Single crystal EPR study of Mn(II)-doped magnesium potassium Tutton's salt

Single crystal EPR studies of Mn(II)-doped magnesium potassium Tutton's salt, MgK₂(SO₄)₂.6H₂O, was studied at room temperature. The spin-Hamiltonian parameters obtained are: g=2.0036(3), A = −96(3), D = 350(5), a = 14(2) and F = −5(1) (A, D, a and F are in units of 10⁻⁴ cm⁻¹). The tetragonal distortion axis corresponds to one of the MgO bond directions. The zero-field splitting parameter (D) shows a linear dependence in the temperature range 77–370 K. The percentage of covalency of the MnO bond has been estimated to be 8 per cent.     

Single crystal electron spin resonance of bis(tetraphenylphosphonium) bis(quinoxaline-2,3-dithiolato)oxovanadate(IV) as a pure compound and diluted in the isomorphous molybdenum(IV) compound

Single crystal e.s.r. spectra at room temperature and Q-band frequencies on [PPh₄]₂ [(Mo/V)O(qdt=₂] (qdt = quinoxaline-2,3-dithiolate) containing ca 3% vanadium gave the spin-Hamiltonian parameters g₁ = 1.977 ± 0.001, g₂ = 1.985 ± 0.001, g₃ = 1.987 ± 0.001, A₁ = (−38.5 ± 0.3) × 10⁻⁴, A₂ = (−45.1 ± 0.3) × 10⁻⁴, A₃ = (−133.2 ± 0.3) × 10⁻⁴ cm⁻¹, and Q′ = −(0.15±0.05) × 10⁻⁴ cm⁻¹. The g and A tensor axes are not coincident. The principal values of the g and A tensors have been analysed via an angular overlap treatment. Proton spin-flip transitions were observed in the spectra at X-band frequencies. Single crystal e.s.r. spectra of undiluted [PPh₄]₂ [VO(qdt)₂] at both X- and Q-band frequencies are interpreted in terms of a two-dimensional weak exchange model with J₀ = −48 ± 2G (ferromagnetic).     

The nature of the growing species in living cationic polymerization: Principles,stereochemistry, and in-situ NMR analysis

The objective of this paper is to discuss: (i) the general approaches to living cationic polymerizations; (ii) the nature of the growing species thus generated. For the first, it is concluded that three general methods are currently available which involve the nucleophilic stabilization of the growing carbocations by (a) a suitable counteranion, (b) an added Lewis base, or (c) an added neutral salt. According to this view, a variety of initiating systems are classified. For the second, findings are presented for the recently developed living cationic polymerization of vinyl ethers by the HCl/SnCl₄ initiating system in the presence of an added salt (nBu₄N⁺Cl⁻). The nature of the growing species therein is discussed on the basis of the steric structure of the living polymers, relative to nonliving counterparts, and the in-situ ¹³C NMR spectroscopic analysis of model reactions where the interaction of the growing end model [CH₃CH(OR)Cl] with SnCl₄ and the added salt is analyzed.     

The carbenium ion - oxonium ion exchange processes related to vinyl and ring-opening polymerization

It has previously been proposed (Ref. 1) that in the cationic vinyl polymerizations, proceeding with termination due to the collapse of ion pairs, addition of bases increases “livingness”, because of the fast convertion of the otherwise dead (within the time of polymerization) covalent species into the onium ions; these, in turn, fast convert into carbenium ions, the actually propagating species. Equilibria between carbenium ions (CH₃OCH₂⁺A⁻ has been used as a model) and their onium counterparts ((CH₃)₂O taken as a model base) as well as between covalent species (CH₃OCH₂OSO₂CF₃) and the corresponding oxonium ion (with a (CH₃)₂O ligand) have been studied by dynamic ¹H and ¹⁹F NMR. Total ionization of methoxymethyl triflate (CH₃OCH₂OSO₂CF₃) has been shown to increase indeed from 10⁴ (-10°C) to 10⁶ (-70°C) times when 1,0 mol·L⁻¹ of (CH₃)₂O is added. Although this model system better describes polymerization of cyclic acetals than that of vinyl ethers, it shows at least qualitatively the importance of bases in ionization of covalent species, which may be responsible for reinitiation in the cationic polymerization of vinyl ethers.     

