Kinetics and mechanism of the oxygenation of potassium nitronates. Evidence for a single electron transfer (SET) mechanism ??

Summary The oxygenation of the potassium salts of alkylnitronates in absolute DMSO leads to potassium nitrite (nitrate) and the corresponding aldehydes or ketones at 40°C. Kinetic measurements for potassium 1-propylnitronate (K-1-PN) resulted in the rate law -d[K-1-PN]/dt = k₂[K-1-PN][O₂]. The rate constant, activation enthalpy, and entropy at 313.16 K are as follows: k/M^-1 s^-1 = (1.43±0.04), DH^‡/kJ mol^-1 = 57±10 DS^‡/J mol^-1 K^-1 = -60±32. The presence of superoxide ion as a result of a SET from the nitronate to dioxygen was proved by NBT test.     

Kinetics and mechanism of electron transfer via bridge metal-ion

The kinetics and mechanism of electron transfer between stable verdazyl radicals via bridge Mn(II), Co(II), Ni(II), Cu(I), and Zn ions have been studied in the range 280–330 K using the stopped-flow technique. It was found that the kinetic features of the reactions could be described in terms of the Marcus theory. The reorganization energy of the inner coordination sphere (ICS) of intermediate complexes was estimated from the reported data on the energy of release of various ligands from the ICS of similar complexes. The rest of the kinetic parameters (solvent reorganization energy, transmission coefficients, enthalpy, and entropy of activation) were determined from Marcus equations and from plots of rate constant versus temperature, “standard” free energy, and polarity of the medium. It was also found that the reactions under consideration are nonadiabatic and the transmission coefficient value depends on the mean lifetimes of ligands in the ICS.     

Evidence against the hopping mechanism as an important electron transfer pathway for conformationally constrained oligopeptides

The rate constant of intramolecular electron transfer through oligopeptides based on the alpha-aminoisobutyric acid residue was determined as a function of the peptide length and found to depend weakly on the donor-acceptor separation. By measuring the electron-transfer activation energy and estimating the energy gap between donor and bridge, we were able to discard the electron hopping mechanism.     

Kinetics and mechanism of metal ion catalysis: Part I. Kinetics of electron transfer from cycloheptanone in aqueous alkali

The kinetics of the electron transfer from cycloheptanone to OsO₄ in alkaline medium has been studied spectrophotometrically. The oxidation of cycloheptanone by Os^VIII, continuously regenerated by Fe(CN)^3– ₆ in alkaline medium in the 0.00123–0.01 M range, is zeroth order with respect to Fe(CN)^3– ₆ and first order with respect to Os^VIII. A suitable mechanism, based on rate data analysis, is proposed.     

A study on the mechanism of reaction between BNAH and chloranil:Evidence for electron transfer

The reaction between chloranil and N-benzyldihydronicotinamide(BNAH)in boratebuffer/DMF was investigated.The reaction mixture gave a strong esr signal,which is consistentwith that of chloranil anion radical,and tetrachlorohydrophenol(QH_2)and N-benzylnicotinamide(BNA~+)were obtained as the sole products.When the reaction was run in benzene solution,a greencoloured charge-transfer complex between the reactants could be isolated,which decomposed in polarsolvents to give BNA-+ and QH_2.Based on kinetic studies by esr spectroscopy by the stopped-flowtechnique,a two-step electron-transfer mechanism for the reactionis proposed in contrast to thehydride-transfer mechanism reported in the literature.     

On the electron transfer mechanism between cytochrome C and metal electrodes. Evidence for dynamic control at short distances

Cytochrome c was coordinatively bound to self-assembled monolayers of pyridine-terminated alkanethiols on Au and Ag electrodes. The mechanism of heterogeneous electron transfer of the immobilized protein was investigated by cyclic voltammetry and time-resolved surface-enhanced resonance Raman spectroelectrochemistry. The temperature, distance, and overpotential dependencies of the electron transfer rates indicate a change of mechanism from a tunneling controlled reaction at long distances (thicker films) to a solvent/protein friction controlled reaction at smaller distances (thinner films).     

