Borate-catalyzed reactions of hydrogen peroxide: kinetics and mechanism of the oxidation of organic sulfides by peroxoborates

The kinetics of the oxidation of substituted phenyl methyl sulfides by hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. Second-order rate constants at 25 degrees C for the reaction of methyl 4-nitrophenyl sulfide and H(2)O(2), monoperoxoborate, HOOB(OH)(3) (-), or diperoxoborate, (HOO)(2)B(OH)(2) (-), are 8.29 x 10(-5), 1.51 x 10(-2) and 1.06 x 10(-2) M(-1) s(-1), respectively. Peroxoboric acid, HOOB(OH)(2), is unreactive. The Hammett rho values for the reactions of a range of substituted phenyl methyl sulfides and hydrogen peroxide, monoperoxoborate or diperoxoborate are -1.50 +/- 0.1, -0.65 +/- 0.07 and -0.48 (two points only), respectively. The rho values for the peroxoborates are of significantly lower magnitude than expected from their reactivity compared to other peroxides. Nevertheless the negative rho values indicate positive charge development on the sulfur atom in the transition state consistent with nucleophilic attack by the organic sulfides on the peroxoborates as with the other peroxides. The kinetic parameters, including the lack of reactivity of peroxoboric acid, are discussed in terms of the differences in the transition state of reactions involving peroxoboron species with respect to those of other peroxides.     

Photochemistry of peroxoborates: borate inhibition of the photodecomposition of hydrogen peroxide

The UV absorbance and photochemical decomposition kinetics of hydrogen peroxide in borate/boric acid buffers were investigated as a function of pH, total peroxide concentration, and total boron concentration. At higher pH borate/boric acid inhibits the photodecomposition of hydrogen peroxide (molar absorptivity and quantum yield of H(2)O(2) and HO(2) (-), (19.0+/-0.3) M(-1) cm(-1) and 1, and (237+/-7) M(-1) cm(-1) and 0.8+/-0.1, respectively). The results are consistent with the equilibrium formation of the anions monoperoxoborate, K(BOOH)=[H(+)][HOOB(OH)(3) (-)]/([B(OH)(3)][H(2)O(2)]), 2.0 x 10(-8), R. Pizer, C. Tihal, Inorg. Chem. 1987, 26, 3639-3642, and monoperoxodiborate, K(BOOB)=[BOOB(2-)]/([B(OH)(4) (-)][HOOB(OH)(3) (-)]), 1.0+/-0.3 or 4.3+/-0.9, depending upon the conditions, with molar absorptivity, (19+/-1) M(-1) cm(-1) and (86+/-15) M(-1) cm(-1), respectively, and respective quantum yields, 1.1+/-0.1 and 0.04+/-0.04. The low quantum yield of monoperoxodiborate is discussed in terms of the slower diffusion apart of incipient (.)OB(OH)(3) (-) radicals than may be possible for (.)OH radicals, or a possible oxygen-bridged cyclic structure of the monoperoxodiborate.     

Basic ancillary ligands promote O-O bond formation in iridium-catalyzed water oxidation: a DFT study

The cationic iridium complex [Ir(OH(2))(2)(phpy)(2)](+) (phpy = o-phenylpyridine) is among the most efficient mononuclear catalysts for water oxidation. The postulated active species is the oxo complex [Ir(O)(X)(phpy)(2)](n), with X = OH(2) (n = +1), OH(-) (n = 0) or O(2-) (n = -1), depending on the pH. The reactivity of these species has been studied computationally at the DFT(B3LYP) level. The three [Ir(O)(X)(phpy)(2)](n) complexes have an electrophilic Ir(v)-oxo moiety, which yields an O-O bond by undergoing a nucleophilic attack of water in the critical step of the mechanism. In this step, water transfers one proton to either the Ir(V)-oxo moiety or the ancillary X ligand. Five different reaction pathways associated with this acid/base mechanism have been characterized. The calculations show that the proton is preferably accepted by the X ligand, which plays a key role in the reaction. The higher the basicity of X, the lower the energy barrier associated with O-O bond formation. The anionic species, [Ir(O)(2)(phpy)(2)](-), which has the less electrophilic Ir(V)-oxo moiety but the most basic X ligand, promotes O-O bond formation through the lowest energy barrier, 14.5 kcal mol(-1). The other two active species, [Ir(O)(OH)(phpy)(2)] and [Ir(O)(OH(2))(phpy)(2)](+), which have more electrophilic Ir(V)-oxo moieties but less basic X ligands, involve higher energy barriers, 20.2 kcal mol(-1) and 25.9 kcal mol(-1), respectively. These results are in good agreement with experiments showing important pH effects in similar catalytic systems. The theoretical insight given by the present study can be useful in the design of more efficient water oxidation catalysts. The catalytic activity may increase by using ligand scaffolds bearing internal bases.     

