Electrochemical and quantum-chemical studies of chromium(III,II) fluoride complexes in alkali chloride melts

The standard rate constants (k _s) of charge transfer on a glass carbon electrode were determined for the Cr(III)/Cr(II) redox pair in the NaCl-KCl-K₃CrF₆, KCl-K₃CrF₆, and CsCl-K₃CrF₆ systems at 973–1173 K by cyclic voltammetry. The k _s constant was found to increase at elevated temperatures and the following nonmonotonic dependence of k _s on the nature of the outer-spheric cation was found: k _s (CsCl) > k _s (NaCl-KCl) > k _s (KCl). On the basis of quantum-chemical data for the M₃CrF₆ + 18MCl (M = Na, K) model systems, it was shown that the complex chromium particles with four or five outer-spheric sodium or potassium cations had maximum thermodynamic stability. Quantum-chemical calculations were performed to interpret the experimental data on the effect of the second coordination sphere of the complexes on the standard charge transfer rate constants.     

Standard rate constants of charge transfer for Nb(V)/Nb(IV) redox couple in chloride-fluoride melts: Experimental and calculation methods

Standard rate constants k _s of charge transfer for Nb(V)/Nb(IV) redox couple in NaCl-KCl (the equimolar mixture) -K₂NbF₇, KCl-K₂NbF₇, and CsCl-K₂NbF₇ melts are determined by using cyclic voltammetry. An unconventional series of the standard rate constants is found: k _s (KCl) < k _s (CsCl) < k _s (NaCl-KCl). Ab initio calculations carried out by using a PC Gamess/Firefly quantum-chemical program showed that the charge transfer activation energy can change not monotonically in the Na-K-Cs series, in compliance with the reorganization energy relationship. This leads, in its turn, to nonmonotonic change in the charge transfer rate constants.     

Standard charge transfer rate constants for Nb(V)/Nb(IV) redox pairs in fluoride melts

Standard charge transfer rate constants (k _s) were defined for Nb(V)/Nb(IV) redox couples in NaF-KF (eutectic)-K₂NbF₇, KF-K₂NbF₇, and CsF-K₂NbF₇ melts using the cyclic voltammetry technique. It was established that in fluoride melts, the k _s values change in the following order depending on the composition of the second coordination sphere: k _s (CsF) < k _s (KF) < k _s (NaF-KF).     

Standard rate constants of charge transfer of Nb(V)/Nb(IV) redox pair in equimolar NaCl-KCl melt

Standard rate constants of charge transfer (k _s) on platinum and glassy carbon electrodes for Nb(V)/Nb(IV) redox pair in the NaCl-KCl-K₂NbF₇ melt are determined using the method of cyclic voltammetry in the temperature range of 973 to 1123 K. It is found that k _s increases with increasing temperature and when we pass from glassy carbon to platinum electrode. The “apparent” activation energies of charge transfer are determined; it is shown that the charge transfer between the Nb(V) and Nb(IV) complexes is quasi-reversible and is controlled predominantly by the diffusion.     

Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Studies on the electrode reaction of Cr(III)CyDTA/Cr(II)CyDTA by the coulostatic method in various alcohol + water mixtures

Standard rate constants (k_s)_ms and cathodic transfer coefficients (α_c)_ms of the electrode reaction of Cr(III)CyDTA/Cr(II)CyDTA have been obtained by the coulostatic method in methanol + water, ethanol + water, n-propanol + water and n-butanol + water mixtures, where CyDTA means trans-1,2-cyclohexanediaminetetraacetate. The values of (k_s)_ms obtained at low concentrations of the alcohols are nearly equal to that obtained in aqueous solutions. With increasing concentrations of alcohols, they decrease, in the cases of methanol and n-butanol, in the concentration ranges in which the measurements were made, while, in the cases of ethanol and n-propanol, they decrease to reach minima at certain concentrations and then increase again. The values of (α_c)_ms obtained at high concentrations of n-propanol and n-butanol are much larger than those obtained in aqueous solutions.     

