SENSITIZED TRIPLET FORMATION OF CHLOROPHYLL-A AND ß-CAROTENE

The naphthalene-sensitized formation of triplet excited chlorophyll-a (Chl-a) and all-transß-carotene has been studied by pulse radiolysis. The rate constants for transfer of triplet energy from naphthalene to Chl-a and all-transß-carotene in benzene at 25°C are (3.6 ± 0.6)·10⁹M^-1 s^-1 and (10.7 ± 1.2)·10⁹M^-1 s^-1, respectively. The decays of the excited triplet states of naphthalene, Chl-a and all-transß-carotene all follow a mixed first-and second-order mechanism. The first-order rate constant for triplet decay is strongly dose dependent for naphthalene but only slightly dependent and independent of dose for Chl-a and all-transß-carotene, respectively. The rate constants for triplet-triplet annihilation are (1.4 ± 0.3)·10⁹M^-1 s^-1 for Chl-a and (3.6 ± 0.4)·10⁹M^-1 s^-1 for all-transß carotene. The nearly constant ratio k(ß-carotene)/k(Chl-a) for the bimolecular triplet energy transfer rate constants is discussed in terms of the molecular shapes of the two molecules. The energetics of the triplet-triplet annihilation of all-transß-carotene are discussed, and it is proposed that production of the excited ¹A_B state may be a major route in the annihilation process.     

Mechanistic studies of the sensitized photoreduction of bis(acetylacetonato)copper(II) by phenylalkyl ketones

Bis(acetylacetonato)copper(II) (Cu(acac)₂) interacts with both the triplet excited state and the triplet biradical of phenylalkyl ketones which undergo the Norrish type II reaction. Mechanistic studies by static quenching methods show that the triplet biradicals interact with the paramagnetic copper species, leading to the preferential formation of cyclobutanols without the formation of new products; in the presence of Ph₃P the former interaction causes the known reduction of Cu(acac)₂) to Cu(acac)(Ph₃P)₂, with a rate constant of about 6 × 10⁹ M⁻¹ s⁻¹. It is shown that Ph₃P interacts with one reactive intermediate, the triplet excited state ketone. The results of extensive kinetic analysis strongly support the proposed reaction mechanism.     

The fluorescence quenching of 1,4-bis[2-(5-phenyl-oxazolyl)]-benzene by bromomethanes

The interaction between the ground and excited states of 1,4-bis[2-(5-phenyloxazolyl)]-benzene and bromomethanes such as CBr₄, CHBr₃ and CH₂Br₂ were investigated in benzene. Distinct complex formation was not observed either in the ground state or in the excited states. The excited singlet and triplet states are deactivated by these bromomethanes. The triplet yield is increased on the addition of CHBr₃ or CH₂Br₂, whereas it is decreased on the addition of CBr₄. The fluorescence quenching rate constants k_q at 23 °C were determined to be 1.6 × 10¹⁰ M⁻¹ s⁻¹, 3.6 × 10⁸M⁻¹s⁻¹ and 2.4 × 10⁷M⁻¹s⁻¹ for CBr₄, CHBr₃ and CH₂Br₂ respectively. The rate constants k_ST′ of the enhanced intersystem crossing associated with the fluorescence quenching were evaluated from emission—absorption flash photolysis experiments as 3.0 × 10⁸ M⁻¹s⁻¹, 1.9 × 10⁸ M⁻¹s⁻¹ and 5.1 × 10⁷ M⁻¹s⁻¹ for CBr₄, CHBr₃ and CH₂Br₂ respectively. k_ST′ increases with increasing number of bromine atoms contained in the quencher, so that the enhanced intersystem crossing is due to the external heavy-atom effect of the quencher. The apparent triplet yield for the quenching system depends not only on k_ST′ but also on the rates of the other non-radiative processes. This is the reason why the apparent triplet yield does not necessarily increase on fluorescence quenching by bromomethanes.     

