The redox systems Ru(IV-III) at platinum electrodes and Ru(III-II) at mercury electrodes

The chemistry of Ru(IV)tetramer is described. The redox system Ru(IV-III) and the redox system Ru(III-II) are studied with electrochemical techniques, respectively at platinum and mercury electrodes. The decomposition of a tetrameric Ru(III) as a reduction product and the relationship between the tetrameric and monomeric ruthenium species are discussed and may give a new vision on the chemistry of Ru(IV)tetramer.     

Photosensitized aquation reactions of Cr(NH3 5(NCS)2+

Biacetyl and acridinium ion but not Michler's ketone sensitize the aquation of Cr(NH_{3 5}(NCS)²⁺ in 0.1 N sulfuric acid solution. Phosphorescence quenching occurs in first system, and the kinetics of this and of the sensitized reaction in the presence and absence of oxygen indicate that transfer of excitation energy occurs from the biacetyl first triplet state. The reaction is entirely one of ammonia aquation, indicating that the first quartet excited state of the complex is populated.Acridinium ion sensitizes both ammonia and thiocyanate aquation, the presence of oxygen affecting the latter but not the former mode. The kinetics of the fluorescence quenching by the complex and that of the sensitized ammonia aquation indicate that the latter process is induced by the first singlet excited state of acridinium ion, along with a small component of thiocyanate aquation. The oxygen dependence results indicate that the remainder of the thiocyanate aquation is induced by the first triplet excited state of the donor. It is concluded that, in this system, the singlet excited state of acridinium ion populates the first quartet excited state of the complex ion while the triplet state populates the doublet excited state of the complex. Some of the complexities of excited state processes of Cr(III) complexes are discussed.

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Zusammenfassung Das Biacetyl und das Acridinium, nicht aber Michler's Keton sensibilisieren die Aquotisierung von Cr(NH_{3 5}(NCS)²⁺ in 0,1 n-Schwefelsäure-Lösung. Phosphoreszenzlöschung tritt im erstgenannten System auf und seine Kinetik, sowie die der sensibilisierten Reaktion, in Gegenwart sowie unter Ausschluß von Sauerstoff weist darauf hin, daß der Übergang der Anregungsenergie vom ersten Triplettzustand des Biacetyls stattfindet. Die Reaktion führt anschließend zur Aquotisierung von Ammoniak. Diese Tatsache weist darauf hin, daß der erste angeregte Quartettzustand des Komplexes besetzt wird. Das Acridinium sensibilisiert sowohl die Ammoniakals auch die Thiocyanat-Aquotisierung, wobei die Gegenwart von Sauerstoff zwar letztere, jedoch nicht erstere beeinflußt. Die Kinetik der Fluoreszenzlöschung durch den Komplex sowie die der sensibilisierten Ammoniak-Aquotisierung deuten darauf hin, daß letzterer Prozeß durch den ersten angeregten Singulettzustand des Acridiniums induziert wird, zusammen mit einem kleinen Teil der Thiocyanat-Aquotisierung. Die Sauerstoffabhängigkeit zeigt, daß der Rest der Thiocyanat-Aquotisierung durch den ersten angeregten Triplettzustand des Donors induziert wird. Daraus wird der Schluß gezogen, daß in diesem System der angeregte Singlettzustand des Acridiniums zur Besetzung des ersten angeregten Quartettzustands des Komplexes führt, wohingegen die Energie des Triplettzustands auf den angeregten Dublettzustand des Komplexes übertragen wird. Einige Probleme der Prozesse zwischen angeregten Zuständen von Cr(III)-Komplexen werden diskutiert.

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Résumé Les ions biacétyle et acridinium, mais non pas la cétone de Michler, sensibilisent l'hydratation de Cr(NH₃)₅(NCS)²⁺ dans une solution d'acide sulforique 0,1 N. L'extinction de la phosphorescence se produit dans le premier système; sa cinétique ainsi que celle de la réaction sensibilisée en présence et en l'absence d'oxygène indiquent que le transfert d'énergie d'excitation se produit à partir du premier état triplet du biacétyle. La réaction est totalement celle de l'hydratation de l'ammoniac, ce qui indique que le premier quartet excité du complexe se trouve peuplé.L'ion acridinium sensibilise l'hydratation de l'ammoniac et du thiocyanate seule cette dernière réaction étant sensible à l'oxygène. La cinétique de l'extinction de fluorescence par le complexe ainse que celle de l'hydratation sensibilisée de l'ammoniac indiquent que ce dernier processus est induit par le premier état excité singulet de l'ion acridinium, avec parallèlement une faible hydratation du thiocyanate. Les résultats sur le rôle de l'oxygène montrent que le restant de l'hydratation du thiocyanate est induit par le premier triplet excité du donneur. On en conclue que dans ce système l'état excité singulet de l'ion acridinium peuple le premier quartet excité de l'ion complexe tandis que l'état triplet peuple l'état excité doublet du complexe. On discute certains détails complexes des processus d'excitation des complexes de Cr (III).

