fields lift some of the orbital degeneracy (2L -)- 1) of the terms of d" ions, so they lift some of the 2J -)- 1 degeneracy of the sates of P ions, though in this case only by the order of 100cm This produces fine structure in some bands of Ln spectra. 

Having discussed the theoretical principles of crystal field theory, theories of intensity of the hypersensitive transitions, it is logical to examine the absorption spectral features of lanthanide ions in aqueous solutions. It is probably worthwhile looking into the historical development of the spectra of lanthanides. The most prominent era in the development is the 1930 s. Prandtl and Scheiner [118] presented a complete collection of absorption spectra of trivalent lanthanides in solution. The covered region is 7000-2000 A. An apparent symmetry in the region of absorption with band structure shifting toward the ultraviolet in approaching the centre of the series from both ends was observed. 

The EQ oscillator strengths are calculated to be between 10 1° to 10-13 [113] for individual ZPL transitions of Ln3+ in Cs2NaMX6. Much attention has previously been paid to the relationship of hypersensitive and EQ transitions. It appears, however, that the spectra of many such transitions were usually recorded (in solution) at room temperature so that the distinction was not possible between vibronic and pure electronic structure (see for example [129-132]), with the total oscillator strengths being of the order 10 5. Indeed, the only conclusive resolution of the electric quadrupole mechanism in contributing the entire ZPL intensity in a crystal concerns the uranyl ion. 

Work is being carried out in collaboration with Dr. W.T. Carnall in an effort to connect hypersensitivity to the structures of the crystal lattices where it is exhibited. The mechanism based on an inhomogeneous dielectric leads to an expression for the intensities of hypersensitive lines that is proportional to (12)... 

The wave functions used in the expressions for the line strengths are precisely those deduced by an analysis of the free-ion energy level structure. Therefore, only three new parameters, the s, have been introduced to account for the line strengths. This scheme has been remarkably successful in modeling experimental observations in both crystal and solution environments. It also accommodates the existence of the "hypersensitive" transitions. Peacock (30) has recently reviewed the field with regard to lanthanide f-f transitions. The simplicity of this scheme has been utilized by Krupke (31) and Caird (32) to predict potential laser transitions in the lanthanides. 

An additional feature in Fig. 2 worth noting is the amino-terminal 160 amino acids of mercuric reductase that lacked a fixed position in the crystal and therefore were not part of the solved structure. These contain the sequence that is homologous to MerP and postulated to be a mercurybinding domain. This region is drawn in Fig. 2 as an extension from the protein perhaps it functions like a baseball mitt that catches Hg from the membrane transport proteins and delivers Hg " to the carboxyl-terminal catalytic binding site, so that, as in the bucket brigade model above, Hg " is never found free within the cell. Mutant strains with the transport system but lacking the MerA detoxification enzyme are hypersensitive to mercury salts, as they accumulate Hg " but cannot get rid of it. After reduction by NADH (via FAD and the active site cysteine pair), metallic Hg is released... 

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