The kinetics of complexation of nickel(II) and cobalt(II) with cinchomeronate in aqueous solution

The pressure-jump method has been used to determine the rate constants for the formation and dissociation of nickel(II) and cobalt(II) complexes with cinchomeronate in aqueous solution at zero ionic strength. The forward and reverse rate constants obtained are k_f = 2.27 × 10⁶ M^?1 s^?1 and k_r = 3.81 × 10¹ s^?1 for the nickel(II) complex and k_f = 1.23 × 10⁷ M^?1 s^?1 and k_r = 2.66 × 10² s^?1 for the cobalt(II) complex at 25°C. The activation parameters of the reactions have also been obtained from the temperature variation study. The results indicate that the rate determining step of the reaction is a loss of a water molecule from the inner coordination sphere of the cation for the nickel(II) complex and the chelate ring closure for the cobalt(II) complex. The influence of the pyridine ring nitrogen atom of the cinchomeronate ligand on the complexation of cobalt(II) ion is also discussed.     

Spectroscopic characterization of 2-naphthylcarbene

The 308 nm excimer laser flash photolysis of 2-naphthyldiazomethane produces triplet 2-naphthylcarbene (λ_max = 362 nm) which decays with the observed pseudo-first-order rate constants (k_exptl) of 5.54 ± 0.03 × 10⁶; 3.33 ± 0.4 × 10⁶; 1.64±0.02 × 10⁷; and 3.05±0.4 × 10⁶ s^-1 in n-pentane, 2,2,4-trimethylpentane (2,2,4-TMP), benzene and Freon 113 respectively. In hydrocarbon solvents the observed decay of triplet 2-naphthylcarbene is correlated with the pseudo-first-order growth of the 2-naphthylmethyl radical (λ_max = 378 nm). Direct kinetic measurements of the reaction of triplet 2-naphthylcarbene in 2,2,4-TMP with cyclohexane, styrene, methanol and carbon tetrachloride yielded bimolecular quenching rate constants of 1.48 ± 0.04 × 10⁶;4.33 ± 0.1 × 10⁷;7.25 ± 0.5 × 10⁶; and 3.35 ± 0.07 × 10⁶M^-1S^-1. It is also found that 2-naphthylcarbene reacts with acetonitrile (k_q = 5.28 ± 0.1 × 10⁵ M^-1 s^-1) to form a nitrile ylide intermediate with a λ_max = 372 nm. These results are interpreted in terms of a rapid singlet-triplet 2-naphthylcarbene equilibrium.     

Kinetic studies of a catalyzed metalloporphyrin formation

Kinetics of the incorporation of mercury(II) ion in tetra (p-trimethylammoniumphenyl)porphine have been investigated in aqueous solution at 30.0°C and 0.2 M (NaNO₃) ionic strength. The reaction was found to be first order each in mercury(II) and the porphyrin. The forward (formation) and the reverse (dissociation) rate constants were found to be 1.9 ± 0.2 × 10³ M^?1 s^?1 and 7 ± 2 × 10⁶ M^?1 s^?1, respectively. Kinetics of zinc(II) incorporation in tetra(p-trimethylammoniumphenyl)porphine catalyzed by mercury(II) were also investigated. This catalysis is explained in terms of steady-state formation of mono mercury(II) porphyrin followed by zinc(II) displacement of mercury(II) ion from the porphyrin. Such a mechanism also illustrates the importance of porphyrin core deformation to metal incorporation.     