Kinetics and mechanism of bimolecular electron transfer reaction in quinone-amine systems in micellar solution

Photoinduced electron transfer (ET) reactions between anthraquinone derivatives and aromatic amines have been investigated in sodium dodecyl sulphate (SDS) micellar solutions. Significant static quenching of the quinone fluorescence due to high amine concentration in the micellar phase has been observed in steady-state measurements. The bimolecular rate constants for the dynamic quenching in the present systems k(q) (TR), as estimated from the time-resolved measurements, have been correlated with the free energy changes DeltaG(0) for the ET reactions. Interestingly it is seen that the k(q) (TR) vs DeltaG(0) plot displays an inversion behavior with maximum k(q) (TR) at around 0.7 eV, a trend similar to that predicted in Marcus ET theory. Like the present results, Marcus inversion in the k(q) (TR) values was also observed earlier in coumarin-amine systems in SDS and TX-100 micellar solutions, with maximum k(q) (TR) at around the same exergonicity. These results thus suggest that Marcus inversion in bimolecular ET reaction is a general phenomenon in micellar media. Present observations have been rationalized on the basis of the two-dimensional ET (2DET) theory, which seems to be more suitable for micellar ET reactions than the conventional ET theory. For the quinone-amine systems, it is interestingly seen that k(q) (TR) vs DeltaG(0) plot is somewhat wider in comparison to that of the coumarin-amine systems, even though the maxima in the k(q) (TR) vs DeltaG(0) plots appear at almost similar exergonicity for both the acceptor-donor systems. These observations have been rationalized on the basis of the differences in the reaction windows along the solvation axis, as envisaged within the framework of the 2DET theory, and arise due to the differences in the locations of the quinones and coumarin dyes in the micellar phase.     

硝基苯甲酸酯水解反应单电子转移机理的证据: ESR和自旋捕获的研究

对-硝基苯甲酸甲酯(1), 对-硝基苯甲酸正丁酯(2), 对-硝基苯甲酸叔丁酯(3), 对-硝基苯甲酸苄酯(4), 对-硝基苯甲酸苯酯(5), 对-硝基苯甲酸(对-硝基)苯酯(6), 间-硝基苯甲酸甲酯(7), 间-硝基苯甲酸乙酯(8), 间-硝基苯甲酸苯酯(9)和3,5-二硝基苯甲酸甲酯(10)与氢氧化钾在二甲亚砜中反应, 反应产物分别为相应的对-硝基苯甲酸和间-硝基苯甲酸。反应液用ESR检测, 得到1-10自由基负离子的ESR谱。用自旋捕获技术证明反应过程中有OH自由基生成, 自由基捕获剂亚硝基叔丁烷(TNB), 苯基叔丁基硝酮(PBN)和氧气使产物硝基苯甲酸的产率降低, 结果表明, 1-10与KOH与DMSO中反应存在单电子转移机理。     

Kinetics of the aminolysis and hydrolysis of alkyl nitrites: Evidence for an orbital controlled mechanism

Summary The kinetics of the nitrosation of piperidine by propyl,iso-propyl, butyl,iso-butyl,sec-butyl, andtert-butyl nitrites in 0.1M NaOH and of the hydrolysis of the nitrite esters were studied spectrophotometrically by monitoring the absorbance of the nitrites at 381 nm. The observed correlation between logk ₂ and * (*=4.5) shows the reaction to proceedvia electrophilic attack by the nitrites; the existence of an isokinetic relationship suggests a single mechanism for the whole series. Comparison of the relative reactivities of the alkyl nitrites (primary>secondary>tertiary) with characteristic parameters of theirR groups (vertical ionization potentials and heats of formation ofR ⁺) suggests that these reactions are orbital controlled. All hydrolysis reactions were slower than the corresponding aminolysis reactions. This is attributed to a retardation of the former reaction by unfavourable interactions between the lone pairs of the nucleophile and the nitroso nitrogen atom.

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Kinetik der Aminoloyse und Hydrolyse von Alkylnitriten: Hinweise auf einen orbitalkontrollierten Mechanismus

