The Solution Chemistry of Cu2+–tren Complexes Revisited: Exploring the Role of Species That Are Not Trigonal Bipyramidal

Potentiometric and spectrophotometric titrations indicate that aqueous solutions that contain equimolar amounts of Cu2+ and tren contain the HCuL3+, CuL2+ and CuL(OH)+ species and that their relative concentrations depend on the pH of the solution. The stability constants and the UV/Vis and EPR spectra of the three species have been determined. The position of the absorption maximum clearly corresponds to a trigonal bipyramidal (tbp) geometry for CuL2+, whereas for HCuL3+ and CuL(OH)+ there are also bands that could correspond to square pyramidal (sp) complexes, but the EPR spectra indicate that only HCuL3+ can be considered to be sp. When any of these species is mixed with an excess of acid, an intermediate is formed within the mixing time of the stopped-flow technique. This intermediate undergoes complete decomposition in a second slower step. Interestingly, the spectrum of this intermediate is typical of sp geometry. Kinetic studies on complex formation in general indicate that complexation occurs in a single step, although under certain conditions an additional step has been observed that probably corresponds to the conversion of CuL2+ to HCuL3+, and the spectral changes indicate that the process involves structural reorganization from tbp to sp geometry. DFT and TDDFT calculations have been carried out for the three stable species, as well as for species in a higher protonation state. The results indicate that CuL2+ exists as a species with tetradentate tren and tbp geometry, although a wide range of distortions between the ideal tbp geometry and a geometry closer to sp is possible with a very modest energy cost. The energy change associated with hydrolysis of one of the Cu– N bonds to give a species with tridentate tren was found to be slightly higher than that previously found for a related ligand, which contains a substituent at one of the terminal amino groups. For CuL(OH)+, the calculations suggest that an equilibrium exists between species with essentially the same energy but different geometries, each one of the species is closer to one of the ideal tbp and sp limits. For HCuL3+, the relevance of the sp geometry was confirmed by the calculations

High resolution profile of inorganic aqueous geochemistry and key redox zones in an arsenic bearing aquifer in Cambodia

Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models

Latvian Photographers in the Age of Glasnost = Latviesu Fotografi Glasnost Laikmeta

Ingelevics points out how Glasnost has re-opened the social realm for seven Latvian photographers. Gulens reflects upon the impact of the "Latvian Renaissance" of 1988. Includes a chronology of Latvian history and biographical notes