George V. Buxton

Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅OH/⋅O− in Aqueous Solution

Kinetic data for the radicals H⋅ and ⋅OH in aqueous solution,and the corresponding radical anions, ⋅O− and eaq−, have been critically pulse radiolysis, flash photolysis and other methods. Rate constants for over 3500 reaction are tabulated, including reaction with molecules, ions and other radicals derived from inorganic and organic solutes.

Re-evaluation of the thiocyanate dosimeter for pulse radiolysis

The thiocyanate dosimeter (10–2 mol dm–3 SCN– in O2-saturated water) has been standardised against the super-Fricke dosimeter (10–2 mol dm–3 FeII in O2-saturated 0.4 mol dm–3 H2SO4) using the hexacyanoferrate(II) dosimeter [5 × 10–3 mol dm–3 Fe(CN)64– in O2-saturated water] as a secondary standard. On the basis that G(FeIII)= 1.67 × 10–6 mol J–1 and IµFeIII= 220.4 m2 mol–1 at 304 nm and 25 °C in the super-Fricke dosimeter, we obtain GIµ[Fe(CN)63–]=(3.47 ± 0.06)× 10–5 m2 J–1 at 420 nm and GIµ(SCN)2˙–=(2.59 ± 0.05)× 10–4 m2 J–1 at 475 nm. These values remain unchanged when the solutions are saturated with air instead of O2 and are doubled in N2O-saturated solution.

Estimation of rate constants for near-diffusion-controlled reactions in water at high temperatures

Rate constants measured over the temperature range 20–200 °C are reported for the following reactions: (a) reaction of the hydrated electron with oxygen, the proton, hydrogen peroxide, nitrate, nitrite, nitrobenzene and methyl viologen; (b) reaction of the hydroxyl radical with another hydroxyl radical and ferrocyanide; (c) reaction of the hydrogen atom with permanganate and oxygen. To evaluate methods of estimating rate constants at high temperatures these rate constants and others in the literature have been fitted to the following equation: kobs=kdiff/(1 +kdiff/kreact), where kobs is the measured rate constant for the bimolecular reaction in solution, kdiff is the encounter rate constant of the two reacting species, and kreact is the rate constant that would be measured if diffusion of the species was not rate influencing. With the exception of reactions of the hydrated electron with nitrate and nitrite ions and nitrous oxide, good fits have been obtained to the above equation, and the results demonstrate that few, if any, of the reactions which are pertinent to water radiolysis are truly diffusion controlled at elevated temperatures.

The reactivity of chlorine atoms in aqueous solution Part II.The equilibrium SO4-+Cl-ClNsbd+SO42-

A study of the two simultaneous equilibria, reactions (1) and (2), has been performed by pulse radiolysis in Ar-saturated aqueous solution at 25°C. The forward and reverse rate constants of equilibrium (2) were determined at 0.3 mol dm-3 ionic strength to be k2=(6.1±0.2)×108 and k-2=(2.1±0.1)×108 d mol-1 s-1. These yield the equilibrium constant K2=k2/k-2=(2.9±0.2) at 0.3 mol dm-3 ionic strength, or 1.2 corrected to zero ionic strength. As part of the study, the reactions of SO4- with S2O82-, t-BuOH and water were found to be <1.5×103, (7.8±0.2)×105 d mol-1 s-1 and (690±120) s-1, respectively. The relevance of these results to cloud chemistry is discussed.

A study of the spectra and reactivity of oxysulphur-radical anions involved in the chain oxidation of S(IV): A pulse and γ-radiolysis study

Absorption spectra of the radicals SO3−., SO4−. and SO5−. have been redetermined and their molar absorptivities at λmax evaluated as: ϵ(SO3−.)250 = (114±2) m2mol−1, ϵ(SO4−.)450 = (163 ± 5) m2 mol−1 and ϵ(SO5−.)260 = (79 + 2) m2 mol−. The rate constants for each of the steps in the freeradical-chain oxidation mechanism at pH 4 and 9 have been determined either by pulse radiolysis or a combination of pulse and steady-state radiolysis. The main chain terminating reaction is SO5−. + SO5−. → S2O82− + O2 for which 2κ16 = (9.6 ± 0.3) x 107 dm3mol−1 s−1 The rate determining propagation steps at pH 4 are SO5−. + HSO3− → HSO5−. + SO3−. (reaction 7) and SO5−. + HSO3−. → SO4−. + H+ + SO42− (reaction 9) with κ7 = (8.3 ± 1.0) x 103 dm3 mol−1 s−1 and κ9 = (3.4 ± 0.4) x 102 dm3 mol−1 s−1 and at pH 9 SO5−. + SO32− (+ H+) → HSO5− + SO3−. (reaction 8) and SO5−. + SO32− → SO4− + SO42− (reaction 10) for which κ8 = (3.6 ± 0.8) × 105 dm3 mol−1 s−1 and κ10 = (1.4 ± 0.8) × 105 dm3 mol−1 s−1 A previous report (Huie and Neta, 1987, Atmospheric Environment 21, 1743–1747) gave (κ7 + κ9) ≤ 3 x 105 dm3 mol−1 s− and (κ8 + κ10) = 1.3 x 107 dm3 mol−1s−1. The much lower values reported here suggest that the simple free-radical-chain oxidation of S(IV) may be of less significance in the oxidation of S(IV) in cloud droplets than previously estimated.

Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate?

The rates of reaction of OH with benzene, chlorobenzene, nitrobenzene, ion and benzoic acid have been measured in aqueous solution up to 200 °C using pulse radiolysis to generate OH. The temperature dependence of the observed rate constant, kobs, is essentially the same for each compound and kobs changes by less than three-fold between 20 °C and 200 °C. The kinetic data are consistent with a mechanism whereby OH reversibly forms a π-complex with the aromatic compound, irrespective of the substituent on the ring, which then transforms to a σ-bonded hydroxycyclohexadienyl radical. The values of kobs were determined from the rate of formation of this radical. There is no evidence for dissociation of the σ-bonded radical nor for H atom abstraction from the ring which have been reported for the gas phase. The apparent mechanistic differences between the two phases may be due to the different timescales over which the kinetics measurements were made.

The Radiolysis of aqueous Solutions of Oxybromine Compounds; the Spectra and Reactions of BrO and BrOFormula

Pulse radiolysis supplemented by steady state radiolysis of aqueous solutions containing some of the following solutes: N2O, Br-, BrO-, BrO-2, BrO-3, CO2-3, and OH- has been used to identify the absorption spectra of BrO (λmax = 350 nm) and BrO2(λmax = 475 nm) and to evaluate the following rate constants (units, M-1 s-1): e-aq. + BrO- → Br- + O- (2.3 ± 0.5 x 1010); e-aq. + BrO-2 → BrO + O2- (1.8 ± 0.2 x 1010); e-aq. + BrO-3 → BrO2 + O2- (4.1 ± 0.2 x 109); OH + BrO- → OH- + BrO (4.5 x 109); O- + BrO- → BrO + O2- (4.6 x 109); OH + BrO-2 → OH- + BrO2 (1.9 x 109); Br-2 + BrO- → BrO + 2Br- (8.0 ± 0.7 x 107); Br-2 + BrO-2 → BrO + Br- + BrO- (8.0 ± 0.8 x 107); BrO + BrO-2 → BrO- + BrO2 (3.4 ± 0.7 x 108); 2BrO2 ⇌ Br2O4 (k = 1.4 x 109 and K = 19 M-1); Br2O4 + OH- → H+ + BrO-2 + BrO-3 (7 x 108); 2BrO + H2O → BrO- + BrO-2 + 2H+ (4.9 ± 1.0 x109); CO-3 + BrO- → CO2-3 + BrO (4.3 ± 0.4 x 107); CO-3 + BrO-2 → CO2-3 + BrO2 (1.1 ± 0.1 x 108). In contrast to their chlorine analogues, little is known of the chemistry of the bromine oxides in aqueous solution. In this paper we describe the methods of formation and identification of the radicals BrO and BrO2, and their reactions with various oxybromine species, as elucidated by the techniques of pulse radiolysis and kinetic spectroscopy applied to aqueous solutions containing oxybromine anions.

