Surface control of oxidation by an adsorbed Ru(IV)-oxo complex.

When adsorbed to optically transparent, thin films of TiO(2) nanoparticles on glass, the aqua complex [Ru(II)(tpy)(bpy(PO(3)H(2))(2))(OH(2))](2+) (bpy(PO(3)H(2))(2) is 2,2'-bipyridyl-4,4'-diphosphonic acid; tpy is 2,2':6',2' '-terpyridine) is oxidized by Ce(IV)(NH(4))(2)(NO(3))(6) in 0.1 M HClO(4) to its Ru(IV)=O(2+) form as shown by UV-visible measurements and analysis of oxidative equivalents by oxidation of hydroquinone to quinone. Kinetic studies on the oxidations of cyclohexene, benzyl alcohol, phenol, and trans-stilbene by surface-bound Ru(IV)=O(2+) by UV-visible monitoring reveal direct evidence for initial 2-electron steps to give Ru(II) intermediates in all four cases. These steps are masked in solution where Ru(IV) --> Ru(II) reduction is followed by rapid reactions between Ru(II) intermediates and Ru(IV)=O(2+) to give Ru(III). Reactions between Ru(II) and Ru(IV)=O(2+) on the surface are inhibited by binding to the surface, which restricts translational mobility. Rate constants on the surface and in solution are comparable, pointing to comparable reactivities. The surface experiments give unprecedented insight into oxidation mechanism with important implications for achieving product selectivity in synthesis by limiting oxidation to two electrons.     

On the Spreading of Glycerol Trioleate

The benzoquinone/hydroquinone (Q/H(2)Q) redox reaction has been studied by electrochemical-scanning tunneling microscopy (EC-STM) at a Pd(111)-(square3xsquare3)R30 degrees -I electrode surface in a solution that contained 10(-4) M H(2)Q in 0.05 M H(2)SO(4); iodine-pretreatment of the Pd(111) surface was to prevent chemisorption of organic-derived species. The molecule-resolved EC-STM images indicated that: (i) at a potential where only H(2)Q is present in solution, a self-assembled (square21xsquare21)R10.9 degrees -eta(6)-H(2)Q monolayer is produced in which the H(2)Q molecules are oriented parallel to the surface; (ii) at a potential where partial oxidation (to Q) occurs, a self-assembled (square21xsquare21)R10.9 degrees -eta(6)-QH adlayer is generated, where QH represents quinhydrone, an equimolar mixture of Q and H(2)Q; in this structure, the Q and H(2)Q molecules are oriented vertically, face-to-face, and arranged alternately along a given row, reminiscent of the crystal structure of quinhydrone. The partial oxidation-induced molecular reorientation, which is reversible, most likely arises from favorable Q-H(2)Q face-to-face interactions; that is, complete oxidation would yield only flat-oriented Q species. Unfortunately, at potentials where only Q would be present in solution, I-catalyzed corrosion of the Pd starts to occur, which leads to noise-laden EC-STM images. Copyright 2001 Academic Press.     

纤维素纤维接枝β-环糊精对苯二酚类分子的包络识别性能

用分光光度滴定法在不同温度、不同pH值条件下测定了纤维素纤维接枝β-环糊精与苯二酚类分子形成超分子包合物的表观热力学参数. 化学计量法表明, 纤维素纤维接枝β-环糊精上环糊精与客体苯二酚类分子形成了1∶1的超分子包合物. 从主-客体间包合物的表观热力学参数、尺寸关系、溶液pH值、包合时间等因素讨论了纤维上环糊精对客体苯二酚类分子的分子识别机制. 结果表明, 纤维素纤维接枝β-环糊精上环糊精对苯二酚类分子不同位置羟基具有分子选择性和识别能力.     

Synthesis of nitrilotriacetic acid terminated tethers for the binding of His-tagged proteins to lipid bilayers and to gold

Nitrilotriacetic acid terminated tethers for trapping of His-tagged proteins have been synthesized and characterized. Compound 1, containing a lipophilic cholesterol anchor, hydrophilic oligoethoxy chain linker, and nitrilotriacetic acid terminus, can be used for attaching His-tagged proteins to phospholipid bilayers. Compound 2, containing a gold binding thioacetate anchor, hydrophilic oligoethoxy chain, and nitrilotriacetic acid terminus, can be used to tether His-tagged proteins to a gold surface.     