Relaxation of vibrationally excited gas-phase Cr(CO) 5 −

Radiative relaxation of Cr(CO)₅⁻ was investigated by two techniques: a standard two-pulse photodissociation experiment and by using the branching ratio of its reaction with oxygen as an ion thermometric probe. Photoexcitation at 1064 nm was used to prepare highly vibrationally excited Cr(CO)₅⁻. Although the overall oxidation rate changes only slightly upon excitation (actually decreasing by a factor of 1.2 ± 0.1), the primary product distribution shifts dramatically, from Cr(CO)₃O⁻ (the thermodynamic product) to Cr(CO)₃O₂⁻ (the kinetic product). The two-pulse photodissociation measurement gave a radiative relaxation rate constant (k _rad) of 15 ± 2 s⁻¹, whereas the branching ratio experiments gave a k _rad value of 3. 3 ± 0.7 s⁻¹. The large difference between these two values is due to the difference in Cr(CO)₅⁻ internal energy ranges probed by the two techniques. In the high internal energy regime interrogated by the two-pulse measurements (about 12,000 to 6000 cm⁻¹), the strongly emitting C-O stretching modes are populated and contribute to fast relaxation. In contrast, the branching ratio measurements remain sensitive to internal energy changes all the way down to thermal energies, where the C-O stretches are depopulated and thus unavailable for radiative relaxation.     

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru^II complexes containing two 2,2′‐bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridines, for one complex containing no 2,2′‐bipyridine, but 2 of these different ligands, for three multinuclear Ru^II complexes containing 2 or 4 [Ru(bpy)₂] moieties and also coordinated via 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridine ligands, and for the complex [(Ru(bpy)₂(L)]²⁺ where L is N,N′‐([2,2′‐bipyridine]‐4,4′‐diyl)bis[3‐methoxypropanamide]. Absorption maxima are red‐shifted with respect to [Ru(bpy)₃]²⁺, as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal‐to‐ligand charge transfer states ³MLCT in de‐aerated MeCN are equal to or longer than for [Ru(bpy)₃]²⁺ and vary considerably, i.e., from 0.86 to 1.71 μs. Rate constants k_q for quenching by O₂ of the ³MLCT states were measured and found to be well below diffusion‐controlled, ranging from 1.2 to 2.0⋅10⁹ dm³ mol⁻¹ s⁻¹. The efficiencies f of singlet‐oxygen formation during oxygen quenching of these ³MLCT states are relatively high, namely 0.53 – 0.89. The product of k_q and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and k_q−k equals k, the net rate constant for quenching due to energy dissipation of the excited ³MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔG^CT, the free‐energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.     

Kinetic and mechanistic studies on the formation of bis(dicarboxylato)carbonatochromate(III) complexes

The reaction of the Cr(xx)₂(H₂O)₂ ⁻ (xx = oxalate, malonate and methylmalonate) complexes with dissolved CO₂ was studied by stopped-flow spectrophotometry in the 7 < pH < 9 range and between 20 to 30°C at an ionic strength of 0.5 mol dm⁻³ (NaCl). Under the experimental conditions the aqua complex ion consists of a pH-dependent mixture of Cr(xx)₂(H₂O)₂ ⁻, Cr(xx)₂(OH) (H₂O)²⁻ and Cr(xx)₂(OH)₂ ³⁻. The monohydroxo and dihydroxo species undergo CO₂ uptake and subsequent intramolecular carbonate ligand chelation independently, at rates which are readily distinguishable and are governed by the uptake rate constants k ₁ and k ₂ and chelation rate constants k ₃ and k ₄, respectively. Only the k ₁ values for oxalato, malonato and methylmalonato complexes could be calculated; k ₁ = 1084 and 1333 and 1650 mol⁻¹ dm³ s⁻¹, respectively. The results obtained were compared with those obtained from other systems that have either cobalt(III), iridium(III) or rhodium(III) as central atoms. This revised version was published online in June 2006 with corrections to the Cover Date.     

