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The adsorption of thiram and disulfiram onto alpha-Al2O3 and montmorillonite clay has been studied in the presence of small carboxylate anions, bicarbonate, formate, and oxalate. At natural concentrations, HCO3- enhances dramatically the adsorption of both pesticides on alpha-Al2O3 and clay. An analogous significant enhancement of pesticide adsorption is also observed in the presence of formate and oxalate. Density functional theory calculations demonstrate that in solution a stable molecular complex between one molecule of thiram and one molecule of HCO3- is formed with interaction energy -35.6 kcal/mol. In addition, two H20 molecules further stabilize it by an interaction energy of -3.6 kcal/mol. This clustering [thiram- HCO3- -2H2O] leads to a change of the electronic structure and the ultraviolet-visible spectrum of thiram that is observed experimentally. Surface complexation modeling shows that the molecular cluster [thiram-HCO3- -2H2O], which bears a total net charge of -1, is responsible for the observed enhanced adsorption on the charged surface of alumina and clay at pH below their points of zero surface charge. The results reveal a novel pervasive role of carboxylate anions and particularly HCO3- on the adsorption of dithiocarbamate pesticides in natural waters.

[1] , which contains a total of 46 valence electrons and hence a Pt À Pt bond. To our surprise, when the starting compound comprises at least one palladium center the analogous Pd III -containing derivatives cannot be isolated from, or identified in, the reaction mixtures. [1] To understand the different response of the heteronuclear species towards oxidation by Ag + ions, and guide our experimental efforts towards identifying and finally isolating the reaction products, we searched the potential energy surface (PES) of a model reaction system with electronic structure calculation methods at the B3LYP level of theory [2,3] using the LANL2DZ basis set, as implemented in the Gaussian 03 series of programs.[4] To obtain a computationally convenient size we used a model reaction system where the phenyl groups of the phosphanide ligands are substituted by H atoms and the C 6 F 5 ligands by CN ligands. The decision to substitute the C 6 F 5 ligands by CN is based on electronic structure calculations of the coordinating ability (s-donor and p-acceptor capacity) of the ligands and has been described previously.[5] The geometric and energetic profile of the oxidation reaction of M1 b, along with the HOMO of all stationary points, computed at the B3LYP/LANL2DZ level of theory is depicted schematically in Figure 1.One-electron oxidation of complex M1 b yields the intermediate open-shell (doublet) anionic species M1 b1, which is a symmetric (D 2h point group), trinuclear PtPdPt complex without any intermetallic interaction. This oneelectron oxidation step requires about 49.3 kcal mol À1 at the B3LYP/LANL2DZ level, which is provided by the reduction of the Ag + ions used as the oxidant (the electron affinity of the Ag + ions was predicted to be 178.7 kcal mol À1 ). Interestingly, the one-electron oxidation involves a strong coordination of the Ag + ion to Pd and one terminal Pt atom to afford complex M1 bAg, which adopts a chairlike structure (see Supporting Information) with a binding energy predicted to be 171.9 kcal mol À1 , thus illustrating that the Pd···Ag + and Pt···Ag + intermetallic bonds are relatively strong. These strong intermetallic interactions are also mirrored in the short Pd···Ag + and Pt···Ag + bond distances of 2.756 and 2.746 , respectively. Phosphanido Pt/Pd/Ag complexes of this type have recently been isolated by us.[6] It should be noted that the transfer of one electron is a favorable process because the LUMO of M1 bAg is localized purely on the coordinated Ag + ion. The salient feature of the structure of M1 b1 is the remarkable shortening of the P···P distance by about 0.29 with respect to the starting M1 b species. The localization of the spin density on the P atoms of the bridging phosphanido ligands (Figure 1) is also noteworthy as it indicates that P···P interactions should start to evolve during the course of the oxidation process. Further one-electron oxidation of M1 b1 leads to a second-order saddle point (probably an effective monkey-saddle-type transition state [7] possessing a doubly dege...

An NMR trans-philicity ladder was built for octahedral [Cr(CO)5L]−/0/+ complexes employing a 13C NMR probe and quantified by Δσ13C NMR descriptors.

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