Periodic density functional theory study of methane activation over La(2)O(3): activity of O(2-), O(-), O(2)(2-), oxygen point defect,and Sr(2+)-doped surface sites

Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at O(s)(2-), O(s)(-), O(2)(s)(2-) point defect, and Sr(2+)-doped surface sites on La(2)O(3)(001). The results show that the anionic O(s)(-) species is the most active surface oxygen site. The overall reaction energy to activate methane at an O(s)(-) site to form a surface hydroxyl group and gas-phase (*)CH(3) radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen at an site O(s)(-) is -102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium into the oxide by a direct Sr(2+)/La(3+) exchange at the surface. The O(-)-like nature of the surface site is reflected in a calculated hydrogen binding energy of -109.7 kcal/mol. Calculations indicate that surface peroxide (O(2(s))(2-)) sites can be generated by adsorption of O(2) at surface oxygen vacancies, as well as by dissociative adsorption of O(2) across the closed-shell oxide surface of La(2)O(3)(001). The overall reaction energy and apparent activation barrier for the latter pathway are calculated to be only 12.1 and 33.0 kcal/mol, respectively. Irrespective of the route to peroxide formation, the O(2)(s)(2-) intermediate is characterized by a bent orientation with respect to the surface and an O-O bond length of 1.47 A; both attributes are consistent with structural features characteristic of classical peroxides. We found surface peroxide sites to be slightly less favorable for H-abstraction from methane than the O(s)(-) species, with DeltaE(rxn)(CH(4)) = 39.3 kcal/mol, E(act) = 47.3 kcal/mol, and DeltaE(ads)(H) = -71.5 kcal/mol. A possible mechanism for oxidative coupling of methane over La(2)O(3)(001) involving surface peroxides as the active oxygen source is suggested.     

Oxidative reactivity difference among the metal oxo and metal hydroxo moieties: pH dependent hydrogen abstraction by a manganese(IV) complex having two hydroxide ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of nature's choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).     

Oxidation of vanadium(III) by hydrogen peroxide and the oxomonoperoxo vanadium(V) ion in acidic aqueous solutions: a kinetics and simulation study

The reaction between vanadium(III) and hydrogen peroxide in aqueous acidic solutions was investigated. The rate law shows first-order dependences on both vanadium(III) and hydrogen peroxide concentrations, with a rate constant, defined in terms of -d[H(2)O(2)]/dt, of 2.06 +/- 0.03 L mol(-)(1) s(-)(1) at 25 degrees C; the rate is independent of hydrogen ion concentration. The varying reaction stoichiometry, the appreciable evolution of dioxygen, the oxidation of 2-PrOH to acetone, and the inhibition of acetone formation by the hydroxyl radical scavengers, dimethyl sulfoxide and sodium benzoate, point to a Fenton mechanism as the predominant pathway in the reaction. Methyltrioxorhenium(VII) does not appear to catalyze this reaction. A second-order rate constant for the oxidation of V(3+) by OV(O(2))(+) was determined to be 11.3 +/- 0.3 L mol(-)(1) s(-)(1) at 25 degrees C. An overall reaction scheme consisting of over 20 reactions, in agreement with the experimental results and literature reports, was established by kinetic simulation studies.     

Olefin epoxidation with bis(trimethylsilyl) peroxide catalyzed by inorganic oxorhenium derivatives. Controlled release of hydrogen peroxide.