THE PHOTOPHYSICS OF BONELLIN: A CHLORIN FOUND IN MARINE ANIMALS

The photophysical properties of bonellin, a free-base chlorin, were studied in ethanolic solution. For the singlet excited state the following data were determined: an energy level, E^B_S= 187 ± 2kJ mol^-1, a lifetime, τ_f= 6.3± 0.1ns at 298 K, and fluorescence quantum yields, φ_r= 0.07 ± 0.02 (298 K) and 0.20 ± 0.04 (77 K). The S₁→ T intersystem crossing quantum yield was φ_isc= 0.85 ± 0.1. No phosphorescence was observed at 298 K and 77 K. Based on quenching experiments the triplet state energy level was determined to be E^B_T= 180 ± 20 kJ mol^-1. A unimolecular decay rate constant, k₁= (2.3 ± 0.5)· 10³ s^-1 at room temperature, and a molar absorption coefficient, ε^T₄₄₃= 9500 ± 500 M^-1 cm^-1, were obtained for the triplet state. This species was quenched by O₂ with ko₂= (1.7 ±0.3)· 10⁸M^-1 s^-1, and by benzoquinone with k_q= (5.2 ± 0.3)-10⁹M^-1 s^-1. The latter value, as well as the high value determined for the triplet annihilation rate constant, k₂= (2 ± 0.5)· 10⁹M^-1 s^-1, might reflect an electron transfer mechanism. Copper bonellin had a shorter triplet lifetime (>20 ns), which offers a possible explanation for its lack of photodynamic action.     

Efficiency and excited state processes in a three-component system,based on thioxanthene derived dye/amine/additive,usable in photopolymer plates

The excited state interactions occurring when a three-component system of thioxanthene derived dye TXD/amine/additive (diphenyliodonium salt, CBr₄, benzoyl peroxide, cumene hydroperoxide) is subjected to sensitization processes in the visible range, were investigated through time-resolved absorption spectroscopy, spectrofluorometry, and photolysis. The rate constants of the various operative processes were measured together with the values of the fluorescence quantum yields (e.g. ϕ _f = 0.75 ± 0.07 in methanol) and the lifetimes of the singlet excited state of the dye (e.g. 6 ns in methanol). Singlet state quenching by methyldiethanolamine (MDEA) occurs with a rate constant k = 10⁹ M⁻¹ s⁻¹ in methanol. The reactivity of the triplet excited state of the dye is very low (k = 5.6 × 10⁴ M⁻¹s⁻¹ for the TXD/MDEA interaction). The ketyl radicals that arise from the interaction of the singlet state of the dye with the amine, are quenched by such additives as CBr₄ (k = 6.7 × 10⁵M⁻¹s⁻¹), or the onium salts (k = 5.7 × 10⁵M⁻¹ s⁻¹). The calculations of the yields of interaction of the singlet state of the dye with the two components of the system demonstrate that the process of quenching by the amine is the main one (ϕ_int = 0.5) compared to that by, e.g., an onium salt (ϕ_int = 0.07). Sensitivity of 0.3 mJ cm⁻² obtained in a laser scanning system is also reported. © 1996 John Wiley & Sons, Inc.     

MECHANISMS OF QUENCHING OF THE FLUORESCENCE OF A BENZO[a]PYRENE TETRAOL METABOLITE MODEL COMPOUND BY 2'-DEOXYNUCLEOSIDES

Abstract— The hydrophobic interactions of bulky polycyclic aromatic hydrocarbons with nucleic acid bases and the formation of noncovalent complexes with DNA are important in the expressions of the mutagenic and carcinogenic potentials of this class of compounds. The fluorescence of the polycyclic aromatic residues can be employed as a probe of these interactions. In this work, the interactions of the (+)-trans stereoisomer of the tetraol 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (BPT), a hydrolysis product of a highly mutagenic and carcinogenic diol epoxide derivative of benzo[a]pyrene, were studied with 2′-deoxynucleosides in aqueous solution by fluorescence and UV spectroscopic techniques. Ground-state complexes between BPT and the purine derivatives 2′-deoxyguanosine (dG), 2′-deoxyadenosine (dA), and 2′-deoxyinosine (dI) are formed with association constants in the range of ～40–130 M^?1 Complex formation with the pyrimidine derivatives 2′-deoxythymidine (dT), 2′-deoxycytidine (dC), and 2′-deoxyuridine (dU) is significantly weaker. Whereas dG is a strong quencher of the fluorescence of BPT by both static and dynamic mechanisms (dynamic quenching rate constant k_dyn= [2.5 ± 0.41 × 10⁹M¹ s ¹, which is close to the estimated diffusion-controlled value of ～ 5 × 10⁹M^{? 1} s^?1), both dA and dI are weak quenchers and form fluorescenceemitting complexes with BPT. The pyrimidine derivatives dC, dU, and dT are efficient dynamic fluorescence quenchers (K_dyn～ [1.5–3.0] × 10⁹M^?1 s^?1), with a small static quenching component due to complex formation evident only in the case of dT. None of the four nucleosidcs dG, dA, dC and dT are dynamic quenchers of BPT in the triplet excited state; the observed lower yields of triplets are attributed to the quenching of single excited states of BPT by 2′-deoxynucleosides without passing through the triplet manifold of BPT. Possible fluorescence quenching mechanisms involving photoinduced electron transfer are discussed. The strong quenching of the fluorescence of BPT by dG, dC and dT accounts for the low fluorescence yields of BPT-native DNA and of pyrene-DNA complexes.     