Zusammenfassung Die Kinetik der Nitrosierung von Piperidin durch Propyl-,iso-Propyl-, Butyl-,iso-Butyl-,sec-Butyl- undtert-Butylnitrit sowie die Hydrolyse der entsprechenden Nitritester wurde in alkalischem Medium (NaOH, 0.1M) spektrophotometrisch (=381 nm) untersucht. Die beobachtete Relation zwischen logk ₂ und * (*=4.5) zeigt, daß die Reaktion durch nucleophile Attacke des Amines erfolgt. Die Existenz einer isokinetischen Relation läßt einen einheitlichen Mechanismus für die gesamte untersuchte Serie vermuten. Aus dem Vergleich der gefundenen Reaktivitätssequenzen für die Alkylnitrite (primär>sekundär>tertiär) mit den strukturellen Parametern ihrer ResteR (Ionisationspotentiale, Bildungswärme vonR ⁺) schließen wir, daß die untersuchten Reaktionen orbitalkontrolliert verlaufen. In allen Fällen wurde bei gleichen Bedingungen eine im Vergleich zur Aminolyse entsprechend langsamere Hydrolyse beobachtet. Der Unterschied ist einer ungünstigen Wechselwirkung zwischen den einsamen Elektronenpaaren der Nucleophile und des Stickstoffatoms der NO-Gruppe während der Reaktion mit der OH^–-Gruppe zuzuschreiben.

     

Understanding the mechanism of short-range electron transfer using an immobilized cupredoxin

The hydrophobic patch of azurin (AZ) from Pseudomonas aeruginosa is an important recognition surface for electron transfer (ET) reactions. The influence of changing the size of this region, by mutating the C-terminal copper-binding loop, on the ET reactivity of AZ adsorbed on gold electrodes modified with alkanethiol self-assembled monolayers (SAMs) has been studied. The distance-dependence of ET kinetics measured by cyclic voltammetry using SAMs of variable chain length, demonstrates that the activation barrier for short-range ET is dominated by the dynamics of molecular rearrangements accompanying ET at the AZ-SAM interface. These include internal electric field-dependent low-amplitude protein motions and the reorganization of interfacial water molecules, but not protein reorientation. Interfacial molecular dynamics also control the kinetics of short-range ET for electrostatically and covalently immobilized cytochrome c. This mechanism therefore may be utilized for short-distance ET irrespective of the type of metal center, the surface electrostatic potential, and the nature of the protein-SAM interaction.     

Evidence for a single electron transfer mechanism in reactions of lithium diorganocuprates with organic halides

It has been demonstrated by means of spectroscopic studies involving cyclizable alkyl halides that lithium dimethylcuprate can react with organic halides by a single electron transfer pathway.     

Evidence for through-space electron transfer in the distance dependence of normal and inverted electron transfer in oligoproline arrays

Four new helical oligoproline assemblies containing 16, 17, 18, and 19 proline residues and ordered arrays of a Ru(II)-bipyridyl chromophore and a phenothiazine electron-transfer donor have been synthesized in a modular fashion by solid-phase peptide synthesis. These arrays are illustrated and abbreviated as CH(3)CO-Pro(6)-Pra(PTZ)-Pro(n)()-Pra(Ru(II)b(2)m)(2+)-Pro(6)-NH(2), where PTZ is 3-(10H-phenothiazine-10)propanoyl and (Ru(II)b'(2)m)(2+) is bis(4,4'-diethylamide-2,2'-bipyridine)(4-methyl,4'-carboxylate,2,2'-bipyridine)ruthenium(II) dication with n = 2 (2), 3 (3), 4 (4), and 5 (5). They contain PTZ as an electron-transfer donor and (Ru(II)b'(2)m)(2+) as a metal-to-ligand charge transfer (MLCT) light absorber and are separated by proline-to-proline through-space distances ranging from 0 (n = 2) to 12.9 A (n = 5) relative to the n = 2 case. They exist in the proline-II helix form in water, as shown by circular dichroism measurements. Following laser flash Ru(II) --> b'(2)m MLCT excitation at 460 nm in water, excited-state PTZ --> Ru(2+) quenching (k(2)) occurs by reductive electron transfer, followed by Ru(+) --> PTZ(+) back electron transfer (k(3)), as shown by transient absorption and emission measurements in water at 25 degrees C. Quenching with DeltaG degrees = -0.1 eV is an activated process, while back electron transfer occurs in the inverted region, DeltaG degrees = -1.8 eV, and is activationless, as shown by temperature dependence measurements. Coincidentally, both reactions have comparable distance dependences, with k(2)( )()varying from = 1.9 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4) and k(3) from approximately 2.0 x 10(9) (n = 2) to 2.2 x 10(6) s(-)(1) (n = 4). For both series there is a rate constant enhancement of approximately 10 for n = 5 compared to n = 4 and a linear decrease in ln k with the through-space separation distance, pointing to a significant and probably dominant through-space component to intrahelical electron transfer.     

