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Electrodeposited manganese oxide films are promising catalysts for promoting the oxygen evolution reaction (OER), especially in acidic solutions. The activity of these catalysts is known to be enhanced by the introduction of Mn3+ We present in situ electrochemical and X-ray absorption spectroscopic studies, which reveal that Mn3+ may be introduced into MnO2 by an electrochemically induced comproportionation reaction with Mn2+ and that Mn3+ persists in OER active films. Extended X-ray absorption fine structure (EXAFS) spectra of the Mn3+-activated films indicate a decrease in the Mn-O coordination number, and Raman microspectroscopy reveals the presence of distorted Mn-O environments. Computational studies show that Mn3+ is kinetically trapped in tetrahedral sites and in a fully oxidized structure, consistent with the reduction of coordination number observed in EXAFS. Although in a reduced state, computation shows that Mn3+ states are stabilized relative to those of oxygen and that the highest occupied molecular orbital (HOMO) is thus dominated by oxygen states. Furthermore, the Mn3+(Td) induces local strain on the oxide sublattice as observed in Raman spectra and results in a reduced gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). The confluence of a reduced HOMO-LUMO gap and oxygen-based HOMO results in the facilitation of OER on the application of anodic potentials to the δ-MnO2 polymorph incorporating Mn3+ ions.

Benzimidazolines (dihydrobenzimidazoles) are shown for the first time to eliminate hydrogen (H2) by catalyzed reaction with protic compounds.

We report the activation of a 2,2′-bipyridine ligand within a class of (mono)cyclopentadienyl lanthanide complexes when reacted with carbon monoxide.

We have studied the quenching of the uranyl ion in aqueous acetonitrile with a series of alkenes. The quenching rate constant (kq) increases linearly with decreasing ionization potential (IP) for a series of hexene isomers, as seen in a plot of ln(kq) vs IP; no correlation is observed between the quenching rate and the bond dissociation energy of the allylic C−H bond. These results demonstrate that the mechanism of quenching for alkenes is electron transfer from the alkene to uranyl.

Les ions alcoolates subissent une elimination de molecules neutres pour donner des ions enolates

An ion cyclotron resonance (ICR) spectrometer can be used to generate, trap, and detect gas phase ions which are irradiated with an infrared laser. Infrared multiple photon (IRMP) photochemistry can be conveniently studied under these conditions. Results of several recent studies are discussed.

Metallacycles are key intermediates in organometallic chemistry and catalysis as well as useful reagents for organic synthesis. Despite numerous studies on the preparation and thermal chemistry of these complexes, few photochemical investigations have been reported. In this paper, they report photochemical studies of the bis(cyclopentadienyl)titanacyclobutanes 1. Photochemical activation of 1 results in clean reductive elimination to form cyclopropanes and titanocene which can be trapped by a number of added reagents. Stereochemical studies strongly suggest a stepwise mechanism in which the primary photochemical step involves metal-carbon bond homolysis to produce a metal-centered 1,4-biradical intermediate.

Acetone enolate ion trapped in a pulsed ion cyclotron resonance spectrometer cell was irradiated with a pulsed CO2 laser. Both unimolecular decomposition to give HC2O− and CH4, and vibrationally induced electron autodetachment were observed. The data are consistent with a scheme in which a vibrationally excited enolate ion decomposes through an adiabatic (decomposition) pathway and a nonadiabatic (autodetachment) pathway. From an analysis of the branching using statistical reaction rate theory the autodetachment rate constant ka is estimated to be ∼107 s−1 at ∼1 eV above the electron detachment threshold.

Infrared multiple photon (IRMP) photochemical activation of gas-phase ions trapped in an ion cyclotron resonance (ICR) spectrometer has been used to study the mechanism of a gas-phase negative ion unimolecular decomposition. Upon irradiation with a CO/sub 2/ laser (both high-power pulsed and low-power continuous wave (CW)), tert-butoxide anion, trapped in a pulsed ICR spectrometer, decomposes to yield acetone enolate anion and methane. The mechanism of this formal 1,2-elimination reaction was probed by measuring competitive hydrogen isotope effects (both primary and secondary) in the IR laser photolysis of 2-methyl-2-propoxide-1,1,1-d/sub 3/ (1) and 2-methyl-2-propoxide-1,1,1,3,3,3-d/sub 6/ (2) anions. Unusually large secondary isotope effects (pulsed laser, 1.9 for 1 and 1.7 for 2; cw laser, 8 for (1) and small primary isotope effects (pulsed laser, 1.6 for 1 and 2; cw laser, 2.0 for (1) were observed. These isotope effects, particularly the large difference in energy dependence of the primary and secondary effects, are consistent only with a stepwise mechanism involving initial bond cleavage to an intermediate ion-molecular complex followed by a hydrogen transfer within the intermediate complex. The observed secondary isotope effect have been modelled by using statistical reaction rate (RRKM) theory. The implications of this study for several previously reported unimolecular ionmore » decompositions are also discussed.« less

Metallacycles are key intermediates in organometallic chemistry and catalysis as well as useful reagents for organic synthesis. Despite numerous studies on the preparation and thermal chemistry of these complexes, few photochemical investigations have been reported. In this paper, they report photochemical studies of the bis(cyclopentadienyl)titanacyclobutanes 1. Photochemical activation of 1 results in clean reductive elimination to form cyclopropanes and titanocene which can be trapped by a number of added reagents. Stereochemical studies strongly suggest a stepwise mechanism in which the primary photochemical step involves metal-carbon bond homolysis to produce a metal-centered 1,4-biradical intermediate.