Oxidation of manganese(II) by ozone and reduction of manganese(III) by hydrogen peroxide in acidic solution

Manganese(II) is oxidized by ozone in acid solution, k=(1.5±0.2)×10³ M⁻¹ s⁻¹ in HClO₄ and k=(1.8±0.2)×10³M⁻¹ s⁻¹ in H₂SO₄. The plausible mechanism is an oxygen atom transfer from O₃ to Mn²⁺ producing the manganyl ion MnO²⁺, which subsequently reacts rapidly with Mn²⁺ to form Mn(III). No free OH radicals are involved in the mechanism. The spectrum of Mn(III) was obtained in the wave length range 200–310 nm. The activation energy for the initial reaction is 39.5 kJ/mol. Manganese(III) is reduced by hydrogen peroxide to Mn(II) with k(Mn(III)+H₂O₂)=2.8×10³M⁻¹ s⁻¹ at pH 0–2. The mechanism of the reaction involving formation of the manganese(II)-superoxide complex and reaction of H₂O₂ with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 207–214, 1998.     

Absorption spectrum,lifetime and reactivity of the excited triplet state of dimesitylcarbene

The first excited triplet state of dimesitylcarbene has been generated in solution at room temperature. It has a lifetime of 60 ns and shows fluorescence with λ_max = 501 nm and absorption with λ_max = 360 nm. This species is quenched by oxygen and carbon tetrachloride with rate constants of (4.4 ± 0.8) × 10¹⁰ and (7.3 ± 0.6) × 10⁹ M⁻¹ s⁻¹, respectively.     

KINETICS AND MECHANISM OF PHOTOCYCLOADDITION OF DEOXYURIDINES TO 2,3-DIMETHYL-2-BUTENE

The mechanism of photocycloaddition of 2′-deoxyuridine (1a) and thymidine (1b) to 2,3-dimethyl-2-butene (Bu) in acetonitrile by UV irradiation has been studied. The reciprocal quantum yield for the cycloaddition increased linearly with reciprocal concentrations of Bu in acetonitrile to give limiting quantum yields at infinite concentration of Bu as 0.030 and 0.0096 for 1a and 1b , respectively. This shows that the cycloaddition proceeds in a two-step mechanism between the triplet state of 1 and Bu through biradical intermediates. Addition of cis-1,3-pentadiene quenched the reaction obeying the Stern–Volmer equation. The above quenching experiments and laser transient spectroscopy revealed that the triplet state of 1a reacts with Bu with much larger rate constant (1.3–1.6 × 10⁹ M^?1 s^?1) than that of 1b (4–5 × 10⁷ M^?1 s^?1) reflecting larger steric hindrance exerted in the reaction of 1b than that of 1a .     

Quenching of the fluorescence of ditolyl aminoacridine solutions by tetrabromomethane

It has been shown that fluorescence quenching of 2,7-dimethyl-N,N-di-p-tolylacridine-9-amine (9-DTAA) solutions in hexane and acetone by tetrabromomethane (TBM) was dynamic in nature with the quenching rate constants of 1.5 × 10¹⁰ and 0.6 × 10¹⁰ M⁻¹s⁻¹, respectively. The difference in the constants was explained by the possibility of competition between the processes of the formation of solvation shell around the excited singlet 9-DTAA* molecule and the formation of the 9-DTAA*/TBM encounter complex.     