Bioadsorption and ion exchange of Cr3+ and Pb2+ solutions with algae

Heavy metals can be removed from effluents and recovered using physico-chemical mechanisms as biosorption processes. In this work “Arribada” seaweed biomass was employed to assess its biosorptive capacity for the chromium (Cr³⁺) and lead (Pb²⁺) cations that usually are present in waste waters of plating industries. Equilibrium and kinetic experiments were conducted in a mixed reactor on a batch basis. Biosorption equilibrium and fluid-solid mass transfer constants data were analyzed through the concept of ion exchange sorption isotherm. The respective equilibrium exchange constants (K _eqCr=173.42, K _eqPb=58.86) and volumetric mass transfer coefficients ((k _mCr a)′=1.13×10⁻³ s⁻¹, (k _mPb a)′=0.89×10⁻³ s⁻¹) were employed for the dynamic analysis of Cr and Pb sorption in a fixed-bed flow-through sorption column. The breakthrough curves obtained for both metals were compared with the predicted values by the heterogeneous model (K _eqCr=171.29, K _eqPb=60.14; k _mCr a=7.81×10⁻² s⁻¹, k _mPb a=2.43×10⁻² s⁻¹), taking into account the mass transfer process. The results suggest that these algae may be employed in a metal removal/recovery process at low cost. An erratum to this article can be found at     

Ag(I)‐Catalyzed Chlorination of Linezolid during Water Treatment: Kinetics and Mechanism

The kinetic and mechanistic study of Ag(I)‐catalyzed chlorination of linezolid (LNZ) by free available chlorine (FAC) was investigated at environmentally relevant pH 4.0–9.0. Apparent second‐order rate constants decreased with an increase in pH of the reaction mixture. The apparent second‐order rate constant for uncatalyzed reaction, e.g., k″_app = 8.15 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 0.076 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C and for Ag(I) catalyzed reaction total apparent second‐order rate constant, e.g., k″_app = 51.50 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 1.03 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C. The Ag(I) catalyst accelerates the reaction of LNZ with FAC by 10‐fold. A mechanism involving electrophilic halogenation has been proposed based on the kinetic data and LC/ESI/MS spectra. The influence of temperature on the rate of reaction was studied; the rate constants were found to increase with an increase in temperature. The thermodynamic activation parameters E_a, ΔH^#, ΔS^#, and ΔG^# were evaluated for the reaction and discussed. The influence of catalyst, initially added product, dielectric constant, and ionic strength on the rate of reaction was also investigated. The monochlorinated substituted product along with degraded one was formed by the reaction of LNZ with FAC.     

Kinetics of the Temperature-Dependent eaq− and ⋅OH Radical Reactions with Cr(III) Ions in Aqueous Solutions

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (e_aq⁻) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr³⁺ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)²⁺, did with a rate coefficient of k= (7.2±0.3)×10⁸ M⁻¹ s⁻¹ at 25 °C. The oxidation of Cr(OH)²⁺ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.     

Ligand exchange reactions of some Cr(III) complexes investigated by cyclic voltammetry

From cyclic linear sweep voltammograms of some Cr(III) complexes it is evident that after electron transfer ligand groups are expelled relatively slowly inDMSO and the tetra coordinated complex is formed. The rate constants could be determined in some cases by the methods of cyclic voltammetry. The following compounds were examined: [Cr(en)₃]³⁺, [Cr(acac)(en)₂]²⁺, [Cr(acac)₃]. [Cr(dien)₂]³⁺, [Cr(DMSO)₆]³⁺, [Cr(ur)₆]³⁺. For [Cr(en)₃]³⁺ the energy of activation could be determined as well.The dependence of the velocity of ligand elimination on complex structure is discussed.