Photoelectrochemistry on Ru(II)-2,2'-bipyridine-phosphonate-derivatized TiO2 with the I3-/I- and quinone/hydroquinone relays. Design of photoelectrochemical synthesis cells

Photocurrent measurements have been made on nanocrystalline TiO2 surfaces derivatized by adsorption of a catalyst precursor, [Ru(tpy)(bpy(PO3H2)2)(OH2)]2+, or chromophore, [Ru(bpy)2 (bpy(PO3H2)2)]2+ (tpy is 2,2':6',2' '-terpyridine, bpy is 2,2'-bipyridine, and bpy(PO3H2)2 is 2,2'-bipyridyl-4,4'-diphosphonic acid), and on surfaces containing both complexes. This is an extension of earlier work on an adsorbed assembly containing both catalyst and chromophore. The experiments were carried out with the I3-/I- or quinone/hydroquinone (Q/H2Q) relays in propylene carbonate, propylene carbonate-water mixtures, and acetonitrile-water mixtures. Electrochemical measurements show that oxidation of surface-bound Ru(III)-OH2(3+) to Ru(IV)=O(2+) is catalyzed by the bpy complex. Addition of aqueous 0.1 M HClO4 greatly decreases photocurrent efficiencies for adsorbed [Ru(tpy)(bpy(PO3H2)2)(OH2)]2+ with the I3-/I- relay, but efficiencies are enhanced for the Q/H2Q relay in both propylene carbonate-HClO4 and acetonitrile-HClO4 mixtures. The dependence of the incident photon-to-current efficiency (IPCE) on added H2Q in 95% propylene carbonate and 5% 0.1 M HClO4 is complex and can be interpreted as changing from rate-limiting diffusion to the film at low H2Q to rate-limiting diffusion within the film at high H2Q. There is no evidence for photoelectrochemical cooperativity on mixed surfaces containing both complexes with the IPCE response reflecting the relative surface compositions of the two complexes. These results provide insight into the possible design of photoelectrochemical synthesis cells for the oxidation of organic substrates.     

Molecular adsorption at well-defined electrode surfaces: hydroquinone on Pd(111) studied by EC-STM

The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.     

Fabrication of steady junctions consisting of alpha,omega-bis(thioacetate) oligo(p-phenylene vinylene)s in nanogap electrodes

For obtaining molecular devices using metal-molecule-metal junctions, it is necessary to fabricate a steady conductive bridge-structure; that is stable chemical bonds need to be established from a single conductive molecule to two facing electrodes. In the present paper, we show that the steadiness of a conductive bridge-structure depends on the molecular structure of the bridge molecule for nanogap junctions using three types of modified oligo(phenylene vinylene)s (OPVs): alpha,omega-bis(thioacetate) oligo(phenylene vinylene) (OPV1), alpha,omega-bis(methylthioacetate) oligo(phenylene vinylene) (OPV2), and OPV2 consisting of ethoxy side chains (OPV3). We examined the change in resistance between the molecule-bridged junction and a bare junction in each of the experimental Au-OPV-Au junctions to confirm whether molecules formed steady bridges. Herein, the outcomes of whether molecules formed steady bridges were defined in terms of three types of result; successful, possible and failure. We define the ratio of the number of successful junctions to the total number of experimental junctions as successful rate. A 60% successful rate for OPV3 was higher than for the other two molecules whose successful rates were estimated to be approximately 10%. We propose that conjugated molecules consisting of methylthioacetate termini and short alkoxy side chains are well suited for fabricating a steady conductive bridge-structure between two facing electrodes.     

Electrochemical release of amine molecules from carbamate-based, electroactive self-assembled monolayers

In this paper, carbamate-based self-assembled monolayers (SAMs) of alkanethiolates on gold were suggested as a versatile platform for release of amine-bearing molecules in response to the electrical signal. The designed SAMs underwent the electrochemical oxidation on the gold surface with simultaneous release of the amine molecules. The synthesis of the thiol compounds was achieved by coupling isocyanate-containing compounds with hydroquinone. The electroactive thiol was mixed with 11-mercaptoundecanol [HS(CH(2))(11)OH] to form a mixed monolayer, and cyclic votammetry was used for the characterization of the release behaviors. The mixed SAMs showed a first oxidation peak at +540 mV (versus Ag/AgCl reference electrode), indicating the irreversible conversion from carbamate to hydroquinone groups with simultaneous release of the amine molecules. The analysis of ToF-SIMS further indicated that the electrochemical reaction on the gold surface successfully released amine molecules.     

