Correlation of singlet energies of aromatic hydrocarbons with the rates of protonation of their anion radicals

Formation and protonation of aromatic anion radicals in 2-propanol were studied by kinetic spectrophotometric pulse radiolysis. All polycyclic hydrocarbons studied were found to react very rapidly with e^?_solv. Those with relatively high electron affinity were also reduced by (CH₃)₂CO^?. The anion radicals formed undergo protonation by direct reaction with the alcohol molecule. The rate constants for this protonation vary from ≈ 6 × 10⁵ s^?1 for cis-stilbene and naphthalene down to 20 s^?1 for perylene. The variations in rates are discussed in terms of changes in singlet energy separation (ΔE_{S1 ← S0}). The logarithm of the protonation rate constant for alternant hydrocarbons is linearly dependent on ΔE_{S1 ← S0}.     

Rate constants for hydrogen abstraction reactions of the sulfate radical,SO4−. Alkanes and ethers

Rate constants have been determined for the reactions of SO₄^? with a series of alkanes and ethers. The SO₄^? radical was produced by the laser-flash photolysis of persulfate, S₂O₈^2?. For methane, only an upper limit of 1 × 10⁶ M^?1 s^?1 could be determined. For ethane, propane, and 2-methylpropane, rate constants of 0.44, 4.0, and 10.5 × 10⁷ M^?1 s^?1 were found. For ethyl and n-propyl ether, rate constants of 1.3 × 10⁸ and 2.2 × 10⁸ M^?1 s^?1 were found and for 1,4-dioxane and tetrahydrofuran, rate constants of 7.2 × 10⁷ and 2.8 × 10⁸ were obtained. The reaction of SO₄^? with allyl alcohol was also studied and found to have a rate constant of 1.4 × 10⁹ M^?1 s^?1.     

Degradation of n-butylparaben and 4-tert-octylphenol in H2O2/UV system

The degradation of two endocrine disrupting compounds: n-butylparaben (BP) and 4-tert-octylphenol (OP) in the H₂O₂/UV system was studied. The effect of operating variables: initial hydrogen peroxide concentration, initial substrate concentration, pH of the reaction solution and photon fluency rate of radiation at 254 nm on reaction rate was investigated. The influence of hydroxyl radical scavengers, humic acid and nitrate anion on reaction course was also studied. A very weak scavenging effect during BP degradation was observed indicating reactions different from hydroxyl radical oxidation. The second-order rate constants of BP and OP with OH radicals were estimated to be 4.8×10⁹ and 4.2×10⁹ M^?1 s^?1, respectively. For BP the rate constant equal to 2.0×10¹⁰ M^?1 s^?1was also determined using water radiolysis as a source of hydroxyl radicals.     

Kinetic study of the reaction of meso-tetrakis (p-trimethylammoniumphenyl)porphinatodiaquorhodate(II) with chloride,bromide, iodide,hexacyanocobaltate(III) and thiocyanate ions in aqueous solution

The reaction of Cl^?, Br^?, I^?, Co(CN)₆^3? and NCS^? with meso-tetrakis (p-trimethylammoniumphenyl)porphinatodiaquorhodate(III), [RhTAPP(H₂O)₂]⁵⁺, has been studied at 15, 25 and 35°C in 0.1 M [H⁺] with μ = 1.00 M (NaNO₃). The value of the acidity constant, K_al, at 25°C is 4.39 × 10^?9 M. The reactions are first order in anion concentration up to 0.9 M. The values of the stability constants, K₁, and the second order rate constants, k₁, for the reaction with Cl^?, Br^?, I^?, Co(CN)₆^3? and NCS^? are respectively 0.23 M^?1 and 2.5 × 10^?3 M^?1 s^?1, 1.1 M^?1 and 6.92 × 10^?3 M^?1 s^?1, 40.0 M^?1 and 17.0 × 10^?3 M^?1 s^?1, 550 M^?1 and 20.0 × 10^?3 M^?1 s^?1, 3400 M^?1 and 20.9 × 10^?3 M^?1 s^?1. The porphine greatly labilizes the Rh(III). There has been about a 500-fold increase in the rate constant for substitution compared to that of [Rh(NH₃)₅H₂O]³⁺. The substitution rates are however about the same as for [Rh(TPPS)(H₂O)₂]^3?, indicating that the overall charge on the complex plays only a minor role. The kinetic results indicate that dissociative activation is occurring in these reactions.     