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Untersuchung von Ligandenaustauschreaktionen einiger Cr(III)-Komplexe mittels cyclischer Voltammetrie

Zusammenfassung Bei der Reduktion einiger der folgenden Cr(III)-Komplexe inDMSO läßt sich auf Grund der Voltammogramme auf eine Ligandenabspaltung und Bildung der vierfach koordinierten Cr(II)-Komplexe schließen: [Cr(en)₃]³⁺, [Cr(acac)(en)₂]²⁺, [Cr(acac)₃], [Cr(dien)₂]³⁺, [Cr(DMSO)₆]³⁺, [Cr(ur)₆]³⁺. Für das [Cr(en)₃]³⁺ konnten Geschwindigkeitskonstante und Aktivierungsenergie dieser nachgelagerten Reaktion bestimmt werden.Der Einfluß der Struktur des Komplexes auf die Zerfallsgeschwindigkeit wird diskutiert.

     

Kinetics of nucleophilic attack on coordinated organic moieties : VI. Mechanism of addition of tri-n-butylphosphine to [(C7H7)M(CO)3]BF4 (M = Cr, Mo, W) and related cations

Phosphonium adduct formation via attack of tri-n-butylphosphine on the cations [(C₇H₇)M(CO)₃]⁺ (M = Cr, Mo, W) obeys the rate law, Rate = k [complex] [PBu₃]. The very similar rate constants for the Cr, Mo and W complexes confirm the similar electrophilicities of the tropylium rings in these cations, and also support the view that there is direct addition to the rings. The related complexes [(C₆H₇)Fe(CO)₃]BF₄ and [(C₆H₆)Mn(CO)₃]BF₄ also form adducts with PBu₃, and the quantitative reactivity order [(C₆H₇)Fe(CO)₃]⁺ > [(C₇H₇)Cr(CO)₃]⁺ » [(C₆H₆)Mn(CO)₃]⁺ (160:60:1) has been established.     

The nephelauxetic effect and ortho- to para-positronium conversion reactions promoted by Cr(III) complexes

The ortho- (o-Ps) to para-positronium (p-Ps) conversion reactions promoted by ten Cr(III) complexes were investigated at several temperatures. The results obtained are discussed together with those of three other Cr(III) complexes measured previously.It was found that all the reactions studied are diffusion controlled and that their rate constants,k _SE, increased as the corresponding ratiosB/B _o between the Racah interelectronic repulsion parameters of Cr(III) complexes and Cr(III) gaseous ion decrease; that is,K _SE increases as 3d electron cloud expansion promoted by ligands, increases. Therefore the o-Ps to p-Ps conversion reactions can be considered a complementary probe for the spectroscopic method in investigating the ligand capabilities to cause 3d cloud expansion. The effect is called nephelauxetic which means expanding (electron) cloud.Finally an empirical average environmental rule, similar to that hypothesized for B/B_o ratios, is proposed for estimating the rate constants,K _SE, of complexes formed with mixed ligand sets.This work was supported by Ministero Universitá e Ricerca Scientifica e Tecnologica (M.U.R.S.T.) and by Consiglio Nazionale Ricerche (C.N.R.).     

Quadrupolar Ultrafast Charge Transfer in Diaminoazobenzene-Bridged Perylenediimide Triads

Strong push-pull interactions between electron donor, diaminoazobenzene (azo), and an electron acceptor, perylenediimide (PDI), entities in the newly synthesized A−D−A type triads (A=electron acceptor and D=electron donor) and the corresponding A−D dyads are shown to reveal wide-band absorption covering the entire visible spectrum. Electrochemical studies revealed the facile reduction of PDI and relatively easier oxidation of diaminoazobenzene in the dyads and triads. Charge transfer reversal using fluorescence-spectroelectrochemistry wherein the PDI fluorescence recovery upon one-electron oxidation, deterring the charge-transfer interactions, was possible to accomplish. The charge transfer state density difference and the frontier orbitals from the DFT calculations established the electron-deficient PDI to be an electron acceptor and diaminoazobenzene to be an electron donor resulting in energetically closely positioned PDI ^δ− -Azo ^δ+ -PDI ^δ− quadrupolar charge-transfer states in the case of triads and Azo ^δ+ -PDI ^δ− dipolar charge-transfer states in the case of dyads. Subsequent femtosecond transient absorption spectral studies unequivocally proved the occurrence of excited-state charge transfer in these dyads and triads in benzonitrile wherein the calculated forward charge transfer rate constants, k_f, were limited to instrument response factor, meaning >10¹² s⁻¹ revealing the occurrence of ultrafast photo-events. The charge recombination rate constant, k_r, was found to depend on the type of donor-acceptor conjugates, that is, it was possible to establish faster k_r in the case of triads (∼10¹¹ s⁻¹) compared to dyads (∼10¹⁰ s⁻¹). Modulating both ground and excited-state properties of PDI with the help of strong quadrupolar and dipolar charge transfer and witnessing ultrafast charge transfer events in the studied triads and dyads is borne out from the present study.     

