Kinetics Studies of Aqueous Phase Reactions of Cl Atoms and Cl2- Radicals with Organic Sulfur Compounds of Atmospheric Interest

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

The aqueous reaction of [IrCl6]2- with CH3SO2- is biphasic and yields a 1:1 mixture of [IrCl6]3- and [IrCl5(H2O)]2- and CH3SO2Cl in the initial rapid phase. The next slow phase corresponds to the hydrolysis of CH3SO2Cl to yield CH3SO3- and Cl-. The initial phase shows kinetic inhibition by [IrCl6]3- that can be minimized by the addition of the radical scavenger propiolic acid. A detailed analysis of the kinetics indicates a mechanism with reversible outer-sphere electron transfer from CH3SO2- to [IrCl6]2- as the first step, followed by the irreversible inner-sphere oxidation of CH3SO2• by [IrCl6]2- to yield [IrCl5(H2O)]2- and CH3SO2Cl. Analysis of the inhibition by [IrCl6]3- and the kinetic effects of propiolic acid enable the determination of the equilibrium constant for the first electron-transfer step. This equilibrium constant then yields E° (CH3SO2•/CH3SO2-) = 1.01 V vs NHE at 25 °C. This is the first report of a standard potential for an alkanesulfonyl radical.

Rate constants of chlorine atom reactions with organic molecules in aqueous solutions, an overview

Rate constants of chlorine atom (Cl^•) reactions (k_Cl•) determined using a large variation of experimental methods, including transient measurements, steady-state and computation techniques, were collected from the literature and were discussed together with the reaction mechanisms. The k_Cl• values are generally in the 10⁸-10⁹ mol^-1 dm³ s^-1 range when the basic reaction between the Cl^• and the target molecule is H-atom abstraction. When Cl^• addition to double bonds dominates the interaction, the k_Cl• values are in the 1 × 10⁹-2 × 10¹⁰ mol^-1 dm³ s^-1 range. In the k_Cl• = 1 × 10¹⁰-4 × 10¹⁰ mol^-1 dm³ s^-1 range, single-electron-transfer reactions may also contribute to the mechanism. The Cl^• reactions with organic molecules in many respects are similar to those of ^•OH, albeit Cl^• seems to be less selective as ^•OH. However, there is an important difference, as opposed to Cl^• in the case of ^•OH single-electron-transfer reactions have minor importance. The uncertainty of Cl^• rate constant determinations is much higher than those of ^•OH. Since Cl^• reactions play very important role in the emerging UV/chlorine water purification technology, some standardization of the rate constant measuring techniques and more k_Cl• measurements are recommended.

Rate constants of dichloride radical anion reactions with molecules of environmental interest in aqueous solution: a review

Natural waters, water droplets in the air at coastal regions and wastewaters usually contain chloride ions (Cl-) in relatively high concentrations in the milimolar range. In the reactions of highly oxidizing radicals (e.g., •OH, •NO3, or SO4•-) in the nature or during wastewater treatment in advanced oxidation processes the chloride ions easily transform to chlorine containing radicals, such as Cl•, Cl2•-, and ClO•. This transformation basically affects the degradation of organic molecules. In this review about 400 rate constants of the dichloride radical anion (Cl2•-) with about 300 organic molecules is discussed together with the reaction mechanisms. The reactions with phenols, anilines, sulfur compounds (with sulfur atom in lower oxidation state), and molecules with conjugated electron systems are suggested to take place with electron transfer mechanism. The rate constant is high (107-109 M-1 s-1) when the reduction potential the one-electron oxidized species/molecule couple is well below that of the Cl2•-/2Cl- couple.

Determination and Environmental Implications of Aqueous-Phase Rate Constants in Radical Reactions

Interests in the kinetics of radical-induced reactions in aqueous solution have grown remarkably due to their water engineering significance (e.g., advanced oxidation processes). Although compilations of the rate constants (k) for various radicals have been documented, surprisingly a systematic review has yet to be reported on the development of reliable methods for determining k values. A knowledge gap exists to critically evaluate and screen the various methods to measure them. In this review, we summarize the direct and indirect methods under steady-state and non-steady-state conditions, followed by critical evaluations on their advantages and disadvantages. The radicals of ·OH, [Formula: see text] , [Formula: see text] , and Cl· were chosen based on their significant aquatic environmental relevance. MS excel spreadsheets that demonstrate the determination processes were provided allowing one to reproduce the data and/or to analyze the unprocessed raw data as a "template". We formulated a standard operation procedure for the k determination, although there is simply no "versatile" method fitting for all radical reactions. Finally, existing challenges and future research focus are discussed. This is the first review covering methodological approaches and considerations, aiming to provide a holistic and fundamental basis to choose an appropriate method for determining the k values for bimolecular reactions between target compounds and radicals in the aqueous phase.

