荧光素/层状钇氢氧化物复合体的发光性能及对Hg2+的识别

合成了层状钇氢氧化物(LYH)复合体FN/OS-LYH(FN为荧光素,OS为1-辛烷磺酸钠),研究了复合体的光致发光性能及剥离态胶体对Hg~(2+)的荧光识别性能。固态下,FN的钠盐不发光,FN/OS-LYH复合体发射黄绿光(564 nm);甲酰胺中,剥离为胶体悬浮液后,复合体发射绿光(540 nm,即剥离后蓝移24 nm),比FN阴离子的发射(572 nm)蓝移32 nm。复合体对高毒性Hg~(2+)具有很好的荧光识别能力,优于其他离子(Mg~(2+),Ni~(2+),Co~(2+),Cu~(2+),Zn~(2+),Pb~(2+)和Cd~(2+))。复合体加入Hg~(2+)后发生荧光淬灭,Hg~(2+)检测限为2.73×10-7mol·L-1,淬灭常数(Ksv)为4.82×102 L·mol-1。     

Ion specificity and viscosity of rutile dispersions

The isoelectric point (IEP) of rutile is shifted to higher pH values in the presence of greater than 10⁻⁴ mol dm⁻³ Ba²⁺, Ca²⁺ and Mg²⁺, and when a critical concentration (5 × 10⁻⁴ mol dm⁻³ for Ba²⁺ and 1 × 10⁻³ mol dm⁻³ for Ca²⁺) is exceeded there is no IEP at all and the ζ potential is always positive. A common intersection point for the ζ-potential curves of the different concentrations of salt is found, but for the various salts the point is shifted from ζ = 0 mV for Mg²⁺ up to ζ = 20 mV for Ba²⁺. Between the IEP and the charge-reversal point a rheologically unstable region is discovered. The shear stress of rutile dispersions (2.5 g rutile + 4 g electrolyte solution) at shear rates of 116 s⁻¹ shows the same pH dependence irrespective of the concentration of alkaline-earth metal cations up to 10⁻² mol dm⁻³. The shear stress is less than 1 Pa below pH 3.8 and in the pH range 5–12 it assumes a value between 50 and 80 Pa at 116 s⁻¹ with some scatter; however, no systematic trend with concentration of alkaline-earth metal cations and a rather insignificant decrease with pH at pristine conditions are observed. The acidic branch of the yield stress (pH) and low shear rate viscosity (pH) curves is insensitive to the presence of alkaline-earth metal cations, and the same behaviour is found for the ζ potential. The alkaline-earth metal cations induce an increase in viscosity in the basic region and a shift in the pH of maximum viscosity to high pH values. It was also discovered that the effect different alkaline-earth metal cations have on the rheological properties at the same concentration is different from the effect induced by indifferent electrolytes. When the ζ potential increases the viscosity at high pH is increased in a series which follows the increase in size of the cation. Received: 9 September 1998 Accepted in revised form: 12 January 1999     

Thermal, spectral and magnetic behaviour of 2,3,4-trimethoxybenzoates of Mn(II), Co(II), Ni(II) AND Cu(II)

Four new complexes of 2,3,4-trimethoxybenzoic acid anion with manganese(II), cobalt(II), nickel(II) and copper(II) cations were synthesized, analysed and characterized by standard chemical and physical methods. 2,3,4-Trimethoxybenzoates of Mn(II), Co(II), Ni(II) and Cu(II) are polycrystalline compounds with colours typical for M(II) ions. The carboxylate group in the anhydrous complexes of Mn(II), Co(II) and Ni(II) is monodentate and in that of Cu(II) monohydrate is bidentate bridging one. The anhydrous complexes of Mn(II), Co(II) and Ni(II) heated in air to 1273 K are stable up to 505–517 K. Next in the range of 505–1205 K they decompose to the following oxides: Mn₃O₄, CoO, NiO. The complex of Cu(II) is stable up to 390 K, and next in the range of 390–443 K it loses one molecule of water. The final product of its decomposition is CuO. The solubility in water at 293 K is of the order of 10^–3 mol dm^–3 for the Mn(II) complex and 10^–4 mol dm^–3 for Co(II), Ni(II) and Cu(II) complexes. The magnetic moment values of Mn²⁺, Co²⁺, Ni²⁺ and Cu²⁺ ions in 2,3,4-trimethoxybenzoates experimentally determined in the range of 77–300 K change from 5.64–6.57 μB (for Mn²⁺), 4.73–5.17 μB (for Co²⁺), 3.26–3.35 μB (for Ni²⁺) and 0.27–1.42 μB (for Cu²⁺). 2,3,4-Trimethoxybenzoates of Mn(II), Co(II) and Ni(II) follow the Curie–Weiss law, whereas that of Cu(II) forms a dimer.     

