G. Arthur Salmon

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.

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.

The reactivity of chlorine atoms in aqueous solution. Part III. The reactions of Cl• with solutes

Laser flash photolysis of chloroacetone was used to measure the rate constants and activation energies for the reactions of the Cl• atom with a number of oxygen-containing compounds and inorganic anions in aqueous solution. For the organic compounds there is a strong correlation at 25°C between k(Cl•+RH) and k(•OH+RH), where RH is CH3OH, CH3CH2OH, CH3CH(OH)CH3, (CH3)3COH, HCHO, CH3CHO, CH3CO2H, HCO2H and HCO2−, respectively. For CH3CO2−, k(Cl•+RH)k(•OH+RH), and for CH3COCH3 and CH3COCH2Cl, k(Cl•+RH)k(•OH+RH). Possible reasons for these differences are discussed in terms of preferential attack by Cl• at O–H groups in the neutral molecules, rather than H-abstraction from C–H as with •OH, and electron transfer for the reactions of Cl• with the anions. For the inorganic anions X=OCN−, NO3−, SO42−, ClO3−, SCN−, HCO3−, N3−, NO2−, HSO3, k(Cl•+X) ranges from 1.0×108 (NO3−) to 5.3×109 dm3 mol−1 s−1 (SCN−) but there is no strong correlation between k and the reduction potential of X. Comparison of the reactivity of Cl• with reported rate constants for the reactions of Cl2•− indicates that, in many cases, these rate constants are largely accounted for by the fraction of Cl• present in equilibrium with Cl2•−. The implications of these results for atmospheric chemistry are discussed.

Oxidation of glyoxal initiated by OH inoxygenated aqueous solution

The kinetics and mechanism of the oxidation of glyoxal, which is a constituent of cloud water, initiated by OH in oxygenated solution have been investigated using pulse radiolysis with optical and conductivity detection of the transient species, and steady-state radiolysis with spectrophotometric and ion chromatographic analysis of the permanent products. The data obtained are consistent with glyoxal being present in the form of the dihydrate [CH(OH) 2 ] 2 which is oxidised to glyoxylic acid (pK a =3.4) and hydrogen peroxide via a peroxyl radical O 2 C(OH) 2 CH(OH) 2 that splits off HO 2 in a non-rate determining step. The following rate constants have been determined: k{ OH+[CH(OH) 2 ] 2 }=(1.10±0.04)×10 9 dm3 mol -1 s -1 and k[ C(OH) 2 CH(OH) 2 +O 2 ]=(1.38±0.11)×10 9 dm3 mol -1 s -1 . It is concluded that oxidation of glyoxal by OH in cloud water can proceed by a chain reaction involving H 2 O 2 .

Dissociative electron transfer between arene radical anions and halogenoalkanes: a pulse radiolysis study

Rate constants have been determined at room temperature (20 ± 1 °C) for a number of examples of the dissociative electron-transfer reaction between arene radical anions and the haloalkanes 1-iodobutane, 2-iodo-2-methylpropane and 2-bromo-2-methylpropane in the organic amide solvents N-methylpyrollidin-2-one N,N′-dimethylformamide. The data, when combined with the results of other workers using electrochemical techniques, indicate a linear relationship between log(kET) and the reduction potential of the donor radical anion (E°D) spanning eight orders-of-magnitude of kET up to the diffusion-controlled limit. The effect of temperature on kET was investigated for the reactions of 1-iodobutane with a number of the radical anions.The factors which affect the rates of these reactions and the success of current theory in explaining them are discussed.

Electrons trapped in ethanol-water glasses at ultra-low temperatures

Abstract The trapping of electrons in ethanol-water glasses has been studied by pulse radiolysis at temperatures from 6 to 76 K and by γ-radiolysis from 4.2 to 80 K. Addition of water, or increase in temperature, causes the visible absorption band of the spectrum of trapped electrons to increase relative to the IR band. Closer examination of these bands showed that they behave independently in the temperature range 4.2–50 K. It is concluded that both temperature and water content influence the distribution of electrons between pre-existing traps. The distribution of trap depths is non-random and comprises two sets, i.e. shallow (hydrocarbon) and deep (hydroxyl). G ϵ values measured in γ-radiolysis experiments at 4.2 K are much lower than those measured by pulse radiolysis at 6 K. This difference is probably caused by slow charge recombination by tunnelling which involves e - t mainly from the shallow traps.

Evaluation of the stability constant of Cl2˙– in neutral aqueous solution

The stability constant (K1) of Cl2˙– has been measured in neutral aqueous solution using pulse radiolysis to generate Cl˙ by reaction of SO4˙– with Cl–. K1 is found to be (4.7 ± 0.4)× 103 dm3 mol–1, compared with the literature value of 1.9 × 105 dm3 mol–1. This low value of K1 makes it possible to measure the rates of reaction of Cl˙ using pulse radiolysis. The technique is exemplified for the reaction Cl˙+ HCO2– for which k=(1.3 ± 0.4)× 108 dm3 mol–1 s–1 has been obtained.

Fast decay of the visible band electron in e-irradiated crystalline ice at low temperature: The isotope effects and the role of a mobile proton in the decay

The trapped electron which absorbs in the visible region in crystalline ice e−vis has been studied by pulse radiolysis in the low temperature range 6 to 77 K using 2 μs pulses, and above 77 K using pulses of 40 ps to 6 ns width. The half‐life of e−vis in ice around 77 K is unusually short, 8 ns in H2O ice and 120 ns in D2O ice. The decay of e−vis in ice is found to fit Hummel’s empirical equation for the decay by geminate ion recombination in a spur. Several other pieces of evidence indicate that the decay occurs in a spur. Electron tunneling from e−vis to the OD radical does not occur in D2O ice. It is concluded that a proton (H3O+) or a deuteron (D3O+) produced in a spur by the irradiation migrates through ice to react with immobile e−vis in the same spur, and that the half‐life of e−vis is determined by the mobility of the proton or deuteron. The Arrhenius plot of the half‐life in the range 100 to 6 K is nonlinear and shows an activation energy of 20 meV at higher temperatures and much smaller values a...

Some studies on the formation of excited states of aromatic solutes in hydrocarbons and other solvents

Abstract This paper reviews the work of the author and his co-workers on the radiation-induced formation of excited states of aromatic compounds in solution. The experimental methods used are surveyed and in particular the method of measuring the yields of triplet and singlet excited states of the solute are described. The problems discussed are: (1) the effect of solvent on the yields of excited states, (2) formation of excited states in cyclohexane and other alicyclic hydrocarbons, (3) the formation of excited states in benzene and (4) the identification of T - T absorption spectra.