Halide Photoredox Chemistry

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X-X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-to-ligand charge-transfer excited states. Kosower's salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights.

A computational study on the mechanism and kinetics of the reaction between CH3CH2S and OH

Addition–elimination channels CH₃CHS + H₂O, CH₂CH₂ + HSOH, CH₃CHSO + H₂ and CH₃CH₂SH + O are the major channels in the reaction between CH₃CH₂S and OH.

Reactions of Cl•/Cl2•− Radicals with the Nanoparticle Silica Surface and with Humic Acids: Model Reactions for the Aqueous Phase Chemistry of the Atmosphere

Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic-LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl*, and dichloride radical anions, Cl2*-. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl* and Cl2*- were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k +/- sigma, of (9 +/- 6) x 10(7) M(-1) s(-1) and (7 +/- 4) x 10(5) M(-1) s(-1), respectively. The reaction of Cl* with the surface deprotonated silanols leads to the formation of SiO* defects. HA are also observed to react with Cl* and Cl2*- radicals, with reaction rate constants at pH 4 of (3 +/- 2) x 10(10) M(-1) s(-1) and (1.2 +/- 0.3) x 10(9) M(-1) s(-1), respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA.

Kinetics, Mechanism, and Thermochemistry of the Gas Phase Reaction of Atomic Chlorine with Dimethyl Sulfoxide

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of chlorine atoms with dimethyl sulfoxide (CH3S(O)CH3; DMSO) as a function of temperature (270-571 K) and pressure (5-500 Torr) in nitrogen bath gas. At T = 296 K and P > or = 5 Torr, measured rate coefficients increase with increasing pressure. Combining our data with literature values for low-pressure rate coefficients (0.5-3 Torr He) leads to a rate coefficient for the pressure independent H-transfer channel of k1a = 1.45 x 10(-11) cm3 molecule(-1) s(-1) and the following falloff parameters for the pressure-dependent addition channel in N2 bath gas: k(1b,0) = 2.53 x 10(-28) cm6 molecule(-2) s(-1); k(1b,infinity) = 1.17 x 10(-10) cm3 molecule(-1) s(-1), F(c) = 0.503. At the 95% confidence level, both k1a and k1b(P) have estimated accuracies of +/-30%. At T > 430 K, where adduct decomposition is fast enough that only the H-transfer pathway is important, measured rate coefficients are independent of pressure (30-100 Torr N2) and increase with increasing temperature. The following Arrhenius expression adequately describes the temperature dependence of the rate coefficients measured at over the range 438-571 K: k1a = (4.6 +/- 0.4) x 10(-11) exp[-(472 +/- 40)/T) cm3 molecule(-1) s(-1) (uncertainties are 2sigma, precision only). When our data at T > 430 K are combined with values for k1a at temperatures of 273-335 K that are obtained by correcting reported low-pressure rate coefficients from discharge flow studies to remove the contribution from the pressure-dependent channel, the following modified Arrhenius expression best describes the derived temperature dependence: k1a = 1.34 x 10(-15)T(1.40) exp(+383/T) cm3 molecule(-1) s(-1) (273 K < or = T < or = 571 K). At temperatures around 330 K, reversible addition is observed, thus allowing equilibrium constants for Cl-DMSO formation and dissociation to be determined. A third-law analysis of the equilibrium data using structural information obtained from electronic structure calculations leads to the following thermochemical parameters for the association reaction: delta(r)H(o)298 = -72.8 +/- 2.9 kJ mol(-1), deltaH(o)0 = -71.5 +/- 3.3 kJ mol(-1), and delta(r)S(o)298 = -110.6 +/- 4.0 J K(-1) mol(-1). In conjunction with standard enthalpies of formation of Cl and DMSO taken from the literature, the above values for delta(r)H(o) lead to the following values for the standard enthalpy of formation of Cl-DMSO: delta(f)H(o)298 = -102.7 +/- 4.9 kJ mol(-1) and delta(r)H(o)0 = -84.4 +/- 5.8 kJ mol(-1). Uncertainties in the above thermochemical parameters represent estimated accuracy at the 95% confidence level. In agreement with one published theoretical study, electronic structure calculations using density functional theory and G3B3 theory reproduce the experimental adduct bond strength quite well.