Poly(styrene‐b‐isobutylene) multiarm star‐block copolymers

Multiarm star‐branched polymers based on poly(styrene‐b‐isobutylene) (PS‐PIB) block copolymer arms were synthesized under controlled/living cationic polymerization conditions using the 2‐chloro‐2‐propylbenzene (CCl)/TiCl₄/pyridine (Py) initiating system and divinylbenzene (DVB) as gel‐core‐forming comonomer. To optimize the timing of isobutylene (IB) addition to living PS⊕, the kinetics of styrene (St) polymerization at −80°C were measured in both 60 : 40 (v : v) methyl cyclohexane (MCHx) : MeCl and 60 : 40 hexane : MeCl cosolvents. For either cosolvent system, it was found that the polymerizations followed first‐order kinetics with respect to the monomer and the number of actively growing chains remained invariant. The rate of polymerization was slower in MCHx : MeCl (k_app = 2.5 × 10⁻³ s⁻¹) compared with hexane : MeCl (k_app = 5.6 × 10⁻³ s⁻¹) ([CCl]_o = [TiCl₄]/15 = 3.64 × 10⁻³M; [Py] = 4 × 10⁻³M; [St]_o = 0.35M). Intermolecular alkylation reactions were observed at [St]_o = 0.93M but could be suppressed by avoiding very high St conversion and by setting [St]_o ≤ 0.35M. For St polymerization, k_app = 1.1 × 10⁻³ s⁻¹ ([CCl]_o = [TiCl₄]/15 = 1.82 × 10⁻³M; [Py] = 4 × 10⁻³M; [St]_o = 0.35M); this was significantly higher than that observed for IB polymerization (k_app = 3.0 × 10⁻⁴ s⁻¹; [CCl]_o = [Py] = [TiCl₄]/15 = 1.86 × 10⁻³M; [IB]_o = 1.0M). Blocking efficiencies were higher in hexane : MeCl compared with MCHx : MeCl cosolvent system. Star formation was faster with PS‐PIB arms compared with PIB homopolymer arms under similar conditions. Using [DVB] = 5.6 × 10⁻²M = 10 times chain end concentration, 92% of PS‐PIB arms (M_n,PS = 2600 and M_n,PIB = 13,400 g/mol) were linked within 1 h at −80°C with negligible star–star coupling. It was difficult to achieve complete linking of all the arms prior to the onset of star–star coupling. Apparently, the presence of the St block allows the PS‐PIB block copolymer arms to be incorporated into growing star polymers by an additional mechanism, namely, electrophilic aromatic substitution (EAS), which leads to increased rates of star formation and greater tendency toward star–star coupling. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1629–1641, 1999     

Anionic polymerization of cyclosiloxanes with cryptates as counterions: New results

A kinetic study of the anionic polymerization of hexamethylcyclotrisiloxane ((CH₃)₂SiO)₃ (D₃) was carried out in toluene with the cryptate Li⁺ + [211] as a counterion. The kinetic order with respect to the living end concentration was found to be equal to 1, and the propagation rate constant relative to cryptated ion pairs was determined at several temperatures between −20 and +20°C. The corresponding activation parameters were calculated : Ep = 9.8 kcal.mol⁻¹ and Δ S°^t = −20 cal.mol.⁻¹K⁻¹. The reaction of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetra-siloxane ((CH₃)(CH₂=CH)SiO)₄ (D4^*) was studied under the same conditions, the reactivity towards silanolate active centers was shown to be very close to that of D3. The rate constants of propagation and depropagation of D4^* and of larger cycles D5^* and D6^* were determined from polymerization experiments on D4^*. The propagation rate constants were found to be nearly the same for D4^* and D5^* : 1.2–1.31.mol⁻¹. s⁻¹, D6^* being the less reactive monomer : 0.41.mol⁻¹. s⁻¹. The differences observed in equilibrium constants of the cycles are essentially due to the rate constants of their formation which were found to be 0.42, 0.23, and 0.006 s⁻¹ for D4^*, D5^* and D6^*, respectively.     