基于苯并噻唑Zn2+荧光探针及其细胞造影应用

设计合成基于苯并噻唑Zn²⁺荧光增强型探针BHP,在HEPES缓冲液中测其对Zn²⁺识别性能。实验结果表明,BHP对Zn²⁺有较高的选择性,对其他金属离子如Cd²⁺,Fe²⁺,Ni²⁺,Pb²⁺,Hg²⁺,Al³⁺,Mn²⁺,Ag⁺,Cu²⁺,Co²⁺,Na⁺,K⁺,Mg²⁺和Ca²⁺无明显荧光增强响应。BHP与Zn²⁺按1：1计量比配位,在生理条件下荧光强度不受pH值影响。在HeLa细胞中对Zn²⁺的造影表明BHP可用于生物体Zn²⁺检测。     

Colorimetric Microdetermination of Mercury(II) with Some Quinoxaline Azo Compounds in Presence of an Anionic Surfactant

Summary.  A new simple, rapid, sensitive, and selective method is proposed for the microdetermination of mercury. Mercury(II) forms insoluble complexes with 2,3-dichloro-6-(2-hydroxy-3,5-dinitrophenylazo)-quinoxaline (1), 2,3-dichloro-6-(5-amino-3-carboxy-2-hydroxy-phenylazo)-quinoxaline (2), 2,3-dichloro-6-(2,7-dihydroxynaphth-1-ylazo)-quinoxaline (3), and 2,3-dichloro-6-(3-carboxy-2-hydroxy-naphth-1-ylazo)-quinoxaline (4) in aqueous acidic medium; the complexes can be made soluble by the action of an anionic surfactant. The solution of the pink coloured compounds is stable for at least 24 h. Beer’s law is obeyed over the concentration range from 0.1 to 2.8 μg · cm⁻³ of mercury. For a more accurate analysis, Ringbom optimum concentration ranges were found to be 0.25–2.5 μg · cm⁻³. The molar absorpitivity, Sandell sensitivity, and relative standard deviations were also calculated. A slight interference from Pd²⁺ and Cd²⁺ is exhibited by the first three ligands, whereas the last one is only negligibly affected by these metal ions. Strong interference from Ag(I) is evident for all ligands, whereas alkali, alkaline earth, and other transition metals tested posed negligible interference. 15 μg · cm⁻³ of Cd²⁺ and Pd²⁺ or 10 μg · cm⁻³ of Ag⁺ can be tolerated if 1.0 mg of potassium bromide and 2.0 mg of citrate as masking agents are added for the determination of 1.5 μg · cm⁻³ of mercury(II). The method was applied to the determination of methyl- and ethylmercury chloride and the analysis of environmental water samples. Received August 7, 2000. Accepted (revised) October 18, 2000     

Spectrophotometric Investigation of the Complexing Reaction between Rutin and Titanyloxalate Anion in 50% Ethanol

Summary.  In the present work, rutin (3,3′ ,4′ ,5,7-pentahydrohyflavone-3-rhamnoglucoside) was determinated via a complexing reaction with a titanyloxalate anion. K₂[TiO(C₂O₄)₂] and rutin react in 50% ethanol forming a 1:2 complex in a pH range from 4.00 to 11.50, in which the TiO(C₂O₄)₂ ²⁻ ion is linked to rutin through the 4-carbonyl and 5-hydroxyl group. The thermodynamic stability constant log β₂ ⁰ of the complex is determined to 10.80 at pH = 6.50. The change of the standard Gibbs free energy Δ G⁰ amounts to −61 kJċ mol⁻¹, indicating that the process of complex formation is spontaneous. The optimal conditions for the spectrophotometric determination of microconcentrations of rutin are at pH=6.40 and λ= 430 nm, where the complex shows an absorption maximum with a molar absorption coefficient a ₄₃₀=(60±2)ċ10³ dm³ċ mol⁻¹ċ cm⁻¹. The method is applied rutin determination from tablets. Received January 4, 2000. Accepted (revised) February 17, 2000     