The replacement of organometallic rhenium species (e.g., CH(3)ReO(3)) by less expensive and more readily available inorganic rhenium oxides (e.g., Re(2)O(7), ReO(3)(OH), and ReO(3)) can be accomplished using bis(trimethylsilyl) peroxide (BTSP) as oxidant in place of aqueous H(2)O(2). Using a catalytic amount of a proton source, controlled release of hydrogen peroxide helps preserve sensitive peroxorhenium species and enables catalytic turnover to take place. Systematic investigation of the oxorhenium catalyst precursors, substrate scope, and effects of various additives on olefin epoxidation with BTSP are reported in this contribution.     

A multilateral mechanistic study into asymmetric transfer hydrogenation in water

The mechanism of aqueous-phase asymmetric transfer hydrogenation (ATH) of acetophenone (acp) with HCOONa catalyzed by Ru-TsDPEN has been investigated by stoichiometric reactions, NMR probing, kinetic and isotope effect measurements, DFT modeling, and X-ray structure analysis. The chloride [RuCl(TsDPEN)(p-cymene)] (1), hydride [RuH(TsDPEN)(p-cymene)] (3), and the 16-electorn species [Ru(TsDPEN-H)(p-cymene)] (4) were shown to be involved in the aqueous ATH, with 1 being the precatalyst, and 3 as the active catalyst detectable by NMR in both stoichiometric and catalytic reactions. The formato complex [Ru(OCOH)(TsDPEN)(p-cymene)] (2) was not observed; its existence, however, was demonstrated by its reversible decarboxylation to form 3. Both 1 and 3 were protonated under acidic conditions, leading to ring opening of the TsDPEN ligand. 4 reacted with water, affording a hydroxyl species. In a homogeneous DMF/H(2)O solvent, the ATH was found to be first order in the concentration of catalyst and acp, and inhibited by CO(2). In conjunction with the NMR results, this suggests that hydrogen transfer to ketone is the rate-determining step. The addition of water stabilized the ruthenium catalyst and accelerated the ATH reaction; it does so by participating in the catalytic cycle. DFT calculations revealed that water hydrogen bonds to the ketone oxygen at the transition state of hydrogen transfer, lowering the energy barrier by about 4 kcal mol(-1). The calculations also suggested that the hydrogen transfer is more step-wise in nature rather than concerted. This is supported to some degree by the kinetic isotope effects, which were obscured by extensive H/D scrambling.     

Olefin epoxidation by the hydrogen peroxide adduct of a novel non-heme mangangese(IV) complex: demonstration of oxygen transfer by multiple mechanisms

Olefin epoxidations are a class of reactions appropriate for the investigation of oxygenation processes in general. Here, we report the catalytic epoxidation of various olefins with a novel, cross-bridged cyclam manganese complex, Mn(Me2EBC)Cl2 (Me2EBC is 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), using hydrogen peroxide as the terminal oxidant, in acetone/water (ratio 4:1) as the solvent medium. Catalytic epoxidation studies with this system have disclosed reactions that proceed by a nonradical pathway other than the expected oxygen-rebound mechanism that is characteristic of high-valent, late-transition-metal catalysts. Direct treatment of olefins with freshly synthesized [Mn(IV)(Me2EBC)(OH)2](PF6)2 (pKa = 6.86) in either neutral or basic solution confirms earlier observations that neither the oxo-Mn(IV) nor oxo-Mn(V) species is responsible for olefin epoxidization in this case. Catalytic epoxidation experiments using the 18O labels in an acetone/water (H2(18)O) solvent demonstrate that no 18O from water (H2(18)O) is incorporated into epoxide products even though oxygen exchange was observed between the Mn(IV) species and H2(18)O, which leads to the conclusion that oxygen transfer does not proceed by the well-known oxygen-rebound mechanism. Experiments using labeled dioxygen, (18)O2, and hydrogen peroxide, H2(18)O2, confirm that an oxygen atom is transferred directly from the H2(18)O2 oxidant to the olefin substrate in the predominant pathway. The hydrogen peroxide adduct of this high-oxidation-state manganese complex, Mn(IV)(Me2EBC)(O)(OOH)+, was detected by mass spectra in aqueous solutions prepared from Mn(II)(Me2EBC)Cl2 and excess hydrogen peroxide. A Lewis acid pathway, in which oxygen is transferred to the olefin from that adduct, Mn(IV)(Me2EBC)(O)(OOH)+, is proposed for epoxidation reactions mediated by this novel, non-heme manganese complex. A minor radical pathway is also apparent in these systems.     

