Tailoring the Oxidizing Intermediate in the Fenton Reaction, OH⋅ or Fe=O, by Modifying the pK of the (HO)Fe(HO) Complex, and the Case of PDS - a DFT Study.

A DFT analysis of the Fenton and Fenton-like reactions points out that the pH effect on the nature of the oxidizing intermediate formed is due to a pK of the peroxide when hydroperoxides are used. When SO is used, the pH effect is due to the pK of one of the water ligands of the central iron cation. The results suggest that the choice of the hydroperoxide and the ligands present affects the pH at which the transition from the formation of hydroxyl radicals to the formation of Fe=O occurs.

Oxidation of Methanesulfinate by Hexachloroiridate(IV) and the Standard Electrode Potential of the Aqueous Methanesulfonyl Radical.

The aqueous reaction of [IrCl] with CHSO is biphasic and yields a 1:1 mixture of [IrCl] and [IrCl(HO)] and CHSOCl in the initial rapid phase. The next slow phase corresponds to the hydrolysis of CHSOCl to yield CHSO and Cl. The initial phase shows kinetic inhibition by [IrCl] that can be minimized by the addition of the radical scavenger propiolic acid.

Prediction of Aqueous Reduction Potentials of X, ChH, and XO Radicals with X = Halogen and Ch = Chalcogen.

The aqueous electron affinity and aqueous reduction potentials for F, Cl, Br, I, OH, SH, SeH, TeH, ClO, BrO, and IO were calculated using electronic structure methods for explicit cluster models coupled with a self-consistent reaction field (SMD) to treat the aqueous solvent. Calculations were conducted using MP2 and correlated molecular orbital theory up to the CCSD(T)-F12b level for water tetramer clusters and MP2 for octamer cluster. Inclusion of explicit waters was found to be important for accurately predicting the redox potentials in a number of cases.

Absolute Hydration Free Energy of Small Anions and the Aqueous p of Simple Acids.

Heats of formation and gas phase acidities for the simple acids and their deprotonated anions (A = F, Cl, Br, I, OH, SH, SeH, TeH, OCl, OBr, and OI) were calculated using the Feller-Peterson-Dixon (FPD) method with large basis sets including Douglass-Kroll scalar relativistic corrections. Hydration of the neutral and anionic species was predicted using the supermolecule-continuum approach, resulting in absolute hydration free energies that, when combined with calculated gas phase acidities, produce aqueous acidities and p values for these simple acids that are, in general, in excellent agreement with experimental literature values. Absolute hydration free energy values converged quickly in terms of the experimental values for neutral species, requiring only four explicit HO molecules.

Misconceptions about the chemistry of aqueous chlorine atoms and HClOH˙(aq), and a revised mechanism for the photochemical peroxydisulfate/chloride reaction.

It is widely considered that aqueous chlorine atoms (Cl˙) convert to the species HClOH˙ with a half life of about 3 μs and that this species plays an important role in the chemistry of aqueous chlorine atoms. Here it is shown that there is no firm evidence for the existence of HClOH˙ as a species distinct from Cl˙, that the chemistry attributed to HClOH˙ can be accounted for by other well-established species, and that almost all published mechanisms that include reactions of HClOH˙ violate the principle of detailed balancing. More than 100 publications are identified that violate the principle of detailed balancing with HClOH˙ reactions.

The principle of detailed balancing, the iron-catalyzed disproportionation of hydrogen peroxide, and the Fenton reaction.

The iron-catalyzed disproportionation of HO has been investigated for over a century, as has been its ability to induce the oxidation of other species present in the system (Fenton reaction). The mechanisms of these reactions have been under consideration at least since 1932. Unfortunately, little or no attention has been paid to ensuring the conformity of the proposed mechanisms and rate constants with the constraints of the principle of detailed balancing.

The H•/H Redox Couple and Absolute Hydration Energy of H.

