Adsorption of Nitrate and Bicarbonate on Fe-(Hydr)oxide.

In this work, we used density functional theory calculations to study the resulting complexes of adsorption and of inner- and outer-sphere adsorption-like of bicarbonate and nitrate over Fe-(hydr)oxide surfaces using acidic, neutral, and basic simulated pH conditions. High-spin states that follow the 5N + 1 (N is the number of Fe atoms, each having five unpaired electrons) rule are preferred. Monodentate mononuclear (MM1) surface complexes are shown to lead to the most favorable thermodynamic adsorption for both bicarbonate and nitrate with -63.91 and -28.25 kJ/mol, respectively, under neutral conditions. Our results suggest that four types of regular and charged-assisted hydrogen bonds are involved in the adsorption process; all of them can be classified as closed-shell (long-range or ionic). The formal charges induce unusually short and strong hydrogen bonds. The ability of high multiplicity states of Fe clusters to adsorb oxyanions in solvated environments arises from orbital interactions: the 4s virtual orbitals in Fe have a large affinity for the 2p-type electron pairs of oxygens.

Microsolvation of dimethylphosphate: a molecular model for the interaction of cell membranes with water.

We present an exhaustive stochastic search of the quantum conformational spaces of the (CH(3)O)(2)PO(2)(-) + nH(2)O (n = 1,2,3) systems. We uncover structural, conformational and energetic features of the problem. As in the isolated species, clusters containing the gauche-gauche (gg) conformation of dimethylphosphate (DMP(-)) are energetically preferred, however, contributions from hydrated gauche-anti (ga) and anti-anti (aa) monomers cannot be neglected because such structures are quite common and because they are close in energy to those containing the gg monomer. At least seven distinct types of O∙∙∙H-O-H contacts lead to DMP(-) ↔ water interactions that are always stabilizing, but not strong enough to induce significant changes in the geometries of either DMP(-) or water units. Our results lead us to postulate DMP(-) to be a suitable model to study explicit and detailed aspects of microsolvation of cell membranes.