Insights into the mechanism of O₂ formation and release from the Mn₄O₄L₆ "cubane" cluster

To probe photoinduced water oxidation catalyzed by the Mn?O?L? cubane clusters, we have computationally studied the mechanism and controlling factors of the O? formation from the [Mn?O?L?] catalyst, 6. It was demonstrated that dissociation of an L = H?PO?? ligand from 6 facilitates the direct O-O bond formation that proceeds with a 28.3 (33.4) kcal/mol rate-determining energy barrier at the transition state TS1. This step (the O-O single bond formation) of the reaction is a two-electron oxidation/reduction process, during which two oxo ligands are transformed into to μ2:η2-O?2? unit, and two ("distal") Mn centers are reduced from the 4+ to the 3+ oxidation state. Next two-electron oxidation/reduction occurs by "dancing" of the resulted O?2? fragment between the Mn1 and Mn2/Mn(2')-centers, keeping its strong coordination to the Mn(1')-center. As a result of this four-electron oxidation/reduction process Mn centers of the Mn?-core of I transform from {Mn1(III)-Mn(1')(III)-Mn2(IV)-Mn(2')(IV)} to {Mn1(II)-Mn(1')(II)-Mn2(III)-Mn(2')(III)} in IV. In other words, upon O? formation in cationic complex [Mn?O?L?](+), I, all four Mn-centers are reduced by one electron each. The overall reaction I → TS1 → II → III → TS2 → IV → TS3 → V → VI + O? is found to be exothermic by 15.4 (10.5) kcal/mol. We analyze the lowest spin states and geometries of all reactants, intermediates, transition states, and products of the targeted reaction.     

Electron exchange between alpha-Keggin tungstoaluminates and a well-defined cluster-anion probe for studies in electron transfer

Fully oxidized alpha-AlIIIW12O40(5-) (1ox), and one-electron-reduced alpha-AlIIIW12O40(6-) (1red), are well-behaved (stable and free of ion pairing) over a wide range of pH and ionic-strength values at room temperature in water. Having established this, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1ox (27Al NMR: 72.2 ppm relative to Al(H2O)63+; nu(1/2) = 0.77 Hz) and 1red (74.1 ppm; nu(1/2) = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in 27Al NMR signals as ionic strength, mu, is increased by addition of NaCl at the slow-exchange limit of the NMR time scale. The dependence of k on is plotted using the extended Debye-Hückel equation: log k = log k0 + 2alphaz1z2mu(1/2)/(1 + betarnu(1/2)), where z1 and z2 are the charges of 1ox and 1red, alpha and beta are constants, and r, the distance of closest contact, is fixed at 1.12 nm, the crystallographic diameter of a Keggin anion. Although not derived for highly charged ions, this equation gives a straight line (R2 = 0.996), whose slope gives a charge product, z1z2, of 29 +/- 2, statistically identical to the theoretical value of 30. Extrapolation to mu = 0 gives a rate constant k11 of (6.5 +/- 1.5) x 10(-3) M(-1) s(-1), more than 7 orders of magnitude smaller than the rate constant [(1.1 +/- 0.2) x 10(5) M(-1) s(-1)] determined by 31P NMR for self-exchange between P(V)W12O40(3-) and its one-electron-reduced form, P(V)W12O40(4-). Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1ox and 1red. These results, including independent verification of k11, recommend 1red as a well-behaved electron donor for investigating outer-sphere electron transfer to molecules or nanostructures in water, while addressing a larger issue, the prediction of collision rates between uniformly charged nanospheres, for which 1ox and 1red provide a working model.     

Water oxidation catalyzed by a new tetracobalt-substituted polyoxometalate complex: [{Co4(μ-OH)(H2O)3}(Si2W19O70)]11-

A new polyoxometalate of earth adundant elements [{Co(4)(μ-OH)(H(2)O)(3)}(Si(2)W(19)O(70))](11-) has been synthesized, characterized and shown to be a water oxidation catalyst. The initial catalytic complex is unstable and slowly undergoes hydrolysis. The hydrolysis products have been isolated and characterized, and their catalytic water oxidation activity is assessed.     