The self-assembly mechanism of the Lindqvist anion [W(6)O(19)](2-) in aqueous solution: a density functional theory study

The formation mechanism is always a fundamental and confused issue for polyoxometalate chemistry. Two formation mechanisms (M1 and M2) of the Lindqvist anion [W(6)O(19)](2-) have been adopted to investigate it's self-assembly reaction pathways at a density functional theory (DFT) level. The potential energy surfaces reveal that both the mechanisms are thermodynamically favorable and overall barrierless at room temperature, but M2 is slightly dominant to M1. The formation of the pentanuclear species [W(5)O(16)](2-) and [W(5)O(15)(OH)](-) are recognized as the rate-determining steps in the whole assembly polymerization processes. These two steps involve the highest energy barriers with 30.48 kcal mol(-1) and 28.90 kcal mol(-1), respectively, for M1 and M2. [W(4)O(13)](2-) and [W(4)O(12)(OH)](-) are proved to be the most stable building blocks. In addition, DFT results reveal that the formation of [W(3)O(10)](2-) experiences a lower barrier along the chain channel.     

Surprising coordination for plutonium in the first plutonium(III) borate

The first plutonium(III) borate, Pu(2)[B(12)O(18)(OH)(4)Br(2)(H(2)O)(3)]·0.5H(2)O, has been prepared by reacting plutonium(III) with molten boric acid under strictly anaerobic conditions. This compound contains a three-dimensional polyborate network with triangular holes that house the plutonium(III) sites. The plutonium sites in this compound are 9- and 10-coordinate and display atypical geometries.     

298.15K下Li2B4O7-H2O体系水蒸汽分压及渗透系数的等压测定和离子相互作用模型

在已有研究含硼体系的文献中仅考虑了硼酸根B4O7^-2或B(OH)4^-和H3BO3的存在，而对Li2B4O7-H2O体系具有多种硼物种聚合平衡体系的热力学性质的研究尚未见报道．本文用等压法研究了Li2B4O7-H2O体系于298．15K下浓度由稀到过饱和溶液的平衡气相蒸汽压及渗透系数．考虑了水溶液中多种硼物种的存在，以Pitzer方程为基础，建立了可描述该含硼体系的离子相互作用模型。     

Model of uptake of OH radicals on nonreactive solids

A model of adsorption and recombination of OH radicals was developed for nonreactive solid surfaces of atmospheric interest. A parametrization of this heterogeneous mechanism was carried out to determine the role of the catalytic properties of these solid surfaces, taking into account the adsorption energy, defects, surface diffusion, and chemical reactions in the gas-solid interface. The uptake process was simulated for diffusion-controlled chemical reactions on the surface on the basis of Langmuir-Hinshelwood and Eley-Rideal mechanisms. Using an analytical approach and the Monte Carlo technique, we show the dependencies of the uptake probability of the heterogeneous reactions on the OH concentration and adsorption energy. The model is employed in the analysis of the empirically derived uptake coefficient for water ice, Al(2)O(3), NaCl, NH(4)NO(3), NH(4)HSO(4), and (NH(4))(2)SO(4). We found the following values for the free energy of adsorption of OH radicals: E(ice) = 7.3-7.6 kcal/mol, E(Al)(2)(O)(3) = 11-11.7 kcal/mol, E(NH)(4)(NO)(3) = 10.2 kcal/mol, E(NaCl) = 10.2 kcal/mol, E(NH)(4)(HSO)(4) = 9.8 kcal/mol, and E((NH)(4))(2)(SO)(4) = 9.8 kcal/mol. The atmospheric implications of the catalytic reactions of OH with adsorbed reactive molecules are discussed. The results of the modeling of the uptake process showed that the heterogeneous decay rate can exceed the corresponding gas-phase reaction rate under atmospheric conditions.     