Photoinitiated free radical polymerization of vinyl compounds in aqueous solution

A new method for the photochemical initiation of polymerization of vinyl compounds in aqueous solution is described. The photochemically active species is an ion pair complex of the formula Fe³⁺X⁻(X⁻ = OH⁻, CI⁻, N⁻₃, etc.). The light absorption by the ion pair leads to an electron transfer causing reduction of the cation and oxidation of the anion to an atom or free radical X. The latter leads to the initiation of polymerization in accordance with X + CH₂CHR→XCH₂ CHR . The kinetics of the reaction were studied by the measurement of: (a) ferrous ion formed (colorimetrically), (b) monomer disappearance (by titration and by weighting the polymer), (c) the chain length of the polymer (in the case of methyl methacrylate). The dependence of the quantum yield on the light intensity, light absorption fraction, and the concentration of vinyl monomer and ferrous ion added initially was investigated. A complete mechanism, both with regard to the formation of free radicals and the polymerization reaction, was put forward involving: (1) light absorption, (2) a primary dark back reaction, (3) dissociation of the primary product, (4) a secondary dark back reaction, (5) initiation of polymerization by free radicals, (6) propagation of polymerization, and (7) termination by recombination of active polymer endings. The mechanism was verified by the experimental results and some constant ratios were estimated quantitatively.     

Iodine-Chemisorption,Interpenetration and Polycatenation: Cationic MOFs and CPs from Group 13 Metal Halides and Di-Pyridyl-Linkers

Eight cationic, two-dimensional metal-organic frameworks (MOFs) were synthesized in reactions of the group 13 metal halides AlBr₃, AlI₃, GaBr₃, InBr₃ and InI₃ with the dipyridyl ligands 1,2-di(4-pyridyl)ethylene (bpe), 1,2-di(4-pyridyl)ethane (bpa) and 4,4’-bipyridine (bipy). Seven of them follow the general formula ²_∞[MX₂(L)₂]A, M=Al, In, X=Br, I, A⁻=[MX₄]⁻, I⁻, I₃⁻, L=bipy, bpa, bpe. Thereby, the porosity of the cationic frameworks can be utilized to take up the heavy molecule iodine in gas-phase chemisorption vital for the capture of iodine radioisotopes. This is achieved by switching between I⁻ and the polyiodide I₃⁻ in the cavities at room temperature, including single-crystal-to-single-crystal transformation. The MOFs are 2D networks that exhibit (4,4)-topology in general or (6,3)-topology for ²_∞[(GaBr₂)₂(bpa)₅][GaBr₄]₂⋅bpa. The two-dimensional networks can either be arranged to an inclined interpenetration of the cationic two-dimensional networks, or to stacked networks without interpenetration. Interpenetration is accompanied by polycatenation. Due to the cationic character, the MOFs require the counter ions [MX₄]⁻, I⁻ or I₃⁻ counter ions in their pores. Whereas the [MX₄]⁻, ions are immobile, iodide allows for chemisorption. Furthermore, eight additional coordination polymers and complexes were identified and isolated that elaborate the reaction space of the herein reported syntheses.     

Reactions of SiH2(X̄1A1) with H2, CH4, C2H4, SiH4 and Si2H6 at 298 K

Reaction rate constants of SiH₂(X̄¹A₁) have been directly measured for the first time using the laser photolysis—laser-induced fluorescence method. The preparation of SiH₂ radical in the laser photolysis (193 nm) of phenylsilane and the concentration of the radical is demonstrated by a dye laser at 580.1 nm (X̄¹A₁-Ā¹B₁). The reaction rate constants of SiH₂(X̄¹A₁) with H₂, CH₄, C₂H₄, SiH₄ and Si₂H₆, are 0.001, 0.01, 0.97, 1.1 and 5.7×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively. For SiH₂(Ā¹B₁(0.2,0)), the collision-free lifetime is 0.6 μs and the quenching rate constant for He is 3.8×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹.     

