The intrinsic mechanism of catalytic oxidation of arsenite by hydroxyl-radicals in the H3AsO3-CO32-/HCO3--H2O system: A quantum-chemical examination.

Arsenite is a highly toxic compound present in many water sources around the world. The removal of arsenite from water requires its oxidation to arsenate which is much more amenable to treatment using well attested technologies. Prior research has shown that the oxidation of arsenite by hydroxyl radicals is significantly accelerated in the presence of carbonate ions but the intrinsic mechanisms of the acceleration have not yet been established. The main goal of the present work was to examine the oxidation of arsenite in the framework of the density functional theory, to establish a detailed microscopic level mechanism of interactions between arsenite and hydroxyl radicals and to elucidate the nature of the catalytic effect of carbonate ions. Results of this study demonstrate that the [As(OH)2CO3]- complex is the thermodynamically most stable species formed in the system H3AsO3-CO32-/HCO3--H2O. Interactions of the hydroxyl radical with the [As(OH)2CO3]- complex yield the pre-reaction complex [As(OH)3CO3]-∗ in the reaction of subsequent oxidation of arsenite. The structures of the reactants, products and transition states, as well as pre- and post-reaction complexes corresponding to several possible mechanisms of the first stage of As(III) oxidation to As(IV) intermediate using hydroxyl radicals in the absence and in the presence of [As(OH)2CO3]-, were determined in this study. The data demonstrate that the arsenite-carbonate complexes [As(OH)2CO3]- are characterized by a significantly lower activation energy of the first oxidation stage under the action of a hydroxyl radical (2.8 kcal/mol) compared to that for the free arsenite H3AsO3 (13.6 kcal/mol).

Standard Redox Potentials of Fe(III) Aqua Complexes Included into the Cavities of Cucurbit[ n]urils ( n = 6-8): A DFT Forecast.

An approach for estimating at the DFT level of the standard redox potentials of the inclusion compounds based on Fe(III) and Fe(II) aqua complexes inside the cavities of cucurbit[ n]urils ( n = 6-8) has been proposed. These inclusion compounds were established to have compositions which can be described by the formulas [Fe(H2O)6]3+/2+@CB[6] and [Fe(H2O)6·4H2O]3+/2+@CB[7,8]. Redox potentials E0 relative to the standard hydrogen electrode for the half-reaction Fe(III)/Fe(II) in the CB[ n] cavities calculated at the PBE/TZVP level within the molecular-continuum solvation model are 1.607, 0.949, and 0.847 V for n = 6, 7, and 8, respectively. The obtained values indicate a relative increase of the oxidative ability of Fe(III) aqua-ions in the cavities of the examined CB[ n], especially in CB[6], compared to the calculated value ( E0 = 0.786 V) for the same half-reaction in the bulk of aqueous solution. Possible causes of the detected trend are discussed. The calculations also showed that the Fe(III) aqua complex inside the CB[6] changes its magnetic properties, transforming into a low-spin state with a total spin S = 1/2, whereas for all other systems high-spin states in accord with the classical ligand field theory are realized.

Supramolecular Adducts of Cucurbit[7]uril and Amino Acids in the Gas Phase.

The complexation of the macrocyclic cavitand cucurbit[7]uril (Q7) with a series of amino acids (AA) with different side chains (Asp, Asn, Gln, Ser, Ala, Val, and Ile) is investigated by ESI-MS techniques. The 1:1 [Q7 + AA + 2H](2+) adducts are observed as the base peak when equimolar Q7:AA solutions are electrosprayed, whereas the 1:2 [Q7 + 2AA + 2H](2+) dications are dominant when an excess of the amino acid is used. A combination of ion mobility mass spectrometry (IM-MS) and DFT calculations of the 1:1 [Q7 + AA + 2H](2+) (AA = Tyr, Val, and Ser) adducts is also reported and proven to be unsuccessful at discriminating between exclusion or inclusion-type conformations in the gas phase. Collision induced dissociation (CID) revealed that the preferred dissociation pathways of the 1:1 [Q7 + AA + 2H](2+) dications are strongly influenced by the identity of the amino acid side chain, whereas ion molecule reactions towards N-butylmethylamine displayed a common reactivity pattern comprising AA displacement. Special emphasis is given on the differences between the gas-phase behavior of the supramolecular adducts with amino acids (AA = Asp, Asn, Gln, Ser, Ala, Val, and Ile) and those featuring basic (Lys and Arg) and aromatic (Tyr and Phe) side chains.