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Dedicated to the memory of Professor Hans-Ludwig Schläfer.     

Catalysis of dioxygen reduction at graphite electrodes by an adsorbed cobalt(ii) porphyrin

The electroreduction of dioxygen to hydrogen peroxide is catalyzed when the Co(II) porphyrin shown in Fig. 1 is adsorbed on the surface of graphite electrodes. The electroreduction of the adsorbed catalyst occurs at potentials well separated from those where the dioxygen is reduced showing that more than a simple redox catalysis is involved. The porphyrin also catalyzes the electro-oxidatoin of hydrogen peroxide. In 1 M NaOH catalyst-coated electrodes yield reversible, almost nernstian responses to the $\text{O}{}_2\text{O}{}_2\text{H}{}^{\mathrm{?}}$ couple. Cyclic and rotating disk voltammetry were used to examine the kinetics of the catalyzed reactions and mechanistic schemes are proposed to account for the observed kinetics.     

Theoretical study and rate constant calculation for reaction of CF(3)CH(2)OH with OH

The reaction mechanism of CF(3)CH(2)OH with OH is investigated theoretically and the rate constants are calculated by direct dynamics method. The potential energy surface (PES) information, which is necessary for dynamics calculation, is obtained at the B3LYP/6-311G (d, p) level. The single-point energy calculations are performed at the MC-QCISD level using the B3LYP geometries. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. By means of isodesmic reactions, the enthalpies of the formation for the species CF(3)CH(2)OH, CF(3)CHOH, and CF(3)CH(2)O are estimated at the MC-QCISD//B3LYP/6-311G (d, p) level of theory. The rate constants for two kinds of H-abstraction channels are evaluated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) over a wide range of temperature 200-2000 K. The calculated results are in good agreement with the experimental values in the temperature region 250-430 K. The present results indicate that the two channels are competitive. Below 289 K, hydroxyl-H-abstraction channel has more contribution to the total rate constants than methylene-H-abstraction channel, while above 289 K, methylene-H-abstraction channel becomes more important and then becomes the major reaction channel.     

Electronic and molecular surface structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 adsorbed from solution onto nanostructured TiO2: a photoelectron spectroscopy study

The element specificity of photoelectron spectroscopy (PES) has been used to compare the electronic and molecular structure of the dyes Ru(tcterpy)(NCS)3 (BD) and Ru(dcbpy)2(NCS)2 adsorbed from solution onto nanostructured TiO2. Ru(dcbpy)2(NCS)2 was investigated in its acid (N3) and in its 2-fold deprotonated form (N719) having tetrabutylammonium (TBA+) as counterions. A comparison of the O1s spectra for the dyes indicates that the interactions through the carboxylate groups with the TiO2 surface are very similar for the dyes. However, we observe that some of the dye molecules also interact through the NCS groups when adsorbed at the TiO2 surface. Comparing the N719 and the N3 molecule, the fraction of NCS groups interacting through the sulfur atoms is smaller for N719 than for N3. We also note that the counterion TBA+ is coadsorbed with the N719 and BD molecules although the amount was smaller than expected from the molecular formulas. Comparing the valence levels for the dyes adsorbed on TiO2, the position of the highest occupied electronic energy level is similar for N3 and N719, while that for BD is lower by 0.25 eV relative to that of the other complexes.     

[Cr(H2O)3(NCS)3](18-冠-6)的分子和晶体结构

本文报道了[Cr(H2O)3(NCS)3](18-冠-6)配合物的单晶X射线结构 分析 , 并基于分析结果, 对分子的结构特点和晶胞中的堆积方式进行了讨论.     