Rate constants and Arrhenius parameters for the reactions of Cl atoms with the halogenated ethers: CF3CH2OCHF2, CF3CHClOCHF2, and CF3CH2OCClF2

Rate constants have been determined for the reactions of Cl atoms with the halogenated ethers CF₃CH₂OCHF₂, CF₃CHClOCHF₂, and CF₃CH₂OCClF₂ using a relative‐rate technique. Chlorine atoms were generated by continuous photolysis of Cl₂ in a mixture containing the ether and CD₄. Changes in the concentrations of these two species were measured via changes in their infrared absorption spectra observed with a Fourier transform infrared (FTIR) spectrometer. Relative‐rate constants were converted to absolute values using the previously measured rate constants for the reaction, Cl + CD₄ → DCl + CD₃. Experiments were carried out at 295, 323, and 363 K, yielding the following Arrhenius expressions for the rate constants within this range of temperature:Cl + CF₃CH₂OCHF₂: k = (5.1₅ ± 0.7) × 10⁻¹² exp(−1830 ± 410 K/T) cm³ molecule⁻¹ s⁻¹ Cl + CF₃CHClOCHF₂: k = (1.6 ± 0.2) × 10⁻¹¹ exp(−2450 ± 250 K/T) cm³ molecule⁻¹ s⁻¹ Cl + CF₃CH₂OCClF₂: k = (9.6 ± 0.4) × 10⁻¹² exp(−2390 ± 190 K/T) cm³ molecule⁻¹ s⁻¹ The results are compared with those obtained previously for the reactions of Cl atoms with other halogenated methyl ethyl ethers. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 165–172, 2001     

2-n-propylaminopyridine n-oxide complexes formed from Cu(II) salts

Cu(II) complexes have been prepared with 2-n-propylaminopyridine N-oxide (nP) employing the perchlorate, tetrafluoroborate, nitrate, chloride and bromide salts. Preparative molar ratios of 4:1 and 2:1 ligand to Cu(II) salt yielded the following unique salts: Cu(nP)₄X₂(X=ClO₄⁻, BF₄⁻, NO₃^staggeredt-) and Cu(nP)₂X₂)(X=NO₃⁻, Cl⁻, Br⁻). Characterization has been accomplished primarily by IR, electronic and electron spin resonance measurements of the solid state. nP binds as a monodentate ligand via its N-oxide oxygen in the complexes prepared from the Cu(II) salts of polyatomic anions and as a bidentate ligand in the halogen solids. Anion coordination occurs in the halogen complexes as well as in Cu(nP)₂(NO₃)₂.     

The crystal and molecular structure of octafluorobiphenylene,C12F8

Octafluorobiphenylene was made by heating 1,2-diiodotetrafluorobenzene in a sealed, evacuated tube with either copper, lead or bismuth. The crystal used for the diffraction studies was grown from hexane. Crystal data: C₁₂F₈, M_r = 296.116, monoclinic, A2/a, a = 21.140(2), b = 6.430(6), c = 36.340(3) ÅA, β = 123.76(3)°, U = 4106.73 ÅA³, Z = 16, D_m = 1.884 g cm⁻³, D_x = 1.907 g cm⁻³, λ(Cuka) = 1.5418 ÅA, μ = 1.810 mm⁻¹, F(000) = 2304, measurement temperature = 293K, R = 0.059 for 2230 reflections with I > 3σ(I).     

Kinetics of solvent extraction of metal ions with HDEHP—I Kinetics and mechanism of solvent extraction of Ti(IV) from acidic aqueous solutions with bis-(2-ethyl hexyl) phosphoric acid in benzene

The rate of extraction of Ti(IV) from aqueous sulphuric acid solution into benzene with HDEHP [HDEHP = H₂A₂ = bis-(2-ethyl hexyl)-phosphoric acid] and back extraction of the Ti(IV)-DEHP chelate from the benzene phase to an aqueous phase have been studied under various conditions. The distribution equilibria have also been investigated. The structure of the extracted species has been investigated by IR spectral data, molecular weight determination and phosphorous-titanium atom ratio determination. The extracted species has been shown to be [TiOA₂]₃. From the extraction equilibrium studies, the reaction for the extraction of Ti(IV) has been proposed and the extraction equilibrium constant has been evaluated.The rate of forward extraction is proportional to [(TiO)⁺²]³, [H₂A₂]_(org)⁺³ and [H₂SO₄]⁻³, whereas the rate of backward extraction is proportional to [TiOA₂]_(org)⁺¹, [H₂A₂]_(org)⁻¹ and [H₂SO₄]⁻¹. The rate determining step in the forward extraction involves the reaction of 3 atoms of titanium in a stable chain or ring with 3 molecules of HDEHP dimer molecules. The rate of backward extraction studies indicate that the rate determining step of the backward extraction involves the reaction of [TiOA₂] molecule with one molecule of H₂SO₄. The values of the equilibrium constants obtained from the equilibrium study and rate study are 10^{(3.646±0.16)} and 10^{(10.604±0.194)} respectively. The cause of this discrepancy has been explained.     