Voltammetric study of the interaction of the ofloxacin–copper complex with DNA, and its analytical application

The electrochemical behavior of the ofloxacin–copper complex, Cu(II)L₂, at a mercury electrode, and the interaction of DNA with the complex have been investigated. The experiments indicate that the electrode reaction of Cu(II)L₂ is an irreversible surface electrochemical reaction and that the reactant is of adsorbed character. In the presence of DNA, the formation of the electrochemically non-active complexes Cu(II)L₂-DNA, results in the decrease of the peak current of Cu(II)L₂. Based on the electrochemical behavior of the Cu(II)L₂ with DNA, binding by electrostatic interaction is suggested and a new method for determining nucleic acid is proposed. Under the optimum conditions, the decrease of the peak current is in proportional to the concentration of nucleic acids in the range from 3 × 10⁻⁸ to 3 × 10⁻⁶ g · mL⁻¹ for calf thymus DNA, from 1.6 × 10⁻⁸ to 9.0 × 10⁻⁷ g · mL⁻¹ for fish sperm DNA, and from 3.3 × 10⁻⁸ to 5.5 × 10⁻⁷ g · mL⁻¹ for yeast RNA. The detection limits are 3.3 × 10⁻⁹, 6.7 × 10⁻⁹ and 8.0 × 10⁻⁹ g · mL⁻¹, respectively. The method exhibits good recovery and high sensitivity in synthetic samples and in real samples.     

Chemical origin of the sustained-like pattern formation observed in the bromate — Dual substrate — Dual catalyst oscillatory batch system

The BrO ₃ ⁻ — BrAc — Ru(bpy) ₃ ²⁺ subsystem is shown to represent the core oscillator that serves as source of the long lasting temporal and spatial periodic behaviors observed in the BrO ₃ ⁻ — H₂PO ₂ ⁻ — acetone — Mn²⁺ — Ru(bpy) ₃ ²⁺ — acid “double substrate-double catalyst” oscillatory batch system. The BrAc — the substrate of the core oscillator — is formed and accumulated in the reactions taking place in the six-component system. BrAc was produced in a separate experiment with bromide, acetone, acid and excess bromate and the mixture was used for bringing about patterns in the thin solution layer after adding the Ru(bpy) ₃ ²⁺ catalyst. The two-dimensional reaction-diffusion patterns that appear in the subsystem and its parent system are very similar in wave speed, wavelength, color and in the duration of the pattern evolution, therefore a common chemical origin is supposed to exist in their formation. The role that the BrAc may play in the mechanism of the BrO ₃ ⁻ — reductant — acetone — catalyst type oscillators (∼ 30 variants) is also pointed out.     

Degenerate symmetry-adapted perturbation theory of weak interactions between closed- and open-shell monomers: application to Rydberg states of helium hydride

Symmetry-adapted perturbation theory is extended to the (quasi) degenerate, open-shell case. The new formalism is tested in calculations of the interaction energies for a helium atom in the ground state interacting with an excited hydrogen atom. It is shown that the method gives satisfactory results if the coupling with higher Rydberg states of the dimer is small, as is the case for the A²Σ⁺,B²Π,E²Π,3²Π, and 1²Δ states of HeH. For the C²Σ⁺ state convergence of the method is very slow, but it can be improved by including the n=3 states in the model space. Received: 3 June 1998 / Accepted: 9 September 1998 / Published online: 7 December 1998     

Conductometric and microcalorimetric analysis of the alkaline-earth/alkali-metal ion exchange onto polyacrylic acid