Kinetic studies on the metal(II) tartarate–peroxomonosulfate reaction

The kinetics of oxidation of tartaric acid (TAR) by peroxomonosulfate (PMS) in the presence of Cu(II) and Ni(II) ions was studied in the pH range 4.05–5.20 and also in alkaline medium (pH ～12.7). The rate was calculated by measuring the [PMS] at various time intervals. The metal ions concentration range used in the kinetic studies was 2.50 × 10^?5 to 1.00 × 10^?4 M [Cu(II)], 2.50 × 10^?4 to 2.00 × 10^?3M [Ni(II)], 0.05 to 0.10 M [TAR], and µ = 0.15 M. The metal(II) tartarates, not TAR/tartarate, are oxidized by PMS. The oxidation of copper(II) tartarate at the acidic pH shows an appreciable induction period, usually 30–60 min, as in classical autocatalysis reaction. The induction period in nickel(II) tartarate is small. Analysis of the [PMS]–time profile shows that the reactions proceed through autocatalysis. In alkaline medium, the Cu(II) tartarate–PMS reaction involves autocatalysis whereas Ni(II) tartarate obeys simple first‐order kinetics with respect to [PMS]. The calculated rate constants for the initial oxidation (k₁) and catalyzed oxidation (k₂) at [TAR] = 0.05 M, pH 4.05, and 31°C are Cu(II) (1.00 × 10^?4 M): k₁ = 4.12 × 10^?6 s^?1, k₂ = 7.76 × 10^?1 M^?1s^?1 and Ni(II) (1.00 × 10^?3 M): k₁ = 5.80 × 10^?5 s^?1, k₂ = 8.11 × 10^?2 M^?1 s^?1. The results suggest that the initial reaction is the oxidative decarboxylation of the tartarate to an aldehyde. The aldehyde intermediate may react with the alpha hydroxyl group of the tartarate to give a hemi acetal, which may be responsible for the autocatalysis. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 620–630, 2011     

MECHANISM OF THE PHOTOSENSITIZED REDOX REACTIONS OF ACRIDINE ORANGE IN AQUEOUS SOLUTIONS-A SYSTEM OF INTEREST IN THE PHOTOCHEMICAL STORAGE OF SOLAR ENERGY†

-We have carried out a very detailed study, using fluorescence and optical flash photolysis techniques, of the photoreduction of methyl viologen (MV²⁺) by the electron donor ethylene diamine tetraacetic acid (EDTA) in aqueous solution sensitized by the dye acridine orange (AOH⁺). A complete mechanism has been proposed which accounts for virtually all of the known observations on this reaction. This reaction is novel in that both the triplet and the singlet state of AOH⁺ appear to be active photochemically. We have shown that mechanisms previously proposed for this reaction are probably incorrect due to an artifact. At pH 7 the fluorescence quantum yield φ_s of AOH⁺ is 0.26 ± 0.02 and the fluorescence lifetime is 1.8 ± 0.2 ns. φ_s is pH dependent and reaches a maximum of 0.56 at pH 4. The fluorescence of AOH⁺ is quenched by MV²⁺ at concentrations above 1 mM and the quenching obeys Stern-Volmer kinetics with a quenching rate constant of (1.0 ± 0.1) × 10¹⁰M^?1 s^?1. The quenching of the AOH⁺ excited singlet state by MV²⁺ almost certainly returns the AOH⁺ to its ground state with no photochemistry occurring. EDTA also quenches the fluorescence of AOH· with Stern-Volmer kinetics but with a smaller rate constant (6.4 ± 0.5) × 10⁸M^?1s^?1 at pH 7. In this case the quenching is reactive resulting in the formation of semireduced AOH. In the presence of MV²⁺, flash irradiation of AOH⁺ does result in the reversible formation of the semireduced MV? which absorbs at 603 nm. We attribute this to a photochemical reaction of the triplet state of AOH⁺ with MV²⁺. The initial quantum yield for formation of MV? (φ_MV:)₀ was found to be constant at 0.10 ± 0.05 for [MV²⁺] from 5 × 10^?5 to 1.0 × 10^?3 with [AOH⁺] = 8 × 10^?6M. Previous workers had found that (φ_MV:)₀ appears to decrease with decreasing [AOH⁺]; however, on careful investigation, we found this was most probably due to quenching of the triplet state of AOH⁺ by trace amounts of oxygen. When EDTA is added to a mixture of AOH ⁺ and MV²⁺ at pH 7, the photochemical formation of MV? becomes irreversible as the [EDTA] is increased. The quantum yield for the irreversible formation of MV? exceeds 0.10 becoming as large as 0.16 for [EDTA] = 0.014M. This fact requires that an alternative photochemical process must be operative and we present evidence that this is a reaction of EDTA with the excited singlet state of AOH⁺ to produce the semi-reduced AOH- which then reacts with MV²⁺ to produce MV?. The full kinetic scheme was tested by computer simulation and found to be totally consistent. This also enabled the processing of a full set of rate constants. When colloidal PtO₂ was added to the optimal mixture [EDTA] = 3.4 × 10^?2M; [MV²⁺] = 5 × 10^?4M; [AOH⁺] = 4 × 10^?5M; pH6 H₂ gas was produced at a rate of 0.2μmol H₂h^?1. Thus, acridine orange should serve as an effective sensitizer in reactions designed to use solar energy to photolyze water.     