A homogeneous catalyst for selective O(2) oxidation at ambient temperature. diversity-based discovery and mechanistic investigation of thioether oxidation by the Au(III)Cl(2)NO(3)(thioether)/O(2) system.

A library of inorganic complexes with reversible redox chemistry and/or the ability to catalyze homogeneous oxidations by peroxides, including but not limited to combinations of polyoxometalate anions and redox-active cations, was constructed. Evaluation of library members for the ability to catalyze aerobic sulfoxidation (O(2) oxidation of the thioether, 2-chloroethyl ethyl sulfide, CEES) led to the discovery that a combination of HAuCl(4) and AgNO(3) forms a catalyst that is orders of magnitude faster than the previously most reactive such catalysts (Ru(II) and Ce(IV) complexes) and one effective at ambient temperature and 1 atm air or O(2). If no O(2) but high concentrations of thioether are present, the catalyst is inactivated by an irreversible formation of colloidal Au(0). However, this inactivation is minimal in the presence of O(2). The stoichiometry is R(2)S + (1)/(2)O(2) --> R(2)S(O), a 100% atom efficient oxygenation, and not oxidative dehydrogenation. However, isotope labeling studies with H(2)(18)O indicate that H(2)O and not O(2) or H(2)O(2) is the source of oxygen in the sulfoxide product; H(2)O is consumed and subsequently regenerated in the mechanism. The rate law evaluated for every species present in solution, including the products, and other kinetics data, indicate that the dominant active catalyst is Au(III)Cl(2)NO(3)(thioether) (1); the rate-limiting step involves oxidation of the substrate thioether (CEES) by Au(III); reoxidation of the resulting Au(I) to Au(III) by O(2) is a fast subsequent step. The rate of sulfoxidation as Cl is replaced by Br, the solvent kinetic isotope effect (k(H)(2)(O)/k(D)(2)(O) = 1.0), and multiparameter fitting of the kinetic data establish that the mechanism of the rate-limiting step involves a bimolecular attack of CEES on a Au(III)-bound halide and it does not involve H(2)O. The reaction is mildly inhibited by H(2)O and the CEESO product because these molecules compete with those needed for turnover (Cl(-), NO(3)(-)) as ligands for the active Au(III). Kinetic studies using DMSO as a model for CEESO enabled inhibition by CEESO to be assessed.     

砷化镓光阴极直流高压注入器研究进展

主要介绍自由电子激光相干强太赫兹源（FEL-THz）装置上的砷化镓光阴极直流高压注入器的研究进展,并讨论其驱动未来高重复频率短波长自由电子激光器的差距。通过综合砷化镓阴极寿命的三大影响因素,提出了其工作寿命的定性物理模型;通过该模型对阴极和注入器进行优化,在直流高压电子枪上得到了5 mA, 32 min的连续稳定输出;测量了电子束在4.8 mA下归一化发射度约为4.0 mmmrad,阴极热发射度约为0.6 mmmrad,电子束本征横向能量约为92 meV,250 keV电子束在距离阴极90.6 cm处纵向均方根长度约为11.5 ps。这一束流状态已经基本满足FEL-THz需求。     

Can the ebselen derivatives catalyze the isomerization of peroxynitrite to nitrate?