High resolution Fourier transform spectroscopy of the PbO molecule from investigation of the O2(1Δg)—Pb reaction

From the reaction of Pb with metastable oxygen O₂(¹Δ_g) in a Broida type oven we have analysed at high resolution some vibrational levels of the X0⁺, a1, A0⁺ and B1 states of the ²⁰⁸PbO molecule. The rotational parameters determined allowed us to recalculate the position of the various isotope lines to within 0.01 cm⁻¹. We have found a negative value of ω_eχ_e (−0.123 (25) cm⁻¹) in A0⁺, contrary to previous observations. The Ω type doubling in a1 varies from +1.8 × 10⁻⁴ cm⁻¹ (υ′ = 2) to +2.3 × 10⁻⁴ cm⁻¹ (υ′ = 9) and in B1 from −1.17 × 10⁻⁴ cm⁻¹ (υ′ = 0) to −0.97 × 10⁻⁴ cm⁻¹ (υ′ = 2).     

ESI activity of Br−, BF4−, ClO4− and BPh4− anions in the presence of Li+ and NBu4+ counter‐ions

To improve our understanding of the electrospray ionization (ESI) process, we have subjected equimolar mixtures of salts A⁺X⁻ (A⁺ = Li⁺, NBu₄⁺; X⁻ = Br⁻, ClO₄⁻, BF₄⁻, BPh₄⁻) in different solvents (CH₃CN, tetrahydrofuran, CH₃OH, H₂O) to negative‐ion mode ESI and analyzed the relative ESI activity of the different anionic model analytes. The ESI activity of the large and hydrophobic BPh₄⁻ ion greatly exceeds that of the smaller and more hydrophilic anions Br⁻, ClO₄⁻ and BF₄⁻, which we ascribe to its higher surface activity. Moreover, the ESI activity of the anions is modulated by the action of the counter‐ions and their different tendency toward ion pairing. The tendency toward ion pairing can be reduced by the addition of the chelating ligands 12‐crown‐4 and 2.2.1 cryptand and is, although to a smaller degree, further influenced by the variation of the solvent. Complementary electrical conductivity measurements afford additional information on the interactions of the ionic constituents of the sample solutions. Copyright © 2017 John Wiley & Sons, Ltd.     

ESR characterisation of SO−3 and SO−4 radicals in X-irradiated kainite (KMgClSO4.3H2O)

The ESR spectra of the kainite (KMgClSO₄.3H₂O) crystal revealed an intense isotropic (g = 2.004) peak C attributed to the SO⁻₃ radical and two pairs of lines (A₁, A₂) and (B₁, B₂) bearing intensity ratio 5:3. The intensity and linewidth variation of peak C suggested that the signal contains an unresolved shf structure. The power saturation studies on SO⁻₃ indicate that its ESR line is homogeneously broadened and its line shape is Lorentzian. The spin—lattice and spin—spin relaxation times (T₁ and T₂) of SO⁻₃ have been estimated to be 0.44 s and 656 μs, respectively. The analysis of the anisotropic pairs of lines show that they constitute two sets A and B and are due to two chemically inequivalent SO⁻₄ radical species in the lattice. The ESR spectra of the polycrystalline samples recorded at 300 and 77 K confirm the isotropic behaviour of SO⁻₃ and chemical inequivalence of two types of SO⁻₄ radicals. The principal g-values of the SO⁻₄ radical were evaluated to be: g₁ = 2.007, g₂ = 2.011, g₃ = 2.014 for species A and g₁ = 2.008, g₂ = 2.012, g₃ = 2.015 for species B. The low microsymmetry of the SO²⁻₄ ion in the lattice seems to promote the radiation damage.     