A study of dimethylferrocene diffusion and electrode kinetics using microelectrode voltammetry in poly(ethylene glycol) solvent

The heterogeneous electron transfer rate constant (k _s) of dimethylferrocene (DMFc) was estimated using cyclic voltammetric peak potential separations taken typically in a mixed diffusion geometry regime in a polyelectrolyte, and the diffusion coefficient (D) of DMFc was obtained using a steady-state voltammogram. The heterogeneous electron transfer rate constant and diffusion coefficient are both smaller by about 100-fold in the polymeric solvent than in the monomeric solvent. The results are in agreement with the difference of longitudinal dielectric relaxation time (τ_L) in the two kinds of solvents, poly(ethylene glycol) (PEG) and CH₃CN, indicating that k _s varies inversely with τ_L; k _s is proportional to D of DMFc. Both D and k _s of DMFc in PEG containing different supporting electrolytes and at different temperatures have been estimated. These results show that D and k _s of DMFc increase with increasing temperature in the polyelectrolyte, whereas they vary only slightly with changing the supporting electrolyte. Received: 5 February 1998 / Accepted: 23 July 1998     

Absolute rate constants for the reaction of O(3P) aroms with n-butane and NO(M  Ar) over the temperature range 298–439 K

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.     

Kinetics of electrophilic brominations. Mechanistic significance of the third-order term

The rates of electrophilic bromination of various donors follow complex kinetics which include both first-order and second-order dependences on bromine, especially in the less polar solvents. The second-order rate constant k_s and the third-order rate constant k_t are evaluated for alkene bromination in carbon tetrachloride, and they are compared to those already listed for the electrophilic brominations of substituted styrenes, arenes, and metal carbonyls in the extant literature. Despite the varying magnitudes of the second– and third-order rate constants for these diverse donors (and in different solvents), the ratio log(k_s/k_t) is remarkably invariant. The mechanistic implication of this unique observation is discussed in the context of charge transfer interactions which are common to the activated complexes in the electrophilic brominations of various donors.     

Temperature‐Induced Intersite Charge Transfer Involving Cr ions in A‐Site‐Ordered Perovskites ACu3Cr4O12 (A=La and Y)

Changes in the valence state of transition‐metal ions in oxides drastically modify the chemical and physical properties of the compounds. Intersite charge transfer (ISCT), which involves simultaneous changes in the valence states of two valence‐variable transition‐metal cations at different crystallographic sites, further expands opportunities to show multifunctional properties. To explore new ISCT materials, we focus on A‐site‐ordered perovskite‐structure oxides with the chemical formula AA′₃B₄O₁₂, which contain different transition‐metal cations at the square‐planar A′ and octahedral B sites. We have obtained new A‐site‐ordered perovskites LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ by synthesis under high‐pressure and high‐temperature conditions and found that they showed temperature‐induced ISCT between A′‐site Cu and B‐site Cr ions. The compounds are the first examples of those, in which Cr ions are involved in temperature‐induced ISCT. In contrast to the previously reported ISCT compounds, LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ showed positive‐thermal‐expansion‐like volume changes at the ISCT transition.