Effects of large halides on the structures of lanthanide(III) and plutonium(III) borates

Reactions of LnBr(3) or LnOI with molten boric acid result in formation of Ln[B(5)O(8)(OH)(H(2)O)(2)Br] (Ln = La-Pr), Nd(4)[B(18)O(25)(OH)(13)Br(3)], or Ln[B(5)O(8)(OH)(H(2)O)(2)I] (Ln = La-Nd). Reaction of PuOI with molten boric acid yields Pu[B(7)O(11)(OH)(H(2)O)(2)I]. The Ln(III) and Pu(III) centers in these compounds are found as nine-coordinate hula-hoop or 10-coordinate capped triangular cupola geometries where there are six approximately coplanar oxygen donors provided by triangular holes in the polyborate sheets. The borate sheets are connected into three-dimensional networks by additional BO(3) triangles and/or BO(4) tetrahedra that are roughly perpendicular to the layers. The room-temperature absorption spectrum of single crystals of Pu[B(7)O(11)(OH)(H(2)O)(2)I] shows characteristic f-f transitions for Pu(III) that are essentially indistinguishable from Pu(III) in other compounds with alternative ligands and different coordination environments.     

Flow-injection analysis of hydrogen peroxide based on carbon nanospheres catalyzed hydrogen carbonate-hydrogen peroxide chemiluminescent reaction

A flow-injection chemiluminescence (CL) system with high sensitivity, selectivity, rapidity, and reproducibility is proposed for the determination of hydrogen peroxide (H(2)O(2)) in water samples. The system is based on the reaction of hydrogen peroxide and hydrogen carbonate solution. Carbon nanospheres (CNSs) prepared from aqueous glucose solution are used to enhance the weak CL. The CL intensity was found to be directly proportional to the concentration of H(2)O(2) present in the sample solutions. The effects upon the CL of several physicochemical parameters, including the concentration of the reagents, the mixing order of the reagents, flow rate, pH, particle size of CNSs and other relevant variables, were studied and optimized. The proposed method exhibited advantages in a larger linear range of 5.0 × 10(-8) to 3.0 × 10(-6) mol L(-1) and a lower limit of detection of 1.0 × 10(-9) mol L(-1) (S/N = 3). This method has been successfully applied to the evaluation of H(2)O(2) in tap water and snow water with recoveries from 80 to 110%. The relative standard deviation (RSD) was less than 8% for intra- and inter-assay precision. Based on the kinetic curve, the CL spectrum, fluorescence spectrum, UV-visible spectrum, and electron spin resonance (ESR) spectrum of NaHCO(3)-H(2)O(2)-CNSs system, a possible CL mechanism was proposed. Superoxide ion radical (˙O(2)(-)) and hydroxide radical (˙OH) were generated during the reaction of NaHCO(3) and H(2)O(2). They were the key intermediates for the production of hole-injected and electron-injected CNSs in the CL process.     

Structure and vibrational spectra of the enol form of hexafluoro-acetylacetone. A density functional theoretical study

Molecular and vibrational structure of 1,1,1,6,6,6-hexafluoropentane-2,4-dione (hexafluoro-acetylacetone) have been investigated by means of density functional theory (DFT) calculations and have been compared with those of acetylacetone, the parent molecule. According to the theoretical calculations HFAA has an asymmetric structure with hydrogen bond strength of about 12 kcal mol(-1), about 6 kcal mol(-1) less than that of acetylacetone. This weakening of hydrogen bond is consistent with frequency shifts for OH/OD stretching, OH/OD out of plane bending and O...O stretching modes upon substitution of methyl hydrogen atoms with fluorine atoms. The symmetric structure based on electron diffraction data is interpreted as superposition of two asymmetric structures.     