General expression of the linear potential sweep voltammogram for a surface redox reaction with interactions between the adsorbed molecules: Applications to modified electrodes

The equation of the linear potential sweep voltammogram is derived for any degree of reversibility of the electrochemical reaction for a surface redox system, when interactions between the adsorbed molecules are present. When the reaction is reversible (k_s/v→∞; k_s: rate of the electrochemical reaction; v=sweep rate), the cathodic and anodic peaks are identical. When the reaction is not reversible the shape of the cathodic and anodic peaks can become widely different, depending on the interaction coefficients. The characteristics of the peaks are studied. Applications to submonolayer adsorption and to redox polymer modified electrodes are discussed.     

Frontier electronic structures of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2: a photoelectron spectroscopy study

The frontier electronic structures of Ru(tcterpy)(NCS)3 [black dye (BD)] and Ru(dcbpy)2(NCS)(2) (N719) have been investigated by photoelectron spectroscopy (PES), X-ray absorption spectroscopy (XAS) and resonant photoelectron spectroscopy (RPES). N1s XAS has been used to probe the nitrogen contribution in the unoccupied density of states, and PES, together with RPES over the N1s edge, has been used to delineate the character of the occupied density of states. The experimental findings of the frontier electron structure are compared to calculations of the partial density of states for the nitrogens in the different ligands (NCS and terpyridine/bipyridine) and for Ru4d. The result indicates large similarities between the two complexes. Specifically, the valence level spectra show two well separated structures at low binding energy. The experimental results indicate that the outermost structure in the valence region largely has a Ru4d character but with a substantial character also from the NCS ligand. Interestingly, the second lowest structure also has a significant Ru4d character mixed into the structure otherwise dominated by NCS. Comparing the two complexes the BD valence structures lowest in binding energy contains a large contribution from the NCS ligands but almost no contribution from the terpyridine ligands, while for N719 also some contribution from the bipyridine ligands is mixed into the energy levels.     

The reaction of O(1D) with H2O and the reaction of OH with C3H6

N₂O was photolyzed at 2139 Å to produce O(¹D) atoms in the presence of H₂O and CO. The O(¹D) atoms react with H₂O to produce HO radicals, as measured by CO₂ production from the reaction of OH with CO. The relative importance of the various possible O(¹D )–H₂O reactions is The relative rate constant for O(¹D) removal by H₂O compared to that by N₂O is 2.1, in good agreement with that found earlier in our laboratory. In the presence Of C₃H₆, the OH can be removed by reaction with either CO or C₃H₆: From the CO₂ yield, k₃/k₂ = 75,0 at 100°C and 55.0 at 200°C to within ± 10%. When these values are combined with the value of k₂ = 7.0 × 10^?13exp (–1100/RT) cm³/sec, k₃ = 1.36 × 10^?11 exp (–100/RT) cm³/sec. At 25°C, k₃ extrapolates to 1.1 × 10^?11 cm³/sec.     

Kinetics and mechanism of (CF3)2CHOCH3 reaction with OH radicals in an environmental reaction chamber

The atmospheric chemistry of (CF3)2CHOCH3, a possible HCFC/HFC alternative, was studied using a smog chamber/FT-IR technique. OH radicals were prepared by the photolysis of ozone in a 200-Torr H2O/O3/O2 gas mixture held in an 11.5-dm3 temperature-controlled chamber. The rate constant, k1, for the reaction of (CF3)2CHOCH3 with OH radicals was determined to be (1.40 +/- 0.28) x 10(-12) exp[(-550 +/- 60)/T] cm3 molecule(-1) s(-1) by means of a relative rate method at 253-328 K. The value of k1 at 298 K was (2.25 +/- 0.04) x 10(-13) cm3 molecule(-1) s(-1). The random errors are reported with +/-2 standard deviations, and potential systematic errors of 15% could increase k(1). In considering OH-radical reactions, we estimated the tropospheric lifetime of (CF3)2CHOCH3 to be 2.0 months using the rate constant at 288 K. The degradation mechanism of (CF3)2CHOCH3 initiated by OH radicals was also investigated using FT-IR spectroscopy at 298 K. Products (CF3)2CHOC(O)H, CF3C(OH)2CF3, CF3C(O)OCH3, and COF(2) were identified and quantified. The branching ratio, k1a/k1b, was estimated to be 2.1:1 for reactions (CF3)2CHOCH3 + OH --> (CF3)2CHOCH2*+ H2O (k1a) and (CF3)2CHOCH3 + OH --> (CF3)2C*OCH3 + H2O (k1b).     

