Reaction of perrhenate with the aquated electron in neutral aqueous solution yields ReO42? (kf 1.3 × 1010 M?1 s?1), with an absorption maximum at 290 nm (ε 1700). This decays by a second-order path (kd 1.5 × 109 M?1 s?1) at a rate ~ 100-fold faster than the decay of TcO42? under similar conditions. 相似文献
The spectroscopic and kinetic data of the short lived intermediates obtained by the attack of H-radicals on fluoro-, chloro-, bromobenzene, benzylchloride and phenethylchloride in aqueous solutions were studied by pulse radiolysis technique. The first three yield cyclohexadienylradicals (k=1–1.5×109 dm3 mol?1 s?1) with characteristic absorption maxima in the region 220–330 nm. In the case of benzylchloride a quantitative abstraction of chlorine by the H-atoms is observed (k=9.5×108 dm3 mol?1 s?1) leading to the formation of the benzylradical (λmax=257, 303, 317.5nm). The attack of H-atoms on phenethylchloride can occur on the aromatic ring forming also a cyclohexadienylradical (k=2.0×109 dm3 mol?1 s?1, λmax=317, 323nm) as well as on the side chain (k=1.5×108 dm3 mol?1 s?1) yielding H2. The intermediates decay according to a second order reaction withk=2 to 4.6×109 dm3 mol?1 s?1. To elucidate reaction mechanisms, steady state radiolysis experiments on the same systems were performed. 相似文献
The reaction of Cl?, Br?, I?, Co(CN)63? 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, Kal, 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)63? 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. 相似文献
Spectrophotometric pulse radiolysis experiments with cis- and trans-stilbene (Sc and St) in 2-propanol show that both isomers react with the solvated electron with a rate constant of 4.5 × 109 M?1 s?1. The absorption spectra of the two anion radicals have maxima at 496 and 486 nm, respectively. The absorbances at 400–550 nm disappear exponentially corresponding to a pseudo first order protonation of the anion radicals. The rate constants for the protonation of the cis isomer is 6.4 × 105 and of the trans isomer 0.7 × 105 s?1. In mixtures of cis- and trans-stilbene the electron transfer has a forward rate constant of 9 × 107 M?1 s?1 while the back reaction has a rate constant of 2.15 × 107 M?1 s?1. An equilibrium constant K = 4.2 is calculated. 相似文献
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. 相似文献
Vanadium(II) ions form with the pyridine-2-carboxylate ligand a deep blue, tris-substituted complex absorbing at 660 nm (ε = 7.2 × 103 M?1) cm?1) with a shoulder at 450 nm. Reversible spectroelectrochemistry and cyclic voltammetry were observed for this complex, with E = ?0.448 V vs NHE, and ΔSrcθ = ?6 cal · mol?1 · deg?1. Electron transfer kinetics with [CO(en)3]3+ led to k12 = 3100 M?1 s?, ΔH≠ = 12.4 kcal · mol?1 and ΔS≠ = ?0.9 cal · mol?1 · deg?1 (I = 0.10 M). For the related [Co(NH3)6]3+ complex, k13 = 1.9 × 104 M?1 s?1. The self-exchange rate constant and activation parameters were analysed in terms of relative Marcus theory. 相似文献
Steady-state fluorescence and single photon timing have been used to study the effect of the presence of hydrogen bonding on the intermolecular quenching of pyrene covalently linked to a guanine-like receptor I by an aliphatic amine (N,N-dimethylpropylamine) covalently linked to cytosine derivative II. By comparing the fluorescence quenching of I by II with that of 10methylpyrene (1-MP) by triethylamine (TEA), as a model system in which no hydrogen bonding can occur, one could possibly analyze the effect of the hydrogen bonding between receptor and substrate as a quenching as it leads to a higher local concentration of donor and acceptor. While the quenching of I by II was observed with an apparent rate constant kq of (1.78 ± 0.10) × 109 M−1 s−1 and (8.72 ± 0.42) × 108 M−1 s−1 in toluene and acetonitrile, respectively, no quenching could be observed in methanol. Upon excitation of 1-MP, no quenching by II could be detected in the same concentration range as used in the quenching of I. Quenching of I and of 1-MP by TEA (⩾ 10−2 M) in toluene leads to exciplex formation with maxima centred at 540 and 514 nm, respectively. The rate constants of exciplex formation and dissociation of I with TEA were analyzed using a global compartmental analysis. The following values were obtained for the rate constants: k01 = (9.70 ± 0.01) × 106s−1, k21 = (1.12 ± 0.003) × 109M−1s−1, k02 = (5.24 ± 0.01) × 107s−1 and k12 = (7.74 ± 0.08) × 106s−1. Quenching of I by TEA in the presence of III, a hydrogen-bonding system without an alkyl amine substituent, leads to exciplex formation centred at 538 nm. The rate constant values for the exciplex formation and dissociation of I with TEA in the presence of III were: k01 = (9.32 ± 0.08) × 106s−1, k21 = (9.32 ± 0.003) × 108M−1s−1, k02 = (6.16 ± 0.03) × 107s−1 and k12 = (21.90 ± 0.3) × 106s−1. The apparent rate constants kq for this system was (7.26 ± 0.56) × 106 M−1 s−1. The observed decrease in the rate of exciplex formation of I with TEA in the presence of III could suggest that the guanine-like moiety in I forms hydrogen bonds with the cytosine-like moiety and this could decrease the electron affinity of I. The rate constant of exciplex dissociation increased, indicating that the exciplex is less stable in the presence of III. Because of the single exponential decay of I in the presence and absence of II and of the agreement between steady-state and transient fluorescence measurements, the information available for quantitative analysis of the association between I and II is limited. 相似文献