A supermolecule-continuum approach with water clusters up to = 16 HO molecules has been used to predict the absolute hydration free energies at 298 K (Δ) of both hydrogen (H•) and hydride (H) to be 4.6 ± 1 and -78 ± 3 kcal/mol, respectively. These values are combined with a high accuracy prediction of the gas-phase electron affinity (Δ = -16.

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process.

Advanced oxidation processes tend to have very complex reaction mechanisms, and models containing over 150 steps have been developed to describe the chemistry. Without the aid of automation, it is extremely difficult to avoid the development of kinetic mechanisms that violate the principle of detailed balancing. Here, we apply DETBAL, a computer application, to systematically identify many violations of the principle of detailed balancing in a model proposed for the UV/chloramine process.

Large-Scale Models of Radiation Chemistry and the Principle of Detailed Balancing.

Large-scale kinetic models containing more than 50 homogeneous reaction steps have been developed to help understand the behavior of actinide solutions in various circumstances. One specific objective is to understand the behaviors in nitrate solutions as such solutions are used in actinide separations. A challenge arises in developing these large-scale models while ensuring compliance with the principle of detailed balancing because it can be very difficult to identify all embedded reaction loops.

Methanesulfonyl Iodide.

Methanesulfonyl iodide is produced in aqueous solutions from the reaction of triiodide with methanesulfinate. Dichroic crystals of (CHSOI)·KI·2I are formed from KI/I solutions with high concentrations of CHSO, while dichroic crystals of (CHSOI)·RbI are formed from RbI/I solutions. X-ray crystallography of these two compounds shows that the CHSOI molecules coordinate through their oxygen atoms to the metal cations and that the S-I bond length is 2.

Systematic Application of the Principle of Detailed Balancing to Complex Homogeneous Chemical Reaction Mechanisms.

It is not uncommon for proposed complex reaction mechanisms to violate the principle of detailed balancing. Here, we draw attention to three ways in which such violations can occur: reversible reaction loops where the rate constants do not attain closure, illegal loops, and reversible steps having rate equations in the forward and reverse directions that are inconsistent with the equilibrium expressions. We present two simple methods to test whether a proposed mechanism is consistent with the first two aspects of the principle of detailed balancing.

Hydration Energies and Reactivity of the Hypohalite Anions.

Herein is a dissection of the energetic contributions to a correlation between the rates and driving forces for oxygen atom transfer from three inorganic peroxides to the halides. Experimental data are used to calculate conventional and absolute hydration enthalpies for OCl, OBr, and OI. It is found that the hydration enthalpies are more exothermic for the OX species in comparison to their X congeners, and it is found that the hydration enthalpies are approximately constant on progressing from OCl to OI.

Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl Versus O Atom Transfer.

Sulfur-containing compounds are known to be susceptible to oxidation by aqueous HOCl, but the factors affecting the rates of these reactions are not well-established. Here we report on the kinetics of oxidation of thiosulfate, thiourea, thioglycolate, (methylthio)acetate, tetrathionate, dithiodiglycolate, and dithiodipropionate at 25 °C and 0.4 M ionic strength.

One-Electron Oxidation of Hydrogen Sulfide by a Stable Oxidant: Hexachloroiridate(IV).

Detailed reports on the oxidation of aqueous H2S by mild one-electron oxidants are lacking, presumably because of the susceptibility of these reactions to trace metal-ion catalysis and the formation of turbid sulfur sols. Here we report on the reaction of [IrCl6](2-) with H2S in acetate buffers. Dipicolinic acid (dipic) is shown to be effective in suppressing metal-ion catalysis.

Kinetics of the Benzaldehyde-Inhibited Oxidation of Sulfite by Chlorine Dioxide.

There has been steady interest in the aqueous reaction of ClO2• with sulfur(IV) since the 1950s, and a wide variety of rate laws and mechanisms have been proposed. In neutral-to-alkaline media, the reaction is challenging to study because of its great rate. Here it is shown that benzaldehyde can be used as an additive to slow the reaction and make its rates more amenable to study.

Kinetics of the initial steps in the aqueous oxidation of thiosulfate by chlorine dioxide.

