Ionic electric field gradient

Calculated free-ion hyperfine fields and ionic electric field gradients (without Sternheimer shielding) in the lanthanides and actinides of interest. 

High-pressure experiments up to 8.3 GPa at 4.2 K were carried out by Kratzer et al. (1986). The observed rise in Curie temperature dTc/dP = 3.5 0.3 K GPa is well within the range predicted by a calculation using a modified RKKY exchange (Jaakkola and Hanninen 1980). The pressure coefficient of is extremely small dB, [/dP= — 0.4 0.1 TGPa Similarly, the ionic electric field gradient decreases by 5% between ambient pressure and 8.3 GPa. Isomer shifts exhibit the linear pressure dependence expected from a compression of mainly s-like conduction electrons. 

Fig. 41. Comparison of measured ionic electric field gradient on Np monopnictides with calculated NpN values, [Taken from Dunlap and Kalvius (1985).]...

The electric field gradient at an ion in an ionic compound is then given as the sum of contributions from individual components surrounding this ion (equation 20),... 

Transport methods constitute the next division. These are methods that involve measurements of diffusion and the velocity of ionic movement under electric field gradients. These approaches provide information on solvation because the dynamics of an ion in solution depend on the number of ions clinging to it in its movements, so that knowledge of the facts of transport of ions in solution can be used in tests of what entity is actually moving. 

There are two conceptually different theories for quadrupolar relaxation of ionic nuclei in solution. Deverell (22) rationalized the electric field gradients at the site of the nucleus as arising from distortions of the closed-shell orbitals in the ion due to collisions with solvent molecules and, at higher concentrations, also counter-ions. In another theory developed by Valiev (23) and by Hertz and his coworkers (24) it is assumed that the electric field gradients are caused by the electric dipoles of the surrounding solvate molecules. It is certainly Hertz to whom we owe the detailed understanding of ionic quadrupole relaxation, and because of the fundamental implications that his work has on ionic solvation the important results are briefly summarized here. 

Both AP and Ga have a tightly bound hydrate shell in aqueous solution and both are prone to hydrolysis. In terms of the Hertz electrostatic model for quadrupolar relaxation of ionic nuclei in electrolyte solution (see Section III.C) one therefore expects effective quenching of the electric field gradient caused by the surrounding water dipoles, due to a nearly perfect coordination symmetry. Any contribution to the e.f.g. should therefore arise from outer-sphere solvent dipoles. In terms of the fully orientated solvation (FOS) model this would correspond to a distribution width parameter approaching zero (/. -> 0) with the first term in equation (4) vanishing. This is indeed what Hertz (24) found for both AF" and Ga ", and the experimental infinite dilution relaxation rates ( AP" 7-5 s Ga 350 s ) are remarkably well matched by the computed ones... 

NMR relaxation method is a very powerful tool for the study of ionic interactions. Many nuclei studied are with spin quantum number I > 1/2 and their relaxations are governed by quadrupole interaction between the nuclear electric quadrupole moment and the electric field gradient (efg) at the nuclear site. Under extreme narrowing condition, the longitudinal... 

Nuclei with spin / > 1 possess an electric quadrupole which can take up different orientations relative to an electric field gradient. These orientations differ in energy, and transitions between them give rise to the nuclear quadrupole resonance (NQR) spectrum. The NQR of ligand atoms such as Cl, Br, and I could, therefore, provide information on the polarity of the metal-ligand bond and the population of the p orbitals (111). The only cobalt(III) complexes that have been studied are trans-[Co-enaXalX HX wHaO, where X = Cl or Br. Hartmann et al. 87) calculated 75-80% ionic character for the Co-Cl bond and also reported data for the bromide complex, from which Kubo and Nakamura (111) calculated a value of 60% ionic character. 

EFG - electric field gradient n - physical (Shannon) ionic radius... 

Counterions. 1. Sodium-23 Alkali metal MIR is a sensitive >robe of the immediate chemical environment and mobility of alkali metal ions in aqueous and nonaqueous solvents (7, 8). The chemical shifts of alkali metal nuclei will respond to" electronic changes only in the immediate environment of the cation since alkali metals rarely participate in covalent bonding (7). All alkali metal nuclei have spins greater than 1/2 and hence have quadrupole moments. The interaction of these moments with electric field gradients, produced by asymmetries in the electronic environment, is modulated by translation and rotational diffusive motions in the liquid. It is via this relaxation mechanism that the resonance line width is a sensitive probe of ionic mobility. 

The electric field gradient is the negative second derivative of the potential at the nucleus of all surrounding electric charge. It therefore embraces contributions from both the valence electrons of the atom and from surrounding ions. In ionic complexes it is customary to consider these separately, and to write q as... 

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