Three single electron charge transfer redox reactions have been studied using the faradaic rectification method. The kinetic parameters obtained for the ferricyanide-ferrocyanide redox couple are α=0.49, ka0=12×10?2 cm s?1; for the chromic-chromous system α=0.47, ka0=2×10?3 cm s?1 and for the titanic-titanous reaction α=0.49 and kao=6×10?4 cm s?1 at 27°C. 相似文献
Absolute rate constants for the reaction of O(3P) atoms with n-butane (k2) and NO(M Ar)(k3) have been determined over the temperature range 298–439 K using a flash photolysis-NO2 chemiluminescence technique. The Arrhenius expressions obtained were k2 = 2.5 × 10?11exp[-(4170 ± 300)/RT] cm3 molecule?1 s?1, k3 = 1.46 × 10?32 exp[940 ± 200)/ RT] cm6 molecule?2 s?1, with rate constants at room temperature of k2 = (2.2 ± 0.4) × 10?14 cm3 molecule?1 s?1 and k3 = (7.04 ± 0.70)×10?32 cm6 molecule?2 s?1. These rate constants are compared and discussed with literature values. 相似文献
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 kf = 2.27 × 106M?1 s?1 and kr = 3.81 × 101 s?1 for the nickel(II) complex and kf = 1.23 × 107M?1 s?1 and kr = 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. 相似文献
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. 相似文献
The luminescence induced by the Hg-photosensitized reaction of NH3 was studied by repetitive fast pulsed excitation. From observations of the decay of the luminescence, the emitter of the 290 nm fluorescence was found to be the precursor of the 340 nm emission (the stabilized complex). The first-order decay rate of the stabilized complex was found to be given by 1/τO + k[NH3], where τ0 = 2.3 μs and k = 3.5 × 10?13 cm3 molecule?1 s?1. 相似文献
Abstract— Semimethylene blue was generated by reductive quenching of triplet methylene blue, 3MBH2+, with diphenylamine at pH 0.62–3.4. A Q-switched ruby laser flash-photolysis-kinetic spectro-photometric apparatus was used to characterize the absorption spectrum of semimethylene blue from 350 to 900 nm and a number of physical constants at 25°C with μ= 0.4 M and Cl? as the anion. The specific rate of quenching of 3MBH2+ by DPA is 2.8 × 109M?1 s?1 in 5% EtOH-95% water and 1.2×109M?1 s?1 in 50 v/v% aq. CH3CN. Corresponding efficiencies of net electron transfer are, respectively, 0.15 and 0.62. Spectral characteristics in 5% EtOH are, for MBH22±, λmax= 375 nm, ε375= 9000 M?1 cm?1; λmax= 880 nm, ε880= 12700 M?1 cm?1; for MBH±, λmax= 410 nm, ε410= 9800 M?1 cm?1, λmax= 880 nm, ε880= 33000 M?1 cm?1; for MBH± in 50 v/v% AN, λmax= 400 nm, ε400= 11000 M?1 cm?1 and λmax= 880 nm,ε880= 39000 M?1 cm?1. The pKa of MBH22ε calculated from the pH dependence of the absorption spectrum is 1.86 × 0.04 in 5% EtOH and 1.15 in 50 v/v% AN. Rate constants, kdecay, for reaction DPAH±+ with MBH22ε and MBH± in 5% EtOH are, respectively, 3.9 × 109 and 9.5 × 109M?1 s?1. The value of pKa of MBH22ε calculated from the dependence of kdecay on pH is 1.75 in 5% EtOH. 相似文献
The rate constant for the reaction of CFCl2 with oxygen is measured in the pressure range 0.2–12 Torr using pulsed-laser photolysis and time-resolved mass spectrometry. CFCl2 radicals are generated by photolysis of CFCl3 at 193 nm. The reaction kinetics are recorded by monitoring the build-up of the CFCl2O2 radical concentration. The reaction is in its fall-off region, and the parameters of the relation for the treatment of the fall-off are for M = N2: k(0) = (5.0 ± 0.8) × 10?30 cm6 molecule?2 s?1. k(∞) = (6.0 ± 1.0) × 10?12 cm3 molecule?1 s?1. This value of k(∞) is consistent with results obtained at low pressure taking Fc = 0.6, but the uncertainty in the high-pressure limit is much higher. The results are compared to measurements performed with CH3 and CF3. Estimates of the relative third-body efficiencies of He and N2 are given for CFCl2 and CF3. 相似文献
ST0 type nuclear spin polarization has been observed in the reaction products of photochemically generated acylalkyl biradicals in the presence of a scavenger. From the magnetic field dependence of biradical CIDNP the rate constant k of proton abstraction from 2-methyl-2-propanethiol by the alkyl site of the biradicals has been determined (k = 5.0 × 107 M?1 s?1). 相似文献