Mechanisms for the deamination reaction of cytosine with H2O/OH(-) and 2H2O/OH(-): a computational study

Mechanisms for the deamination reaction of cytosine with H 2O/OH (-) and 2H 2O/OH (-) to produce uracil were investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at MP2 and B3LYP using the 6-31G(d) basis set and at B3LYP/6-31+G(d) levels of theory. Single point energies were also determined at MP2/G3MP2Large and G3MP2 levels of theory. Thermodynamic properties (Delta E, Delta H, and Delta G), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway investigated. Intrinsic reaction coordinate (IRC) analysis was performed to characterize the transition states on the potential energy surface. Seven pathways for the deamination reaction were found. All pathways produce an initial tetrahedral intermediate followed by several conformational changes. The final intermediate for all pathways dissociates to product via a 1-3 proton shift. The activation energy for the rate-determining step, the formation of the tetrahedral intermediate for pathway D, the only pathway that can lead to uracil, is 115.3 kJ mol (-1) at the G3MP2 level of theory, in excellent agreement with the experimental value (117 +/- 4 kJ mol (-1)).     

Structural features of the electrode/solution interface at the reduction of Cr3+(aq) cations on liquid mercury and solid indium electrodes in acidic media

Characteristics of electroreduction of Cr³⁺ and Cr²⁺ ions on the dropping mercury (literature data) and indium cathodes are analyzed as a function of pH of the medium. A hypothesis that polymeric hydroxocompounds of chromium, which are present in different aggregate states, represent the main or partial reactants in the $ Cr\left( {III} \right)\xrightarrow{{ + e}}Cr\left( {II} \right)\xrightarrow{{ + 2e}}Cr Characteristics of electroreduction of Cr³⁺ and Cr²⁺ ions on the dropping mercury (literature data) and indium cathodes are analyzed as a function of pH of the medium. A hypothesis that polymeric hydroxocompounds of chromium, which are present in different aggregate states, represent the main or partial reactants in the stages is additionally substantiated. Original Russian Text ? V.N. Korshunov, V.A. Safonov, L.N. Vykhodtseva, 2008, published in Elektrokhimiya, 2008, Vol. 44, No. 3, pp. 275–285.     

包覆Y(OH)3的球形Ni(OH)2的电化学性能

利用软嵌式粉末电极技术研究了Y(OH)3包覆对球形Ni(OH)2电化学性能的影响. 循环伏安结果表明, 在球形Ni(OH)2的氧化过程中存在Ni(Ⅲ)和Ni(Ⅳ)的两步氧化反应, 产生的Ni(Ⅳ)不稳定, 能分解产生NiOOH和氧气, 所以可将Ni(Ⅲ)→Ni(Ⅳ)看作副反应. Y(OH)3包覆层对Ni(OH)2氧化过程后期的副反应, 特别是Ni(Ⅲ)→Ni(Ⅳ)具有较好的抑制作用. 由包覆后的Ni(OH)2制成的模拟电池表现出很好的高温性能, 在1C充放电条件下, 当Y的摩尔分数为1.61%时, 在60 ℃时所对应的容量保持率可达到25 ℃的92.7％; 当Y的摩尔分数仅为0.55 %时, 在60 ℃时所对应的质量比容量也可达到241.3 mA·h/g.     

Computational study of the reaction of CH2(X3B1) with CH3OH

The reaction of triplet methylene with methanol is a key process in alcohol combustion but surprisingly this reaction has never been studied. The reaction mechanism is investigated by using various high-level ab initio methods, including the complete basis set extrapolation (CBS-QB3 and CBS-APNO), the latest Gaussian-n composite method (G4), and the Weizmann-1 method (W1U). A total of five product channels and six transition states are found. The dominant mechanism is direct hydrogen abstraction, and the major product channel is CH(3) + CH(3)O, involving a weak prereactive complex and a 7.4 kcal/mol barrier. The other hydrogen abstraction channel, CH(3) + CH(2)OH, is less important even though it is more exothermic and involves a similar barrier height. The rate coefficients are predicted in the temperature range 200-3000 K. The tunneling effect and the hindered internal rotational freedoms play a key role in the reaction. Moreover, the reaction shows significant kinetic isotope effect.