Self electro-catalysis of hydroquinone on gold electrode in aqueous un-buffered media

Redox behavior of hydroquinone (H₂Q) in an un-buffered aqueous medium at gold electrode is reported. In multiple scanned cyclic voltammograms, appearance of a new redox couple, splitting of H₂Q oxidation peak and anodic shift of original H₂Q redox peak were observed. Comparative studies of quinone (Q) under similar conditions establish the formation of surface confined Q as an intermediate. Electro-catalysis by the electro-generated Q monolayer is proposed to be responsible for the splitting of H₂Q oxidative wave in the cyclic voltammetry.     

Self-assembled monolayers of electroactive polychlorotriphenylmethyl radicals on Au(111)

Two new polychlorotriphenylmethyl (PTM) derivatives bearing a thioacetate and a disulfide group have been synthesized to anchor on gold substrate. On the basis of these molecules, three strategies were followed to prepare self-assembled monolayers (SAMs) of electroactive PTMs. The resulting SAMs were fully characterized by contact angle, atomic force microscopy (AFM), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The high coverage surface and stability of the SAMs were demonstrated by cyclic voltammetry. In addition, the electrochemical experiments proved that these SAMs are bistable since it is possible to reversibly switch between the PTM radical state to the corresponding anion. The magnetic response was investigated by electron paramagnetic resonance. We observed that when the PTM SAMs are in their radical form they confer magnetic functionality to the surface, whereas when they are in the anionic state, the surface is diamagnetic. Thus, the PTM-modified substrates are multifunctional surfaces since they combine magnetic and electroactive properties. The reported results show the high potential of these materials for the fabrication of surface molecular devices.     

Theoretical study of the benzyl+O2 reaction: kinetics, mechanism, and product branching ratios

Ab initio calculations at the level of CBS-QB3 theory have been performed to investigate the potential energy surface for the reaction of benzyl radical with molecular oxygen. The reaction is shown to proceed with an exothermic barrierless addition of O2 to the benzyl radical to form benzylperoxy radical (2). The benzylperoxy radical was found to have three dissociation channels, giving benzaldehyde (4) and OH radical through the four-centered transition states (channel B), giving benzyl hydroperoxide (5) through the six-centered transition states (channel C), and giving O2-adduct (8) through the four-centered transition states (channel D), in addition to the backward reaction forming benzyl radical and O2 (channel E). The master equation analysis suggested that the rate constant for the backward reaction (E) of C6H5CH2OO-->C6H5CH2+O2 was several orders of magnitude higher that those for the product dissociation channels (B-D) for temperatures 300-1500 K and pressures 0.1-10 atm; therefore, it was also suggested that the dissociation of benzylperoxy radicals proceeded with the partial equilibrium between the benzyl+O2 and benzylperoxy radicals. The rate constants for product channels B-D were also calculated, and it was found that the rate constant for each dissociation reaction pathway was higher in the order of channel D>channel C>channel B for all temperature and pressure ranges. The rate constants for the reaction of benzyl+O2 were computed from the equilibrium constant and from the predicted rate constant for the backward reaction (E). Finally, the product branching ratios forming CH2O molecules and OH radicals formed by the reaction of benzyl+O2 were also calculated using the stationary state approximation for each reaction intermediate.     

Tethered bisubstrate derivatives as probes for mechanism and as inhibitors of aminoglycoside 3'-phosphotransferases

Aminoglycoside 3'-phosphotransferases [APH(3')s] phosphorylate aminoglycoside antibiotics, a reaction that inactivates the antibiotics. These enzymes are the primary cause of resistance to aminoglycosides in bacteria. APH(3')-Ia operates by a random-equilibrium BiBi mechanism, whereas APH(3')-IIIa catalyzes its reaction by the Theorell-Chance mechanism, a form of ordered BiBi mechanism. Hence, both substrates have to be present in the active site prior to the transfer of phosphate by both mechanisms. Four bisubstrate analogues, compounds 1-4, were designed and synthesized as inhibitors for APH(3')s. These compounds are made of adenosine linked covalently to the 3'-hydroxyl of neamine (an aminoglycoside) via all-methylene tethers of 5-8 carbons. The K(i) values measured for these compounds indicated that affinities of APH(3')-Ia and APH(3')-IIa for compounds 2 and 3 (six- and seven-carbon tethers, respectively) were the best, and the inhibition constants for the two were comparable.     