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.     

Reactivity of selected volatile organic compounds (VOCs) toward the sulfate radical (SO4−)

Rate constants for a series of alcohols, ethers, and esters toward the sulfate radical (SO₄^?) have been directly determined using a laser photolysis set‐up in which the radical was produced by the photodissociation of peroxodisulfate anions. The sulfate radical concentration was monitored by following its optical absorption by means of time resolved spectroscopy techniques. At room temperature the following rate constants were derived: methanol ((1.6 ± 0.2) × 10⁷ M^?1 s^?1); ethanol ((7.8 ± 1.2) × 10⁷ M^?1 s^?1); tert‐butanol ((8.9 ± 0.3) × 10⁵ M^?1 s^?1); diethyl ether ((1.8 ± 0.1) × 10⁸ M^?1 s^?1); MTBE ((3.13 ± 0.02) × 10⁷ M^?1 s^?1); tetrahydrofuran (THF) ((2.3 ± 0.2) × 10⁸ M^?1 s^?1); hydrated formaldehyde ((1.4 ± 0.2) × 10⁷ M^?1 s^?1); hydrated glyoxal ((2.4 ± 0.2) × 10⁷ M^?1 s^?1); dimethyl malonate (CH₃OC(O)CH₂C(O)OCH₃) ((1.28 ± 0.02) × 10⁶ M^?1 s^?1); and dimethyl succinate (CH₃OC(O)CH₂CH₂C(O)OCH₃) ((1.37 ± 0.08) × 10⁶ M^?1 s^?1) where the errors represent 2σ. For the two latter species, we also measured the temperature dependence of the corresponding rate constants. A correlation of these kinetics with the bond dissociation energy is also presented and discussed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 539–547, 2001     

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.     

The relaxation spectra of nickel(II) and cobalt(II) arginine complexes

The temperature-jump method has been used to determine the nickel(II)- and cobalt(II)-arginine complexation kinetics. In the pH range studied, the neutral form of the ligand, HL, is the attacking, as well as the complexed, ligand species. The reactions reported on are of the type where n = 1, 2, 3 and M is Ni or Co. At 25° and ionic strength 0.1M the association rate constants are: for nickel(II) k₁ = 2.3 × 10³(±20%), k₂ = 2.4 × 10⁴(±20%), k₃ = 3.5 × 10⁴(±40%) M^?1 sec^?1; for cobalt(II) k₁ = 1.5 × 10⁵(±20%), k₂ = 8.7 × 10⁵(±20%), k₃ = 2.0 × 10⁵(±40%) M^?1 sec^?1. Arginine binds to metal ions less well than homologous chelating agents due to the electrostatic repulsion arising from the positively charged terminus of the zwitterion. Kinetically, the effect appears in the association rate constants with nickel reactions more strongly influenced than cobalt.