The reaction of ClO2(•) with S2O3(2-) in aqueous solution is a component of the "crazy clock" reaction of ClO2(-) with S2O3(2-), and under conditions of excess S2O3(2-) the absorbance at 360 nm due to ClO2(•) decays with sigmoidal kinetics. A chain reaction mechanism is inferred on the basis that very small concentrations of SO3(2-) accelerate the reaction, and methionine inhibits the reaction. Pseudo-first-order kinetics is observed in the presence of relatively large methionine concentrations, leading to the simple rate law -d[ClO2]/dt = (ka[S2O3(2-)] + kb[S2O3(2-)](2))[ClO2], with ka = 452 ± 16 M(-1) s(-1) and kb = (5.

Oxidation of cysteinesulfinic acid by hexachloroiridate(IV).

We report the results of an experimental study of the oxidation of cysteinesulfinic acid (CysSO2H) by [IrCl6](2-) in aqueous media at 25 °C in order to gain insight into the mechanisms of oxidation of alkylsulfinic acids by simple one-electron oxidants. When the reaction is performed with exclusion of O2 between pH 3 and 5, it is complete in several seconds. The products are [IrCl6](3-) and CysSO3H.

Oxidation of glutathione by hexachloroiridate(IV), dicyanobis(bipyridine)iron(III), and tetracyano(bipyridine)iron(III).

The aqueous oxidations of glutathione (GSH) by [IrCl(6)](2-), [Fe(bpy)(2)(CN)(2)](+), and [Fe(bpy)(CN)(4)](-) are described. All three reactions are highly susceptible to catalysis by traces of copper ions, but this catalysis can be fully suppressed with suitable chelating agents. The direct oxidation by [IrCl(6)](2-) yields [IrCl(6)](3-) and GSO(3)(-); some GSSG is also obtained in the presence of O(2).

Oxidation of phenol by tris(1,10-phenanthroline)osmium(III).

Outer-sphere oxidation of phenols is under intense scrutiny because of questions related to the dynamics of proton-coupled electron transfer (PCET). Oxidation by cationic transition-metal complexes in aqueous solution presents special challenges because of the potential participation of the solvent as a proton acceptor and of the buffers as general base catalysts. Here we report that oxidation of phenol by a deficiency of [Os(phen)(3)](3+), as determined by stopped-flow spectrophotometry, yields a unique rate law that is second order in [osmium(III)] and [phenol] and inverse second order in [osmium(II)] and [H(+)].

Overoxidation of phenol by hexachloroiridate(IV).

It has been previously established that the aqueous oxidation of phenol by a deficiency of [IrCl(6)](2-) proceeds through the production of [IrCl(6)](3-) and phenoxyl radicals. Coupling of the phenoxyl radicals leads primarily to 4,4'-biphenol, 2,2'-biphenol, 2,4'-biphenol, and 4-phenoxyphenol. Overoxidation occurs through the further oxidation of these coupling products, leading to a rather complex mixture of final products.

Electrode potentials of partially reduced oxygen species, from dioxygen to water.

We use published Gibbs energies of formation and equilibrium constants to determine electrode potentials for the partially reduced intermediates along the pathway of reduction of dioxygen to water, as well as of ozone and singlet dioxygen. The results are summarized in an oxidation state (Frost) diagram. Our review of the literature on electrode potentials leads us to revise values for the O(2)/O(2)(*-) couple to E degrees (O(2g)/O(2)(*-))=-0.

Proton-coupled electron-transfer oxidation of phenols by hexachloroiridate(IV).

One-electron oxidation of phenol, 2,4,6-trimethylphenol, and 2,6-dimethylphenol by [IrCl(6)](2-) in aqueous solution has a simple pH dependence, indicating slow bimolecular oxidation of ArOH and faster oxidation of ArO(-). H/D kinetic isotope effects as large as 3.5 for oxidation of ArOH support concerted proton-coupled electron transfer with water as the proton acceptor.

Direct oxidation of L-cysteine by [FeIII(bpy)2(CN)2]+ and [FeIII(bpy)(CN)4]-.

The oxidation of L-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.