One-electron oxidation and demetalation of metalloporphyrins and chlorophyll a in dichloroethane solutions as studied by pulse radiolysis

Formation of cation radicals by pulse radiolysis of metalloporphyrins and chlorophyll a in 1,2-dichloroethane is reported Demetalation of the metalloporphyrin by radiolytically produced HC1 is also observed. Rate constants for demetalation of ZnTPP and Chl a are 1 1 × 10⁸ and ≈ 3 × 10⁸ M^?1 S^?1. Oxidation of Chl a by ZnTPP⁺ has a rate constant of ≈ 4× 10⁹ M^?1S^?1.     

Inorganic reactions of iodine(III) in acidic solutions and free energy of iodous acid formation

An analysis of the former works devoted to the reactions of I(III) in acidic nonbuffered solutions gives new thermodynamic and kinetic information. At low iodide concentrations, the rate law of the reaction IO + I^? + 2H⁺ ? IO₂H + IOH is k_+B [IO][I^?][H⁺]² – k_?B [IO₂H][IOH] with k_+B = 4.5 × 10³ M^?3s^?1 and k_?B = 240 M^?1s^?1 at 25°C and zero ionic strength. The rate law of the reaction IO₂H + I^? + H⁺ ? 2IOH is k_+C [IO₂H][I^?][H⁺] – k_?C [IOH]² with k_+C = 1.9 × 10¹⁰ M^?2s^?1 and k_?C = 25 M^?1s^?1. These values lead to a Gibbs free energy of IO₂H formation of ?95 kJ mol^?1. The pK_a of iodous acid should be about 6, leading to a Gibbs free energy of IO formation of about ?61 kJ mol^?1. Estimations of the four rate constants at 50°C give, respectively, 1.2 × 10⁴ M^?3s^?1, 590 M^?1s^?1, 2 × 10⁹ M^?2s^?1, and 20 M^?1 s^?1. Mechanisms of these reactions involving the protonation IO₂H + H⁺ ? IO₂H and an explanation of the decrease of the last two rate constants when the temperature increases, are proposed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 647–652, 2008     

Magnesium chloride supported high mileage catalysts for olefin (decene-l) polymerization XIV. Propagation and chain transfer

Decene-l was polymerized with the MgCl₂/ethylebenzoate/p-cresol/AIEt₃/TiCl₄-AlEt₃/methyl-p-toluate catalyst at 50° using an A/T ratio of 167 and a range of monomer concentration. The concentration of the two kinds of active sites are [Ti] = 12% and [Ti] = 4% of the total titanium. The rate constants of propagation are 24 M^?1 s^?1. Chain transfers to AIEt₃, monomer, and by β-hydride elimination have rate constant values of 1.7 × 10^?3 M^?1 s^?1, 1.34 × 10^?2 M^?1 s^?1, and 1.7 × 10^?2 s^?1, respectively. Poly(decene-l) have relatively narrow MW which are unchanged during the course of a polymerization. Therefore, the active site concentrations in the CW catalyst for propylene and decene polymerization are identical and their rate constant values agree within a factor of 2. However, the rate of decene polymerization depends on fractional order of monomer concentration and decreases with the increase of activator concentration. Furthermore, the formation of metal polymer bonds has a rate independent of these concentrations. These kinetic behaviors are a manifestation of absorption processes of these species which are not seen in propylene polymerizations.     