Sulphur-nitrogen chelating agent—II. Mixed-metal binuclear (Cu2, Ag+) and mononuclear (Cu2+) solid state complexes of 1,4-diazo-7-thiaoctane and 1,5-diazo-8-thianonane. An infrared and reflectance spectral analysis

A series of CuLX₂, CuL₂X₂, CuLYZ and AgCuL₂Z₃ {X⁻ = Cl⁻, Br⁻, NO⁻₃; Y⁻ = OAc⁻; Z⁻ = NO⁻₃ or ClO⁻₄; L = 1,4-diaza-7-thiaoctane [2,2-NNS(Me)] or 1,5-diaza-8-thianonane [3,2-NNS(Me)]} have been prepared and characterized by means of elementary analysis, i.r. and electronic spectroscopy. All 1 : 1 complexes containing polyanions (NO⁻₃, OAc⁻, ClO⁻₄) are five-coordinated species with the meridionally arranged thiadiamine acting as a tridentate. The remaining coordination positions are occupied by an asymmetric bidentate nitrate or acetate group. In contrast, in the 1 : 2 complexes, the thiadiamines are incompletely coordinated leaving one or even two sulphur donors free. These free sulphide-groups readily coordinate with the Ag(I) ions to form mixed-metal AgCuL₂Z₃-compounds.     

Kinetics of polymerization by activated monomer mechanism

Polymerization of cyclic ethers by activated monomer mechanism involves consecutive additions of protonated monomer molecules to the growing macromolecules fitted with hydroxyl groups at their ends. For oxirane itself and symmetrically substituted oxiranes there is only one kind of hydroxyl groups and one, unique way of ring-opening. Unsymmetrically substituted oxiranes provide however two sites of attack and two different hydroxyls, resulting from these ring-openings. Kinetics of polymerization of epichlorohydrin (chloromethyloxirane) has been studied and all four rate constants determined, namely rate constants of the primary and secondary alcoholate chain ends with a protonated monomer, opening in result of the attack on substituted or unsubstituted carbon atom. These rate constants are (in mol⁻¹·1·s⁻¹ at 25°C, in CH₂Cl₂ solvent): k₁₁ = 0.055, k₁₂ = = 0.41, k₂₂ = 0.135, and k₂₁ = 0.0011 (e.g. k₁₂ is the rate of reaction of the primary alcohol producing the secondary alcohol). Thus, polymerization proceeds almost exclusively on the secondary alcoholate groups, reproducing themselves (k₂₂).     

Tris(trichlorosilyl)tetrelide Anions and a Comparative Study of Their Donor Qualities

Trichlorosilylated tetrelides [(Cl₃Si)₃E]⁻ have been prepared by adding 1 equiv of a soluble Cl⁻ salt to (Cl₃Si)₄Si (E=Si) or 4 Si₂Cl₆/GeCl₄ (E=Ge). To assess their donor qualities, the anions [(Cl₃Si)₃E]⁻ (E=C, Si, Ge) have been treated with BCl₃, AlCl₃, and GaCl₃. Both BCl₃ and GaCl₃ give 1:1 adducts with the anionic centers. AlCl₃ leads to Cl⁻ abstraction from [(Cl₃Si)₃E]⁻ with formation of (Cl₃Si)₄E (E=Si or Ge). (Cl₃Si)₄Ge is cleanly converted to the perhydrogenated (H₃Si)₄Ge by use of Li[AlH₄]. Another case of Cl⁻ abstraction was observed for [(Cl₃Si)₃Ge ⋅ GaCl₃]⁻, which reacts with GaCl₃ to afford the neutral dimer [(Cl₃Si)₃Ge−GaCl₂]₂.     

Chain-length dependent termination in pulsed-laser polymerization, 5. The evaluation of the rate coefficient of bimolecular termination kt for the reference system styrene in bulk at 25°C

Following earlier suggestions the values for the rate coefficient of chain termination k_t in the bulk polymerization of styrene at 25°C were formally calculated (a) from the second moment of the chainlength distribution (CLD) and (b) from the rate equation for laser-initiated pseudostationary polymerization (both expressions originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation k_p. These values were treated as average values, k and k , respectively. They exhibited good mutual agreement, even the predicted gradation (k < k by about 20%) was recovered. The log-log plot of k_t vs. the number-average degree of polymerization of the chains at the moment of their termination yielded exponents b of 0.16–0.18 in the power-law k_t = A · P_n ^−b, A ranging from 2.3 × 10⁸ to 2.7 × 10⁸ L · mol⁻¹ · s⁻¹. These data are only slightly affected if termination is not assumed to occur by recombination only and a small contribution of disproportionation is allowed for.