RESUMEN
Previous published entropies of solid lanthanide trichloride hydrates are reviewed and revised by taking account of the Kramers degeneracy of ground states with half-integral values of J. The revised values necessitate corrections to the standard entropies of the aqueous tripositive lanthanide ions which display an irregular variation with both atomic number and ionic radius. These irregularities can be smoothed out by taking account of contributions from the electronic or magnetic entropy, from structural changes in the coordination that have been revealed by EXAFS and XANES measurements and X-ray crystallography, and from an increasing intrusion of eight coordination across the series. When these contributions are removed, the entropies of the aqueous ions refer to a hypothetical nine-coordinate state in trigonal triprismatic coordination. Within experimental error, they then vary uniformly across the lanthanide series and there is a good linear correlation with the reciprocal of the metal-oxygen distance in the nine-coordinate aqua complexes.
RESUMEN
New estimated standard entropies for some aqueous metal ions are obtained by taking account of magnetic and symmetry contributions. By combining them with an analysis of literature data, improved experimental and estimated values are derived for the standard enthalpies and Gibbs energies of formation of the aqueous ions of titanium, vanadium, chromium, manganese, cerium, and praseodymium. Separate entropy correlations are used for each primary coordination number, and the size dependence is represented by the reciprocal of the metal-oxygen distance in that coordination. The new scheme is consistent with recent work on the coordination of Hg(2+)(aq), Pb(2+)(aq), and tripositive rare earth ions. It differs from its predecessors in indicating a larger variation of the standard molar entropies of aqueous ions with coordination number. The value of S(Θ)(Be(2+), aq) is discussed in this context.
RESUMEN
Information about the three-dimensional structure of solar magnetic fields is encoded in the polarized spectra of solar radiation by a host of physical processes. To extract this information, solar spectra must be obtained in a variety of magnetically sensitive spectral lines at high spatial, spectral, and temporal resolution with high precision. The need to observe many different spectral lines drives the development of Stokes polarimeters with a high degree of wavelength diversity. We present a new paradigm for the design of polarization modulators that operate over a wide wavelength range with near-optimal polarimetric efficiency and are directly applicable to the next generation of multiline Stokes polarimeters. These modulators are not achromatic in the usual sense because their polarimetric properties vary with wavelength, but they do so in an optimal way. Thus, we refer to these modulators as polychromatic. We present here the theory behind polychromatic modulators, illustrate the concept with design examples, and present the performance properties of a prototype polychromatic modulator.
RESUMEN
An analysis of the ligand field stabilization energy which makes allowance for the nephelauxetic effect is shown to give satisfactory results when applied to the hydration enthalpies of the hexaaqua 3+ complexes of the first transition series. The contribution from the nephelauxetic effect is substantial, and the baseline for zero ligand field stabilization energy is displaced below the d(0) point. In these respects, the complexes show a closer resemblance to the hexafluorometalates(III) than to the hexaaqua 2+ ions. In the course of the analysis, the electronic spectra of the cesium alums of vanadium(III), manganese(III), and cobalt(III) are reexamined, and thermodynamic properties of particular complexes are calculated. These include Delta(Ti(3+),aq), E(&crclbr;)(Ti(3+)|Ti(2+)), and the relative stabilities of the high- and low-spin states of Co(3+)(aq), Mn(3+)(aq), and Cr(2+)(aq). The general influence of ligand field stabilization energies on redox potentials is also discussed.