Rate constants for the OH reactions with oxygenated organic compounds in aqueous solution

The kinetics of the oxidation of functionalized organic compounds of atmospheric relevance by the hydroxyl radical (OH) was measured in the aqueous phase. Competition kinetics, using the thiocyanate anion (SCN^?) as competitor, was applied using both a laser flash photolysis long path absorption (LP‐LPA) setup and a Teflon AF waveguide photolysis (WP) system. Both experiments were intercompared and validated by measuring the rate coefficients for the reaction between OH and acetone where values of k₁ = (1.8 ± 0.4) × 10⁸ M^?1 s^?1 were obtained with the WP system, which agrees very well with the rate constant of k₁ = (2.1 ± 0.6) × 10⁸ M^?1 s^?1 determined before by LP‐LPLA [1]. The following temperature dependencies of the rate constants (M^?1 s^?1) for the reactions of OH with ketones, dicarboxylic acids, and unsaturated compounds were obtained in Arrhenius' form: Reaction of OH with: methylisobutyl ketone (MIBK): k₂ = (1.0 ± 0.1) × 10¹²

     

Photocurrent kinetics during electron emission from metal into electrolyte solution: Part III. H3O+ solutions

The photocurrent kinetics in acid solutions have been investigated. The diffusion coefficients of atoms H?((7±2)×10^?5cm²s^?1) and D?((4±1)×10^?5cm²s^?1) and OH^? and OD^? radicals ((1±0.3)×10^?5cm²s^?1) are found. The rate constants of capture of solvated electrons by H₃O⁺ and D₃O⁺ ions are identical and equal to (8±1)×10⁹M^?1s^?1. From the shape of the kinetic curves it follows that electrochemical desorption of atomic hydrogen occurs from the adsorbed state. The rate constant of this process has been measured. It is shown that the rate constant of electrochemical desorption depends only slightly on the potential.     

Rates of addition of tert-butyl and pivaloyl radicals to acrylonitrile measured by modulated ESR and μSR spectroscopy

For the rate constant of addition of tert-butyl radicals to acrylonitrile at T = 300 K in solution modulated ESR spectroscopy and muon spin rotation yield 10⁶ M^?1 s^?1 and 2.4 × 10⁶ M^?1 s^?1. The addition of pivaloyl radical to acrylonitrile proceeds with Arrhenius parameters log A/M^?1 s^?1 = 7.7 and E_a = 11.5 kJ/ mol. The results are discussed in terms of polar effects in radical addition reactions.     

Pulse radiolysis study of the formation and the reactivity of baicalin radical anion,and in comparison with rutin,quercetin and acyrlate ester radical anions in ethanol

The reaction of solvated electrons with baicalin in N₂-saturated ethanol has been studied by pulse radiolysis. The results show that a solvated electron can add to baicalin and generate a baicalin radical anion with a maximum UV absorbance peak at 360 nm. Its molar extinction coefficient at this wavelength is 1.3×10⁴ M⁻¹ cm⁻¹. The rate constant for the build-up of the baicalin radical anion is 1.3(±0.4)×10¹⁰ M⁻¹ s⁻¹. Decay of the radical anion is induced by a proton transfer reaction and a recombination reaction, which involves a pseudo-first-order reaction with rate constant 2.6(±0.4)×10³ s⁻¹ and a second-order reaction with rate constant 1.3(±0.2)×10⁹ M⁻¹ s⁻¹. The effect of acetaldehyde on the decay of the baicalin radical anion was also investigated. Electron transfer between the baicalin radical anion and acetaldehyde was not observed, probably due to the low rate of electron transfer between the baicalin radical anion and acetaldehyde. Reactivity of the rutin, quercetin, baicalin and ethyl acrylate radical anions are also compared.     

Bimolecular self-reactions of ketyl radicals of acetophenone and diphenylaminyl radicals

The rate constants of self-reactions of ketyl radicals of acetophenone in n-heptane [2k = (3.2 ± 0.5) × 10⁹ M^?1 s^?1] and diphenylaminyl radicals in toluene [2k = (3.3 ± 0.5) × 10⁷ M^?1 s^?1] have been determined at 298 K using the flash photolysis technique. The rate constant of ketyl radicals is equal to the calculated diffusion constant and, therefore, this reaction is diffusion-controlled. The aminyl radical recombination rate is independent of the viscosity of the toluene/vaseline oil binary mixture (0.55 ? η ? 12 cP) and this reaction is activation-controlled. Reactivity anisotropy averaging due to the cage effect has been considered for ketyl and some other radicals. On the basis of the analysis it has been proposed that ketyl recombination involves formation of not only pinacol, but also iso-pinacols.     