Biomimetic catalytic system driven by electron transfer for selective oxygenation of hydrocarbon

Hydrocarbon oxyfunctionalization is a crucial industrial process. Most metallic catalysts require higher temperatures and often show lower selectivities. One of the intellectual approaches is the mimicry for bio-oxidation. We have established a biomimetic system with a nonmetallic redox center, composed of anthraquinones, N-hydroxyphthalimide, and zeolite HY, for selective hydrocarbon oxygenation by molecular oxygen. Selectivity of 95.8% for acetophenone and 66.2% conversion were accomplished for oxygenation of ethylbenzene at temperatures as low as 80 degrees C. The redox cycle, driven by one-electron transfer and product orientation by Zeolite HY, opens up the possibility of mimicking bio-oxidation under mild conditions.     

Kinetics and mechanism of gallium discharge and ionization in an alkaline solution of potassium fluoride

Mechanism of gallium discharge and ionization on a liquid gallium electrode in a fluoride-alkaline electrolyte was studied. The exchange currents, transfer coefficients in the cathodic and anodic processes, and the activation energy of the electrode process were determined. The conditions in which gallium(I) ion-intermediates are formed were analyzed. The coordination numbers of the gallium complexes formed in alkaline fluoride solutions were calculated.     

Kinetics and mechanism of the base-catalyzed oxygenation of flavonol in DMSO-H(2)O solution.

The kinetics of the base-catalyzed oxygenation of flavonol have been investigated in 50% DMSO-H(2)O solution in the pH range 6.4-10.8 and an ionic strength of 0.1 mol L(-1) using spectrophotometric techniques at temperatures between 70 and 90 degrees C. The rate law -d[flaH]/dt = k(obs) [OH(-)][flaH][O(2)] (k(obs) = kK(1)/[H(2)O]) describes the kinetic data. The rate constant, activation enthalpy, and entropy at 353.16 K are as follows: k/mol(-1) L s(-1) = (4.53 +/- 0.07) x 10(-2), DeltaH/kJ mol(-1)= 59 +/- 4, DeltaS/J mol(-1) K(-1) = -110 +/- 11. The reaction showed specific base catalysis. It fits a Hammett linear free energy relationship for 4'-substituted flavonols and electron-releasing substituents enhanced the reaction rate. The linear correlation between the oxidation potential of the flavonols and the rate constants supports that a higher electron density on the flavonolate ion makes them more nucleophilic and the electrophilic attack of O(2) easier.     

9,10-Dicyanoanthracene photosensitized oxidation of aryl alkanols: evidence for an electron transfer mechanism

9,10-Dicyanoanthracene (DCA) photosensitizes the oxidation of a series of para substituted aryl alkanols in oxygen-saturated acetonitrile. Product analysis and Hammett correlations support an electron transfer mechanism for the title reaction.     

Kinetics and mechanism of the oxidation of benzyl alcohol by potassium chlorochromate

The kinetics of the oxidation of benzyl alcohol by potassium chlorochromate, KCrClO₃, has been studied in dimethyl sulfoxide-dichloromethane medium. The reaction is catalyzed by acid. The effects of temperature and solvent composition were studied and activation parameters evaluated. Probable mechanisms are discussed.

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, KCrClO₃, . . . .

     

Evidence for orbital-specific electron transfer to oriented haloform molecules

Beams of hyperthermal K atoms cross beams of the oriented haloforms CF(3)H, CCl(3)H, and CBr(3)H, and transfer of an electron mainly produces K(+) and the X(-) halide ion which are detected in coincidence. As expected, the steric asymmetry of CCl(3)H and CBr(3)H is very small and the halogen end is more reactive. However, even though there are three potentially reactive centers on each molecule, the F(-) ion yield in CF(3)H is strongly dependent on orientation. At energies close to the threshold for ion-pair formation ( approximately 5.5 eV), H-end attack is more reactive to form F(-). As the energy is increased, the more productive end switches, and F-end attack dominates the reactivity. In CF(3)H near threshold the electron is apparently transferred to the sigma(CH) antibonding orbital, and small signals are observed from electrons and CF(3)(-) ions, indicating "activation" of this orbital. In CCl(3)H and CBr(3)H the steric asymmetry is very small, and signals from free electrons and CX(3)(-) ions are barely detectable, indicating that the sigma(CH) antibonding orbital is not activated. The electron is apparently transferred to the sigma(CX) orbital which is believed to be the LUMO. At very low energies the proximity of the incipient ions probably determines whether salt molecules or ions are formed.