- Elucidating Proton-Coupled Electron Transfer Mechanisms of Metal Hydrides with Free Energy- And Pressure-Dependent Kinetics
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Proton-coupled electron transfer (PCET) was studied in a series of tungsten hydride complexes with pendant pyridyl arms ([(PyCH2Cp)WH(CO)3], PyCH2Cp = pyridylmethylcyclopentadienyl), triggered by laser flash-generated RuIII-tris-bipyridine oxidants, in acetonitrile solution. The free energy dependence of the rate constant and the kinetic isotope effects (KIEs) showed that the PCET mechanism could be switched between concerted and the two stepwise PCET mechanisms (electron-first or proton-first) in a predictable fashion. Straightforward and general guidelines for how the relative rates of the different mechanisms depend on oxidant and base are presented. The rate of the concerted reaction should depend symmetrically on changes in oxidant and base strength, that is on the overall ?G0 PCET, and we argue that an "asynchronous" behavior would not be consistent with a model where the electron and proton tunnel from a common transition state. The observed rate constants and KIEs were examined as a function of hydrostatic pressure (1-2000 bar) and were found to exhibit qualitatively different dependence on pressure for different PCET mechanisms. This is discussed in terms of different volume profiles of the PCET mechanisms as well as enhanced proton tunneling for the concerted mechanism. The results allowed for assignment of the main mechanism operating in the different cases, which is one of the critical questions in PCET research. They also show how the rate of a PCET reaction will be affected very differently by changes of oxidant and base strength, depending on which mechanism dominates. This is of fundamental interest as well as of practical importance for rational design of, for example, catalysts for fuel cells and solar fuel formation, which operate in steps of PCET reactions. The mechanistic richness shown by this system illustrates that the specific mechanism is not intrinsic to a specific synthetic catalyst or enzyme active site but depends on the reaction conditions.
- Liu, Tianfei,Tyburski, Robin,Wang, Shihuai,Fernández-Terán, Ricardo,Ott, Sascha,Hammarstr?m, Leif
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- Free-energy dependence of electron-transfer rate constants at Si/liquid interfaces
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The interfacial energetics and kinetics of n-type Si electrodes in contact with a series of one-electron, outer-sphere redox couples were investigated using the differential capacitance vs potential and current density vs potential measurements, respectively. The differential capacitance vs potential measurements were essentially independent of the ac frequency imposed on the interface, with linear Bode plots between ≈103 and ≈105 Hz. The current density vs potential plots exhibited first-order kinetic dependence on the concentration of electrons at the semiconductor surface and a first-order kinetic dependence on the concentration of acceptors in the solution.
- Fajardo,Lewis
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- Chiral anion-mediated asymmetric induction onto chiral diquats
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Diquats - which are important electron transfer agents in biological and photocatalytic systems, as well as structural templates for efficient supramolecular synthesis - are noteworthy for their axial chirality and have been so far reported only in racemi
- Pasquini, Chiara,Desvergnes-Breuil, Valérie,Jodry, Jonathan J.,Dalla Cort, Antonella,Lacour, Jér?me
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- Covalent Organic Frameworks Enabling Site Isolation of Viologen-Derived Electron-Transfer Mediators for Stable Photocatalytic Hydrogen Evolution
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Electron transfer is the rate-limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron-transfer mediators (ETMs) to facilitate the rapid electron transfer from photosensitizers to active sites. Nevertheless, the electron-transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic-radical-containing viologen-derived ETMs, by which the electron-transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen-derived ETM, are integrated into a 2,2′-bipyridine-based covalent organic framework (COF) through a post-quaternization reaction. The content and distribution of embedded diquat-ETMs are elaborately controlled, leading to the favorable site-isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 μmol h?1 g?1) and sustained performances when compared to a single-module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi-component cooperation.
- Mi, Zhen,Zhou, Ting,Weng, Weijun,Unruangsri, Junjuda,Hu, Ke,Yang, Wuli,Wang, Changchun,Zhang, Kai A. I.,Guo, Jia
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- Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase
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Effect of chemical structures of some 2,2′-bipyridinium salts (BP2+) as the electron carrier molecules on the visible-light induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc tetraphneylporphyrin tetrasulfonate (ZnTPPS) and formate dehydrogenase (FDH) in the presence of triethanolamine (TEOA) as an electron donor molecule was investigated. Irradiation of a CO2 saturated solution containing TEOA, ZnTPPS, BP2+ and FDH with visible light resulted in production of formic acid. By using 1,1′-ethylene-2,2′-bipyridinium dibromide (DB2+) as an electron carrier molecule, the effective formic acid production was observed compared with the other 2,2′-bipyridinium salt derivatives.
- Amao, Yutaka,Abe, Ryutaro,Shiotani, Sachina
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p. 149 - 153
(2015/10/28)
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- Revisiting the IspH catalytic system in the deoxyxylulose phosphate pathway: Achieving high activity
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(Chemical Equation Presented) From two C5 isoprene building blocks, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP), the more than 30000 members of the isoprenoid family are constructed in nature using two biosynthetic pathways, the mevalonate (MVA) pathway and the deoxyxylulose phosphate (DXP) pathway. IspH of the DXP pathway is a protein containing an iron-sulfur cluster and catalyzes a reductive dehydration reaction of the DXP pathway. In the literature, a wide range of Escherichia coli IspH activities have been reported (2.0 nmol min-1 mg-1 to 3.4 μmol min-1 mg-1). For such a broad range of activities, reaction assays were carried out under many different conditions, preventing direct comparison of the activities and determination of the key factor responsible for such a dramatic difference in IspH activities. In this work, we systematically examined the role of redox mediators in IspH catalysis using E. coli IspH as the enzyme and dithionite as the ultimate electron source. Our studies not only suggest the importance of the iron-sulfur cluster but also improve the E. coli IspH activity by nearly 97-fold relative to that from the E. coli NADPH-flavodoxin reductase-flavodoxin system.
- Xiao, Youli,Chu, Lisa,Sanakis, Yiannis,Liu, Pinghua
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supporting information; experimental part
p. 9931 - 9933
(2009/12/08)
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- ELECTROCHEMISTRY OF METALLOPORPHYRINS AND VIOLOGENS AT ZEOLITE Y MODIFIED ELECTRODES: EVIDENCE FOR ELECTRON TRAPPING BY MONOMOLECULAR PORPHYRIN LAYERS.
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Cyclic voltammetric data are presented for electrodes coated with zeolite Y powder containing porphyrins and viologens. Half-wave potentials for viologen cations (methylviologen, benzylviologen, or N,N prime -(1,3-propenyl)-2,2 prime -bipyridinium) do not change significantly when they are exchanged into zeolite Y, whereas the reduction potentials for cobalt and zinc tetrakis (N-methyl-4-pyridyl)porphyrins shift (relative to aqueous solution) by plus 200 mV. When a viologen cation is ion exchanged into the bulk of the zeolite, and cobalt tetrakis(N-methyl-4-pyridyl)porphyrin is adsorbed onto its outer surface in monolayer quantities, current rectification and charge trapping reactions are observed.
- Li,Wang,Persaud,Mallouk
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p. 2592 - 2597
(2007/10/02)
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