Through bond mechanism versus exciplex formation in the photochemistry of fullerene / ferrocene donor-bridge-acceptor dyads

A systematic fluorescence and flash photolytic investigation of a series of covalently linked fullerene / ferrocene based donor-bridge-acceptor dyads is reported as a function of the nature of the bridge between the donor site and acceptor site. The fluorescence of the investigated dyads 2 (Φ_rel = 0.17 × 10^?4, 3 (Φ_rel = 0.78 × 10^?4), 4 (Φ_rel = 1.5 × 10^?4), 5 (Φ_rel = 0.7 × 10^?4), and 6 (Φ_rel = 2.9 × 10^?4) were substantially quenched, relative to N-methyl fulleropyrrolidine (1) (Φ_rel = 6.0 × 10^?4). Photolysis of N-methyl fulleropyrrolidine (1) in toluene revealed formation of the excited singlet state which was followed by a rapid intersystem crossing to the excited triplet state. On the other hand, the fate of the excited singlet state of 2, 3, 4, 5, and 6 was found to be governed by rapid intramolecular quenching, with rate constants of 28×10⁹ s^?1, 6.9×10⁹ s^?1, and 3.4×10⁹ s^?1, 14×10⁹ s^?1, 2.3×10⁹ s^?1 respectively. The electron transfer process and the charge separation were confirmed by monitoring the characteristic π-radical anion bands at λ_max = 400 and 1055 nm in degassed benzonitrile with τ_1/2 = 1.8 μs (3) and 2.5 μs (4).     

FTIR study of the reaction of ethyl radicals with O2 and Cl2 in air at 295 K

Using Fourier transform infrared spectroscopy, the ethene yield from the reaction of C₂H₅ radicals with O₂ has been determined to be 1.50 ± 0.09%, 0.85 ± 0.11%, and <0.1% at total pressures of 25, 50, and 700 torr, respectively. Additionally, the rate constant of the reaction of C₂H₅ radicals with molecular chlorine was measured relative to that with molecular oxygen. (1) A ratio k₆/k₇ = 1.99 ± 0.14 was measured at 700 torr total pressure which, together with the literature value of k₇ = 4.4 × 10^?12 cm³ molecule^?1s^?1, yields k₆ = (8.8 ± 0.6) × 10^?12 cm³ molecule^?1s^?1. Quoted errors represent 2σ. These results are discussed with respect to previous kinetic and mechanistic studies of C₂H₅ radicals.     

Gas-phase reactions of azulene with OH and NO3 radicals and O3 at 298 ± 2 K

Azulene, which is isomeric with naphthalene, was studied to determine whether it behaves like a polycyclic aromatic hydrocarbon or an alkene in its gas-phase reactions with OH and NO₃ radicals and O₃. Using relative rate methods, rate constants for the reactions of azulene with OH and NO₃ radicals and O₃ of (2.73 ± 0.56) × 10^?10 cm³ molecule^?1 s^?1, (3.9) × 10^?10 cm³ molecule^?1 s^?1, and <7 × 10^?17 cm³ molecule^?1 s^?1, respectively, were obtained at 298 ± 2 K. The observation that the NO₃ radical reaction did not involve NO₂ in the rate determining step indicates that azulene behaves more like an alkene than a polycyclic aromatic hydrocarbon in this reaction. No conclusive evidence for the formation of nitroazulene(s) from either the OH or NO₃ radical-initiated reaction of azulene (in the presence of NO_x) was obtained.     