The specificity of the exchange between divalent (Di²⁺ = Ca²⁺ or Ba²⁺) and monovalent (M⁺ = Li⁺, Na⁺ or K⁺) ions onto a polyacrylic chain is examined using conductometric and microcalorimetric techniques. Assuming the formation of a bidentate complex between the Di²⁺ and the carboxylate groups, the conductometric data give the exchange ratio (Di²⁺/M⁺) and the speciation of the acrylic groups. No significant difference is observed between the three alkali-metal ions for a given Di²⁺ ion. Comparisons between Ca²⁺ and Ba²⁺ show a stronger hydrophobicity of the former as it precipitates at a complexation ratio r = 0.33 versus r = 0.45 for the Ba²⁺ salt. Microcalorimetric data show that all Di²⁺/M⁺ exchange energies are positive and depend significantly on the type of cations. The largest displacement energy (the more positive) is found for the binding of Ca²⁺ with sodium polyacrylate (8.13 kJ · mol⁻¹) and the smallest for Ba²⁺ with lithium polyacrylate (1.88 kJ · mol⁻¹). The interpretation of the data leads to the conclusion that specificity of the Di²⁺ binding originates in the dehydration phenomenon and specificity between the three alkali-metal ions is due to the decrease in the electrostatic bond strength with an increase in the ionic radii. The Di²⁺/M⁺ exchange is entropically driven. Received: 28 January 1999 Accepted in revised form: 7 April 1999     

TOF-SIMS imaging of halide/thiocyanate anions and hydrogen sulfide in mouse kidney sections using silver-deposited plates

In vivo imaging of reactive small molecule metabolites with high spatial resolution and specificity could give clues to understanding pathophysiology of various diseases. We herein applied time of flight-secondary ion mass spectrometry (TOF-SIMS) to newly developed silver-deposited plates that were stamped on mouse tissues, and succeeded in visualization of halide (Cl⁻, Br⁻, and I⁻) and pseudohalide thiocyanate (SCN⁻) anions, a class of substrates for neutrophils/eosinophil peroxidases to produce hypohalous acids (HOX/OX⁻ mixture; X: (pseudo)halides), as well as hydrogen sulfide (H₂S). Forty-micrometer frozen mouse kidney sections on cover glasses were attached to 37 °C preheated silver-deposited plates and incubated at −10 °C for 1 h. After sputter cleaning to remove surface contaminants, the plates were analyzed by TOF-SIMS to identify distribution of Br⁻, AgBr₂⁻, I⁻, AgI₂⁻, SCN⁻, as well as S²⁻ and AgS⁻ as products of tissue-derived H₂S. Br⁻, AgBr₂⁻, I⁻, and SCN⁻ anions were mainly distributed in core regions including the inner medulla and inner stripe of the outer medulla (except for I⁻), rather than outer regions such as the cortex and outer stripe of the outer medulla. AgI₂⁻ anion was spread over the whole kidney, although its levels were relatively low. In contrast, S²⁻ and AgS⁻ anions were mainly present in the outer regions. To our knowledge, this is the first imaging study to reveal the distribution of (pseudo)halides and H₂S in animal tissue sections.     

The extraction of Cr(III) from aqueous thiocyanate solutions and its separation from Co(II) and Ni(II)

For the system liquid anion-exchanger—Cr(III)−NCS⁻, an investigation has been made of the dependence of the percentage extraction of Cr(III) on parameters such as standing time of the Cr(III)−NCS⁻ solution, temperature, pH and type of exchanger. Quantitative extraction of e.g. 4·10⁻⁴ M Cr(III) by 0.1M Aliquat in CCl₄ is easily achieved at room temperature, using 4.75M KNCS−0.05N HCl as aqueous phase. At high Cr(III) concentrations, the complex anion present in the organic phase is Cr(NCS) ₆ ³⁻ ; when working with dilute metal ion solutions, the species extracted is Cr(NCS)₄ (H₂O) ₂ ⁻ . Separations of mixtures containing 10⁻²−10⁻⁴ M Co(II), Ni(II) and Cr(III) have successfully been accomplished.     

Isokinetic relationships in unimolecular heterolysis. Mechanism of ionization of covalent bond

Various types of isokinetic (isoparametric) relationships in heterolytic reactions were summarized and critically analyzed. It was presumed that the series of substrate reactivity is reversed after passing the isoparametric point, and the bimolecular reaction mechanism changes to unimolecular: S_N2-S_N1, S_N2-E1, S_E2-S_E1, S_E2-S_N1, and S_N2(SSIP)-S_N2(C⁺). Three particular cases of isoparametric relationships are discussed: (1) isoentropy (ΔS ^≠ = const) which reflects formation of contact ion pair; (2) isoenthalpy (ΔH ^≠ = const) which reflects formation of space-separated ion pair; and (3) isoenergy (ΔG ^≠ = const), when ΔH ^≠ = ΔG ^≠ = ΔE _r. The rate of heterolysis in cyclohexane does not depend on the substrate nature, and a universal minimal rate of heterolysis exists, k25 ≈ 10⁻¹⁰ s⁻¹, τ_1/2 = 220 years. There is no nucleophilic assistance by the solvent in unimolecular heterolysis.     