The reaction of ebselen and its derivatives (1-7) with peroxynitrite anion (ONOO(-); PN) has been studied in gas phase and in aqueous, dichloromethane, benzene, and cyclohexane solutions using B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and PCM-B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) approaches, respectively. It was shown that the reaction of 2 (R=H) with PN proceeds via 2 + PN --> 2-PN --> 2-TS1 (O-O activation) --> 2-O(NO(2)(-)()) --> 2-SeO + NO(2)(-) pathway with a rate-determining barrier of 25.3 (14.8) kcal/mol at the NO(2)(-) dissociation step (numbers presented without parentheses are enthalpies, and those in parentheses are Gibbs free energies). The NO(3)(-) formation process, starting from the complex 2-O(NO(2)(-)()), requires by (7.9) kcal/mol more energy than the NO(2)(-) dissociation process and is unlikely to compete with the latter. Thus, in the gas phase, the peroxynitrite --> nitrate isomerization catalyzed by complex 2 is unlikely to occur. It is shown that the NO(3)(-) formation process is slightly more favorably than the NO(2)(-) dissociation process for complex 4, with a strongest electron-withdrawing ligand R=CF(3). Therefore, complex 4 (as well as complex 6 with R=OH) is predicted to be a good catalyst for peroxynitrite <--> nitrite isomerization in the gas phase. Solvent effects (a) change the rate-determining step of the reaction 2 + PN from NO(2)(-) dissociation in the gas phase to O-O activation, which occurs with barriers of (13.9), (8.4), (8.4), and (8.2) kcal/mol in water, dichloromethane, benzene, and cyclohexane, respectively, and (b) significantly reduce the NO(2)(-) dissociation energy, while only slightly destabilizing the NO(3)(-) formation barrier, and make the peroxynitrite <--> nitrate isomerization process practically impossible, even for complex 4.     

Formation and redox properties of a complex of phenazine di-N-oxide with a proton

The electrochemical properties of phenazine di-N-oxide (OPO) in acetonitrile solutions in the presence of acids were studied by the method of cyclic voltammetry (CVA). It was shown that OPO forms with a proton a complex having a 2:1 composition, its oxidation and reduction potentials being equal to 1.57 and 0.35 V (saturated calomel electrode), respectively. The same protonated form is also formed in the reaction of OPO^+. with organic substrates, in particular with hydrocarbons and alcohols. The reaction mechanism includes a complexation of OPO^+. with the substrate, followed by splitting off of a hydrogen from the complex by another OPO^+· cation-radical or a proton by the initial OPO. A rapid selective deoxygenation of the protonated form of OPO by iodide ion in acetonitrile in an acid medium has been discovered.Institute of Chemical Physics in Chernogolovka, Russian Academy of Sciences, Chernogolovka 142432. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 845–851, April, 1992.     

Density functional study of the roles of chemical composition of di-transition-metal-substituted gamma-Keggin polyoxometalate anions

The roles of chemical composition (X, M and M(FW)) of di-transition-metal-substituted gamma-Keggin polytungstates and polymolybdates, [(X(n)(+)O(4))M(2)(OH)(2)(M(FW))(10)O(32)]((8-n)-), on the geometry, electronic structure, and magnetic properties of these species have been investigated at the density functional level. It was shown that the change of the heteroatom X via Al(III)-Si(IV)-P(V)-S(VI) slightly stabilizes the broken-symmetry (BS) state over the high-spin (HS) state, increases the antiferromagnetic coupling constant, J, of these species, and lowers the energies of their highest-occupied molecular orbitals (HOMOs) and lowest-unoccupied molecular orbitals (LUMOs). The change of the redox-active center M from Mn to Fe slightly increases the M-(XO(4)) interaction, J-coupling constant, and energy gap between the HS and BS states. Meanwhile, the LUMOs are stabilized, indicating the stronger oxidant character of [(X(n)(+)O(4))M(2)(OH)(2)W(10)O(32)]((8-n)-) for M = Fe than Mn. It was shown that the change of addenda atom M(FW) from W to Mo makes (a) the geometry of Keggin "cage" slightly smaller, (b) the interaction of redox-active centers (Fe) with the central XO(4)-unit slightly stronger, and (c) the J-coupling constant, as well as the energy gap DeltaE(BS-HS), slightly larger.     

Shape resonances in slow electron scattering by aromatic molecules. I. Anthraquinone derivatives

A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.     

Oxidation rate of saturated hydrocarbons by permanganate in aqueous solutions

Conclusions The reaction for the oxidation of alkanes in aqueous permanganate solutions is first order in both substrate and oxidizing agent. The reaction rate constants were measured for a number of C₃-C₇ normal, iso-, and cycloalkanes. The normal type of selectivity is fulfilled: the tertiary C-H bonds are more active, while the primary C-H bonds are less active than the secondary C-H bonds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1654–1656, July, 1982.