The rate constants for the reaction of the hydroxyl radical with methyl,n-propyl,and n-butyl nitrites

The kinetics of the reaction of OH radicals with methyl, n-propyl, and n-butyl nitrite have been studied in a discharge flow system under pseudo first-order conditions. The OH radicals were generated by the reaction of H atoms with NO₂ and the concentration of OH; monitored by resonance fluorescence, was followed as a function of time in an excess of each nitrite. Values of k(CH₃ONO) = (0.6 ± 0.09) × 10⁹ dm³ mol^?1 s^?1 k(n – C₃H₇ONO) = (1.39 ± 0.20) × 10⁹ dm³ mol^?1 s^?1, and k(n – C₄H₉ONO) = (2.89 ± 0.43) × 10⁹ dm³ mol^?1 s^?1 at 295 K were obtained. These results agree with previous relative rate measurements from this laboratory but the value for k (CH₃ONO) is a factor of 7 greater than the value obtained by relative rate measurements elsewhere using a different OH source.     

On the mechanism of reaction of tert-butoxyl radicals with dialkylphosphites

It has been shown by ESR spectroscopy that the title reaction involves abstraction of hydrogen from the phosphite, since at ?10°C the reaction has a kinetic deuterium isotope effect, k_H/k_D, or ～3. The rate constant for hydrogen abstraction is c. 2 × 10⁴ M^?1 s^?1. There is no significant addition of alkoxyl radicals to the phosphite.     

Use of the Rotated Ring Disc Electrode for the Analytical Determination of Dithiaalcanediols in Aquo-Alcoholic Media and Its Rate of Oxidation with Bromine

Abstract

We used bromine generated and detected on the ring-dise electrode for the titration of aquo-alcoholic solution of dithiaalcanediol. The analysis of the ring current vs disc current curve provided a means of determination of the rate constant k of the second order homogeneous and fast reaction of bromine with each of the two sulfide groups of dithiaalcanediol. A value of (3+1)× 10^?6M^?1 _s ^?1 was found for k at 25°C.     

Free-radical chain oxidation of 1,4-dioxane and styrene in the presence of fullerene C60

The antiradical activity of fullerene C₆₀ was studied for the oxidation of 1,4-dioxane and styrene initiated by azobisisobutyronitrile and benzoyl peroxide as model reactions. The effective rate constants of the reaction of peroxyl radicals with fullerene C₆₀ (k ₇) and the stoichiometric inhibition factor (f _eff) were determined in air ( $P_{O_2 }$ = 0.21 atm) and oxygen ( $P_{O_2 }$ = 1.0 atm). The rate of the liquid-phase oxidation of 1,4-dioxane does not depend on $P_{O_2 }$ , and the effective rate constant of inhibition is k ₇ = (2.4 ± 0.2) × 10⁴ L mol^?1 s^?1. Chain termination in the oxidation of styrene occurs when C₆₀ reacts with both the peroxyl radicals (k ₇ = (1.2 ± 0.1) × 10³ L mol^?1 s^?1) and alkyl (k ₈ = 1.07 × 10⁷ L mol^?1 s^?1) radicals.     

The flash photolysis of aqueous solutions of rhodizonic and croconic acids

The flash photolysis of aqueous solutions of rhodizonic and croconic acids has been studied in the presence and absence of electron acceptors. No transient absorption which could be identified with an excited state was observed with either anion. The rate of recovery of the ground state in the absence of additives was a first-order process with both acids and gave rate constants for deactivation of the excited state, k_D, of 2.4 × 10⁵ s^?1 for rhodizonate and 2.8 × 10⁵ s^?1 for croconate. With croconate dianion in the presence of three acceptors, 4-nitrobenzylbromide, methylviologen, and biacetyl, a transient absorption was detected, with a maximum absorbance at 500 nm, and was tentatively identified with the monoanion radical, formed following electron transfer to the acceptor. From the rate of growth of the transient, rate constants for the rate of electron transfer to the acceptor were measured as follows: 4-nitrobenzylbromide: 2.8 × 10⁹ M^?1 s^?1; methyl viologen: 3.7 × 10¹⁰ M^?1 s^?1; and biacetyl: 2.0 × 10⁸ M^?1 s^?1. The significance of the measurements is discussed in relation to the mechanism proposed for the photochemical reactions of these dianions. © 1994 John Wiley & Sons, Inc.