Kinetics and mechanism of the [Ru(III)(edta)(H2O)](-)-mediated oxidation of cysteine by H2O2

The kinetics and mechanism of the [Ru(III)(edta)(H(2)O)](-)-mediated oxidation of cysteine (RSH) by hydrogen peroxide (edta(4-) = ethylenediaminetetraacetate), were studied in detail as a function of both the hydrogen peroxide and cysteine concentrations at pH 5.1 and room temperature. The kinetic traces reveal clear evidence for a catalytic process in which hydrogen peroxide reacts directly with cysteine coordinated to the Ru(III)(edta) complex in the form of [Ru(III)(edta)SR](2-). A parallel process in which [Ru(III)(edta)(H(2)O)](-) first reacts with H(2)O(2) to produce [Ru(V)(edta)O](-) and subsequently oxidizes cysteine, is orders of magnitude slower than the [Ru(III)(edta)(H(2)O)](-)-mediated oxidation in which cysteine rapidly coordinates to [Ru(III)(edta)(H(2)O)](-) prior to the reaction with H(2)O(2). HPLC product analyses revealed the formation of cystine (RSSR) as major product along with cysteine sulfinic acid (RSO(2)H) in the reaction system, and established the catalytic role of [Ru(III)(edta)(H(2)O)](-). Simulations were performed to account for the rather complex kinetic traces in terms of the suggested reaction mechanism. The results of the simulations support the proposed reaction mechanism that involves the oxidation of coordinated cysteine to cysteine sulfenic acid (RSOH), which subsequently rapidly reacts with H(2)O(2) and RSH to form RSO(2)H and RSSR, respectively.     

Theoretical study of the solvation of HgCl2, HgClOH, Hg(OH)2 and HgCl3(-): a density functional theory cluster approach

The determination of the solvation shell of Hg(II)-containing molecules and especially the interaction between Hg(II) and water molecules is the first requirement to understand the transmembrane passage of Hg into the cell. We report a systematic DFT study by stepwise solvation of HgCl(2) including up to 24 water molecules. In order to include pH and salinity effects, the solvation patterns of HgClOH, Hg(OH)(2) and HgCl(3)(-) were also studied using 24 water molecules. In all cases the hydrogen bond network is crucial to allow orbital-driven interactions between Hg(II) and the water molecules. DFT Born-Oppenheimer molecular dynamics simulations starting from the stable HgCl(2)-(H(2)O)(24) structure revealed that an HgCl(2)-(H(2)O)(3) trigonal bipyramid effective solute appears and then the remaining 21 water molecules build a complete first solvation shell, in the form of a water-clathrate. In the HgCl(2), HgClOH, Hg(OH)(2)-(H(2)O)(24) optimized structures Hg also directly interacts with 3 water molecules from an orbital point of view (three Hg-O donor-acceptor type bonds). All the other interactions are through hydrogen bonding. The cluster-derived solvation energies of HgCl(2), HgClOH and Hg(OH)(2) are estimated to be -34.4, -40.1 and -47.2 kcal mol(-1), respectively.     

A theoretical study of surface reduction mechanisms of CeO(2)(111) and (110) by H(2).

Reaction mechanisms for the interactions between CeO(2)(111) and (110) surfaces are investigated using periodic density functional theory (DFT) calculations. Both standard DFT and DFT+U calculations to examine the effect of the localization of Ce 4f states on the redox chemistry of H(2)-CeO(2) interactions are described. For mechanistic studies, molecular and dissociative local minima are initially located by placing an H(2) molecule at various active sites of the CeO(2) surfaces. The binding energies of physisorbed species optimized using the DFT and DFT+U methods are very weak. The dissociative adsorption reactions producing hydroxylated surfaces are all exothermic; exothermicities at the DFT level range from 4.1 kcal mol(-1) for the (111) to 26.5 kcal mol(-1) for the (110) surface, while those at the DFT+U level are between 65.0 kcal mol(-1) for the (111) and 81.8 kcal mol(-1) for the (110) surface. Predicted vibrational frequencies of adsorbed OH and H(2)O species on the surfaces are in line with available experimental and theoretical results. Potential energy profiles are constructed by connecting molecularly adsorbed and dissociatively adsorbed intermediates on each CeO(2) surface with tight transition states using the nudged elastic band (NEB) method. It is found that the U correction method plays a significant role in energetics, especially for the intermediates of the exit channels and products that are partially reduced. The surface reduction reaction on CeO(2)(110) is energetically much more favorable. Accordingly, oxygen vacancies are more easily formed on the (110) surface than on the (111) surface.     

A density functional study of O-O bond cleavage for a biomimetic non-heme iron complex demonstrating an Fe(v)-intermediate

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).