The radiation chemistry of metal ions in aqueous solution

A. Introduction . . . . . . . . . . . . . . . . (i) The radiation chemistry of water . . . _ . . . (ii) Radiation chemical methods . . . _ _ . . . . (iii) Reactivity of e& OH and H with metal ions _ . . . (iv) Reactivity of the carboxyl and hydroxymethyl radicals R. Groups I and II (alkali and aikaiine earth metals) . . . . C. First row transition elements . . . . . . . . . . . (i) Scandium . . . . . . . . . . . . . . . (ii) Titanium . . . . . . . . . . . . . . . (iii) Vanadium _ _ _ . _ _ _ . . _ _ . . . . (iv) Chromium _ . . _ _ . _ . . . _ . . . (v) Manganese _ _ _ _ . . _ . . . . . . . . (vi) Iron . . _ _ . _ . _ _ . . . _ _ . . . D. Second and third :ow transition elements _ . . . . . . (i) Zirconium, niobium and hafnium . . . _ . . . (ii) Molybdenum . . . . . _ . _ . _ _ . . . (iii) Ruthenium _ _ . _ _ _ _ . . . . _ _ . _ (iv) Pafladium _ . . . _ . _ _ . _ _ _ . . . fv) Rhodium . _ _ . . _ _ _ _ _ _ _ _ . . (vi) Silver . . . . . . _ . . _ . . _ . . . (vii) Osmium. _ . _ . . . _ . _ _ _ _ . _ . (viii) Iridium . . . . . . . . . . . _ . . . . (ix) Platinum . . . . . _ _ . _ . . _ . . . (x)Goid _ _ _ _ _ . . _ _ . _ _ _ _ . . . (xi) Mercury _ _ _ . . _ _ _ . _ _ . _ . . E. Lanthanides _ _ . _ _ _ _ _ . _ . _ _ . . . (i) Reduction of the trivalent ions _ _ . _ _ . . . (ii)Cerium _ _ _ . . . . . . . . _ _ _ . . (iii) Praesodymium . . . . _ . . . . _ . . . F. Actinides (i) Gene& rem&s : : : : : : : : : : : : : (ii) Thorium . . . . . . . . . . . . . . . (iii) Uranium _ _ _ . _ _ _ . . . . . . . . . . . . . . . 196 . . . . . . _ * . . . * . . . . . 1 -

Reactivity of chlorine atoms in aqueous solution Part 1The equilibrium ClMNsbd+Cl-Cl2-

The equilibrium constant for reaction (1) has been determined in neutral solution to be (1.4±0.1)×105 d mol-1, with forward and reverse rate constants, k1 and k-1, of (8.5±0.7)×109 d mol-1 s-1 and (6.0±0.5)×104 s-1, respectively. At 25°C the rate constants for the reactions of ClNsbd and Cl2- were measured to be (2.5±0.3)×105 s-1 and (1.3±0.1)×103 s-1 with water and (6.5±0.6)×108 d mol-1 s-1 and ca. 0, within experimental error, with 2-methyl-propan-2-ol. Deviation from the equilibrium values of [ClNsbd] and [Cl2-] in the early stages of the reactions was investigated and shown to account for the discrepancy between the value of K1 determined here and a previous estimate from our laboratory.

Reaction of SO4- withFe2+, Mn2+ and Cu2+ inaqueous solution

The oxidation of Fe 2+ , Mn 2+ and Cu 2+ to the corresponding trivalent ions by SO 4 - has been studied in aqueous solution at pH 3–5 using pulse radiolysis to generate SO 4 - . For Fe 2+ the reaction has a negative energy of activation of -(18±2) kJ mol -1 at low ionic strength, and k obs shows a very small dependence on ionic strength, indicating that a precursor complex, (Fe II SO 4 - ) + , is kinetically significant. The stability constant, K a , of the complex is estimated to be 5.3 dm3 mol -1 at 298 K. The observed rate is first order in [Fe 2+ ] and the overall bimolecular rate constant, at ca. 20°C and an ionic strength of 0.06 mol dm -3 , is (4.6±0.2)×10 9 dm3 mol -1 s -1 . By applying the steady-state approximation to (Fe II SO 4 - ) + , a value of 1.1×10 9 s -1 is obtained for the rate constant for the electron-transfer step. Reaction of SO 4 - with Mn 2+ is also first order in [Mn 2+ ] with a bimolecular rate constant of 1.4×10 7 dm3 mol -1 s -1 , at an ionic strength of 0.065 mol dm -3 at 20°C, and an activation energy of (34±2) kJ mol -1 . The rate constant for electron transfer is obtained as 2.6×10 6 dm3 mol -1 s -1 . For Mn 2+ , like Fe 2+ , k obs shows a small ionic strength dependence consistent with that expected for the formation of an outer-sphere ion-pair complex. Treatment of the data according to the classical Marcus theory for electron transfer yields ΔH°=-277 kJ mol -1 and -54 kJ mol -1 for the reaction of SO 4 - with Fe 2+ and Mn 2+ , respectively. For Cu 2+ , the rate of decay of SO 4 - was independent of [Cu II ] and was largely accounted for by its reaction with Bu t OH which was present to scavenge OH. No rate constant for the oxidation step could be determined; that some oxidation did occur is deduced from spectral changes assigned to the formation of a Cu III species.