Proton coupled electron transfer of ubiquinone Q2 incorporated in a self-assembled monolayer

We present a complete study of the reduction of ubiquinone Q(2) (UQ(2)) in simpler aqueous medium, over a pH range of 2.5 to 12.5. The short isoprenic chain ubiquinones (UQ(2)) were incorporated in a self-assembled monolayer. Under these conditions, the global 2e(-) electrochemical reaction can be described on the basis of a nine-member square scheme. The thermodynamic constants of the system were determined. The global 2e(-) process is controlled by the uptake of the second electron. The elementary electrochemical rate constants obtained by fitting of the experimental rate constant were k(s4) = 1.5 s(-1) for QH˙(+)(2)? QH(2), k(s5) = 1.5 s(-1) for QH˙? QH(-) and k(s6) = 1 s(-1) for Q˙(-)? Q(2-). The three electrochemical reactions QH˙(+)(2)? QH(2), QH˙? QH(-) and Q˙(-)? Q(2-) are successively involved when increasing the pH. Protonations can occur or not, before or after the electron uptake and the reaction paths are, from low to high pH: e(-), H(+)e(-), e(-)H(+), H(+)e(-)H(+), H(+)e(-) and e(-)H(+).     

Synthesis of ferrocenethiols containing oligo(phenylenevinylene) bridges and their characterization on gold electrodes

Ferrocene-terminated oligo(phenylenevinylene) (OPV) methyl thiols have been prepared by orthogonal coupling of phenylene monomers. Ethoxy substituents on the phenyl rings improve the solubility of OPV, enabling the synthesis of longer oligomers. Self-assembled monolayers containing a mixture of a ferrocene OPV methyl thiol and a diluent alkanethiol were deposited on gold. A cyclic voltammetric study of monolayers containing oligomers of the same length with and without ethoxy solubilizing groups reveals that both solubilized and unsolubilized oligomers form well-packed self-assembled monolayers. Changing the position of the solubilizing groups on an oligomer chain does not preclude packing of the oligomer in the monolayer. Conventional chronoamperometry, which can be used to measure rate constants up to approximately 10(4) s(-1), is too slow to measure the electron-transfer rate through these oligomers over distances up to 35 A. OPV bridges are expected to be highly conjugated unlike oligo(phenyleneethynylene) bridges, which may be only partially conjugated because of rotation of the phenyl rings about the ethynylene bonds. Because of its high conjugation, OPV may prove useful as a molecular wire.     

Systematic approach to the quantitative voltammetric analysis of the FeIII/FeII component of the [alpha2-Fe(OH2)P2W17O61]7-/8- reduction process in buffered and unbuffered aqueous media

The one-electron reduction of [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <--> Q (E(1)(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH(+) + e(-) <--> QH(+) (E(2)(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH(+) <--> P + H(+) (K(P) = 3.02 x 10(-6) M), and QH(+) <--> Q + H(+) (K(Q) = 2.35 x 10(-10) M), where P, Q, PH(+), and QH(+) correspond to [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-), [alpha(2)-Fe(II)(OH)P(2)W(17)O(61)](9-), [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), and [alpha(2)-Fe(II)(OH(2))P(2)W(17)O(61)](8-), respectively; E(1)(0)' and E(2)(0)' are the formal potentials, k(1)(0)' and k(2)(0)' are the formal (standard) rate constants, and K(P) and K(Q) are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e(-) <--> red, whose formal potential, E(app)(0)', and standard rate constant, k(app)(0)', are straightforwardly derived functions of pH, as are the values of E(1)(0)', k(1)(0)', E(2)(0)', k(2)(0)', and K(P) (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E(app)(0)' and k(app)(0)', which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E(1)(0)' and k(1)(0)'; K(Q) was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H(2)O. The value of the diffusion coefficient of the [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH is greater than 6, instability of P (i.e., [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-)) led to the loss of the reactant and precluded lengthy experimentation.     