Competition Between O2 and H2O2 in the Oxidation of Fe(II) in Natural Waters

The oxidation rates of nanomolar levels of Fe(II) in seawater (salinity S = 36.2) by mixtures of O₂ and H₂O₂ has been measured as a function of pH (5.8–8.4) and temperature (3–35∘C). A competition exists for the oxidation of Fe(II) in the presence of both O₂ (μ mol⋅L⁻¹ levels) and H₂O₂ (nmol⋅L⁻¹ levels). A kinetic model has been applied to explain the experimental results that considers the interactions of Fe(II) with the major ions in seawater. In the presence of both oxidants, the hydrolyzed Fe(II) species dominate the Fe(II) oxidation process between pH 6 and 8.5. Over pH range 6.2–7.9, the FeOH⁺ species are the most active, whereas above pH 7.9, the Fe(OH)⁰₂ species are the most active at the levels of CO²⁻₃ concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)]₀ = 30nmol⋅L⁻¹ and pH = 8.17 in the oxygen-saturated seawater with [H₂O₂]₀ = 50nmol⋅L⁻¹ (log ₁₀ k = −2.24s⁻¹) is in excellent agreement with the experimental value of log ₁₀ k = −2.29s⁻¹ ([H₂O₂]₀ = 55nmol⋅L⁻¹, pH = 8).     

Multisyringe flow injection analysis hyphenated with liquid core waveguides for the development of cleaner spectroscopic analytical methods: improved determination of chloride in waters

In this work, the hyphenation of the multisyringe flow injection analysis technique with a 100-cm-long pathlength liquid core waveguide has been accomplished. The Cl⁻/Hg(SCN)₂/Fe³⁺ reaction system for the spectrophotometric determination of chloride (Cl⁻) in waters was used as chemical model. As a result, this classic analytical methodology has been improved, minimizing dramatically the consumption of reagents, in particular, that of the highly biotoxic chemical Hg(SCN)₂. The proposed method features a linear dynamic range composed of two steps between (1) 0.2–2 and (2) 2–8 mg Cl⁻ L⁻¹, thus extended applicability due to on-line sample dilution (up to 400 mg Cl⁻ L⁻¹). It also presents improved limits of detection and quantification of 0.06 and 0.20 mg Cl⁻ L⁻¹, respectively. The coefficient of variation and the injection throughput were 1.3% (n = 10, 2 mg Cl⁻ L⁻¹) and 21 h⁻¹. Furthermore, a very low consumption of reagents per Cl⁻ determination of 0.2 μg Hg(II) and 28 μg Fe³⁺ has been achieved. The method was successfully applied to the determination of Cl⁻ in different types of water samples. Finally, the proposed system is critically compared from a green analytical chemistry point of view against other flow systems for the same purpose.     

Outer-Sphere electron-transfer and the effect of linkage isomerism in the titanium(III) reduction of nitro-pentacyanocobaltate(III) and nitro-, and nitrito-pentaamminecobalt(III) complexes in aqueous acidic media

The reductions of [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺, by Ti^III in aqueous acidic solution have been studied spectrophotometrically. Kinetic studies were carried out using conventional techniques at an ionic strength of 1.0 mol dm⁻³ (LiCl/HCl) at 25.0 ± 0.1 °C and acid concentrations between 0.015 and 0.100 mol dm⁻³. The second-order rate constant is inverse—acid dependent and is described by the limiting rate law:- k₂ ≈ k₀ + k[H⁺]⁻¹,where k=k′K_a and K_a is the hydrolytic equilibrium constant for [Ti(H₂O)₆]³⁺. Values of k₀ obtained for [Co(CN)₅NO₂]³⁻, [Co(NH₃)₅NO₂]²⁺ and [Co(NH₃)₅ONO]²⁺ are (1.31 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, (4.53 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ and (1.7 ± 0.08) × 10⁻² dm³ mol⁻¹ s⁻¹ respectively, while the corresponding k′ values from reductions by TiOH²⁺ are 10.27 ± 0.45 dm³ mol⁻¹ s⁻¹, 14.99 ± 0.70 dm³ mol⁻¹ s⁻¹ and 17.93 ± 0.78 dm³ mol⁻¹ s⁻¹ respectively. Values of K _a obtained for the three complexes lie in the range (1–2) × 10⁻³ mol dm⁻³ which suggest an outer-sphere mechanism.     

Thermodynamics of mixed-ligand complex formation of copper (II) ethylenediaminetetraacetate with hexamethylenediamine in an aqueous solution

The mixed-ligand complex formation in the system Cu²⁺−Edta⁴⁻−(CH₂)₆(NH₂)₂ (L), where L is hexamethylenediamine has been calorimetrically, pH-potentiometrically and spectrophotometrically studied in aqueous solution at 298.15 K and the ionic strength of I = 0.5 (KNO₃). The thermodynamic parameters of formation of the CuEdtaL²⁻, CuEdtaHL⁻ (CuEdta)₂L⁴⁻ and (CuEdta)₂En⁴⁻ complexes have been determined. The most probable coordination mode for the complexone and the ancillary ligand in the mixed-ligand complexes was discussed.     

Synthesis, crystal structure and magnetic properties of bimetallic complex of Ni2Co(TTHA)·12H2O

The bimetallic complex of Ni₂Co(TTHA)·12H₂O (TTHA = triethylene tetraminehexaacetic acid) was synthesized and characterized structurally and magnetically. The title complex crystallizes in the triclinic space group P ī with a = 0.7316(2), b = 0.8624(2), c = 1.5041(4) nm; α = 73.38(2), β = 83.97(2), γ = 70.50(2)°. The crystal structure is built up of [Ni₂(TTHA)(H₂O)₂]²⁻, Co(H₂O)₆²⁺ and water molecules. The variable magnetic measurement shows that there is strong antiferromagnetic interaction between two Ni(II) ions in [Ni₂(TTHA) (H₂O)₂]²⁻ with J _Ni−Ni = −141.64 cm⁻¹, g _Ni = 2.21 and that the constant of spin-orbit coupling of Co(II) ion is −134.8 cm⁻¹. __________ Translated from Acta Scientiarum Naturalium Universitatis Nankaiensis, 2007, 40(1): 6–10 [译自: 南开大学学报(自然科学版)]     

A kinetic investigation of the oxidation of <Emphasis Type="SmallCaps">dl</Emphasis>-pipecolinate by bis(hydrogenperiodato)argentate(III) complex anion

Kinetics of oxidation of dl-pipecolinate by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO₆)₂]⁵⁻, has been studied in aqueous alkaline medium in the temperature range of 25–40 °C. The oxidation kinetics is first order in the silver(III) and pipecolinate concentrations. The observed second-order rate constant, decreasing with increasing [periodate] is virtually independent of [OH⁻]. α-Aminoadipate as the major oxidation product of pipecolinate has been identified by chromatographic analysis. A reaction mechanism is proposed that involves a pre-equilibrium between [Ag(HIO₆)₂]⁵⁻ and [Ag(HIO₆)(H₂O)(OH)]²⁻, a mono-periodate coordinated silver(III) complex. Both Ag(III) complexes are reduced in parallel by pipecolinate in rate-determining steps (described by k ₁ for the former Ag(III) species and k ₂ for the latter). The determined rate constants and their associated activation parameters are k ₁ (25 °C) = 0.40 ± 0.02 M⁻¹ s⁻¹, ∆H ₁^≠ = 53 ± 2 kJ mol⁻¹, ∆S ₁^≠ = −74 ± 5 J K⁻¹ mol⁻¹ and k ₂ (25 °C) = 0.64 ± 0.02 M⁻¹ s⁻¹, ∆H ₂^≠ = 41 ± 2 kJ mol⁻¹, ∆S ₂^≠ = −110 ± 5 J K⁻¹ mol⁻¹. The time-resolved spectra, a positive dependence of the rate constants on ionic strength of the reaction medium, and the consistency of pre-equilibrium constants derived from different reaction systems support the proposed reaction mechanism.