Hydroquinone-bridged dinuclear Cu(II) complexes and single-crystalline Cu(II) coordination polymers

Cationic dinuclear Cu(II) complexes 3 and 4 have been prepared using the novel hydroquinone-based imine chelators 2,5-((i)Pr(2)NCH(2)CH(2)N[double bond, length as m-dash]CH)(2)-1,4-(OH)(2)-C(6)H(2) (1) and 2,5-(pyCH(2)CH(2)N[double bond, length as m-dash]CH)(2)-1,4-(OH)(2)-C(6)H(2) (2), respectively (py = 2-pyridyl). X-Ray quality crystals of both complexes were grown from their DMF solutions. The sterically more encumbered compound crystallizes in the form of discrete dinuclear entities with Cu(II) centres in a distorted square-planar ligand environment (one coordination site is occupied by a DMF molecule). The pyridyl derivative 4 features dinuclear hydroquinone-bridged subunits similar to 3. However, the Cu(II) ions are now six-coordinate with two DMF molecules at an axial and an equatorial position of a Jahn-Teller-distorted octahedron. Moreover, the dinuclear subunits are no longer isolated but linked with each other via bridging hydroquinone oxygen atoms which occupy the second apical position of each octahedron. The structure suggests that the magnetic properties of the resulting coordination polymer of 4 could be described by a model valid for dimerized spin chains. As a result of this analysis the antiferromagnetic coupling constants J(1)/k(B) = 9.9 K (intradimer) and J(2)/k(B) = 0.9 K (interdimer) are obtained. Both in 3 and in 4, the hydroquinone --> semiquinone transition of the central bridging unit (E degrees ' = + 0.57 V, 3; E degrees ' = + 0.51 V, 4; DMF; vs. SCE) displays features of chemical reversibility. In the case of , reduction of Cu(II) centres requires a peak potential of E(p) = - 0.42 V.     

Dialkylindium-thioacetate R2InOSCCH3 (R = CH3,C2H5), darstellung,eigenschaften und struktur

Dimethylindium thioacetate has been prepared and the vibrational spectra (IR and Raman) discussed. The X-ray structural determination shows that diethylindium thioacetate belongs to the orthorhombic space group Pnma. The unit cell with lattice constants a = 8.588, b = 13.834, c = 8.345 Å contains four molecules. The indium—oxygen distance within the four-membered ring is nearly the same as the distance between the oxygen and the indium atom of the adjacent ring. Indium has therefore the coordination number 5.     

配位化学中的直线自由能关系XXII: 铜(II)-α-氨基酸-二氧四胺大环三元体系

用pH法在25.0±0.1℃, I=0.1 mol.dm^-^3 KNO3条件下研究了铜(II)-α-氨基酸-13-(2'-羟基-3', 5'-取代苄基)-1, 4,8, 11-四氮杂环十四烷-12, 14-二酮竞争性三元混配型配合物的稳定性。测定了该体系的三元配合物的稳定常数, 并讨论了二氧四胺大环配体的环空腔大小对三元配合物的构型及稳定性的影响以及大环配体上取代基对三元配合物的影响。     

Electrocatalytical oxidation of hydroquinone with ferrocene covalently bound to L-cysteine self-assembled monolayers on a gold electrode

The preparation of a gold electrode modified by ferrocenecarboxylic acid (FcA) covalently bound to L-cysteine self-assembled monolayer (FcA-SAM) is described. The modified electrode shows an excellent electrocatalytic activity for the oxidation of hydroquinone (QH2) and accelerates the electron transfer rate. The anodic overpotential is reduced by ca. 290 mV compared to those obtained at bare gold electrodes. The charge transfer coefficient and the apparent surface electron transfer rate constant for the redox couple of Q/QH2 at the modified electrode are found to be 0.425 and 0.96 s(-1), respectively. The catalytic current response of DPV increases linearly with the QH2 concentration from 5.7 x 10(-7) to 3.2 x 10(-4) M. The estimation of QH2 in a simulative sample is satisfactory. The method is simple, quick, and sensitive.     

Fabrication of nano-network gold films via anodization of gold electrode and their application in SERS

We report here a green and facile one-step method to fabricate nano-network gold films of low roughness via anodization of gold electrodes in an aqueous solution of l-ascorbic acid (AA) or hydroquinone (H₂Q) at the oxidation peak potential. The preparation involves the formation of thin gold oxide layer by anodization of gold and its simultaneous and/or subsequent reduction by AA or H₂Q. The as-fabricated nano-network gold films show very strong SERS activity in comparison with the substrates prepared by some other electrochemical roughening methods.