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1.
After a reminder of concerted/stepwise mechanistic dichotomy and other basic concepts and facts in the field, a series of recent advances is discussed. Particular emphasis is laid on the interactions between the fragments formed upon bond cleavage. These interactions may persist even in polar solvents and have important consequences on dissociative electron transfer kinetics and on the competition between concerted and stepwise pathways. Cleavage of ion radicals and its reverse reaction are examples of single electron transfer reactions concerted with bond cleavage and bond formation, respectively. The case of aromatic carbon–heteroatom bonds is particularly worth examination since symmetry restrictions impose circumventing a conical intersection. Reductive dehalogenases are involved in ‘dehalorespiration’ of anaerobic bacteria in which the role of dioxygen in aerobic organisms is played by major polychloride pollutants such as tetrachloroethylene. They offer an interesting illustration of how the coupling of electron transfer with bond breaking may be an important issue in natural processes. Applications of dissociative electron transfer concepts and models to mechanistic analysis in this class of enzymes will be discussed. 相似文献
2.
Heisenberg’s explanation of how two coupled oscillators exchange energy represented a dramatic success for his new matrix
mechanics. As matrix mechanics transmuted into wave mechanics, resulting in what Heisenberg himself described as “…an extraordinary
broadening and enrichment of the formalism of the quantum theory”, the term resonance also experienced a corresponding evolution.
Heitler and London’s seminal application of wave mechanics to explain the quantum origins of the covalent bond, combined with
Pauling’s characterization of the effect, introduced resonance into the chemical lexicon. As the Valence Bond approach gave
way to a soon-to-be dominant Molecular Orbital method, our understanding of the term resonance, as it might apply to our understanding
the chemical bond, has also changed. 相似文献
3.
We consider recent data on dissociative electron transfer reactions in which the electron transfer causes practically concerted
dissociation of the chemical bond in the reagent. We discuss considerable experimental data on reactions in the gas phase
and in solutions, and also existing theoretical models for describing the kinetics of these complex processes.
Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 2, pp. 67–78, March–April, 1998. 相似文献
4.
Since alkynes have higher symmetry than olefins, it is not easy to infer the mechanism of a triplet carbene’s addition to
an alkyne by traditional product analysis studies. Specifically, no stereochemical information which might offer insight into
the carbene’s spin state can be extracted from the cyclopropene products. In 1971, Hendrick, Baron, and Jones showed that
diphenylcarbene reacts with terminal alkynes in solution to produce indenes via a “self-trapping” vinylcarbene. They also examined the diphenylcarbene reaction with disubstituted alkynes and found at most
trace amounts of the “self-trapping” indene product.
In this work, we report the direct observation by organic matrix EPR of the vinylcarbenes generated from triplet fluorenylidene
and terminal alkynes. Our efforts to confirm the identities of these intermediates by independent synthesis, intermolecular
trapping, and an intramolecular “self-trapping” method-halogen-migration-are also recounted. These findings are among the
few instances in which fluorenylidene undergoes carbon-carbon bond formation rather than atom abstraction reactions in a low-temperature
matrix, and in which the biradical adduct of a triplet carbene and a π-bonded substrate can be directly observed. 相似文献
5.
Mechanism of electroreduction of copper(I) cyanide complexes from aqueous electrolytic solutions is studied within a quantum-chemical method of a density functional and a quantum-mechanical theory of charge transfer in polar environment. The electrochemically active form directly participating in an elementary electroreduction act is shown to be the [Cu(CN)2]– complex. Modeling calculations of the activation energy for an elementary charge transfer act reveal for the first time that the transfer of heavy particles along an adiabatic potential energy curve is a more probable mechanism of electroreduction of copper(I) cyanocomplexes than an outer-sphere electron transfer. 相似文献
6.
An approach is proposed for the quantum-chemical investigation of “hydride ion” transfer based on analysis of the similarity
of the order of variation in the ionization potentials, enthalpies, and free energies of affinity to the hydride ion, the
hydrogen atom, and the proton in the substrate molecules and also the derivatives of their cations, radicals, and ions to
the experimentally established “hydride” series. It was established that the experimental “hydride” mobility series of six
chalcogenopyrans based on “semicyclic” 1,5-diketones agrees with the quantum-chemically calculated ionization potentials of
the molecules and with the affinity of the respective radicals to the hydrogen atom participating in the transfer. It was
found that direct removal of a hydride ion and initial deprotonation of the substrates followed by the removal of two electrons
are unlikely. “Hydride” shift mechanisms, in which the first stage is transfer of an electron or hydrogen atom from the chalcogenopyran
molecules, are feasible.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1305–1311, September, 2005. 相似文献
7.
V.S. Krylov 《Journal of Electroanalytical Chemistry》1981,123(1):95-103
The problem of statistical mechanical averaging for the probability of the elementary act of an irreversible electron transfer reaction occurring at a metal/solution interface in the presence of specifically adsorbed electrochemically inert ions or dipole molecules is discussed in detail. Analytical expressions are derived for the polarization curve of the interface in the cases of localization of the ionic reactant centers both inside and outside the compact part of the double layer. Within the framework of the theory presented the available experimental data on electrochemical kinetics complicated by the specific adsorption of charged and neutral components of the solution are analyzed. 相似文献
8.
Dalibor Hodko Maria Gamboa-Aldeco Oliver J. Murphy 《Journal of Solid State Electrochemistry》2009,13(7):1063-1075
A photopolymerization process that simultaneously deposits electronically conducting polymer films and incorporates nanophase
silver grains within the films, the silver grains having been formed in situ on irradiating cast, photopolymerizable formulations
containing silver salts, was developed. Polymer films produced from formulations containing large organic anions were very
flexible and strongly adherent to substrates. Polypyrrole films containing silver grains were characterized electronically
on measuring their electronic conductivities and electrochemically on recording their cyclic voltammetric profiles. Conductivities
were affected by the chemical identity and concentration of components added to photopolymerizable formulations. The best
photopolymerized films had a conductivity of the order of 1 S cm−1. Electronically conducting films derived from formulations consisting of a monomer, an electron acceptor/“dopant,” and a
photoinitiator were electrochemically active. They possessed long-term stability under extended electrode potential cycling
conditions, acceptable charge storage capacity, and the ability to oxidize or reduce redox couples in solution.
Paper submitted for inclusion in the special issue of the Journal of Solid State Electrochemistry honouring the 85th birthday
of Professor John O’M. Bockris. 相似文献
9.
Charles A. Mebi Joshua J. Trujillo Anwar A. Bhuiyan 《Central European Journal of Chemistry》2012,10(4):1218-1222
A new organometallic complex coupling photoactive coumarin to a diironhexacarbonyl unit has been successfully prepared and its composition and electronic structure confirmed by elemental and spectroscopic analyses. Emission spectral analysis of the complex reveals photoinduced intramolecular electron transfer from coumarin to the iron-carbonyl moiety. The compound is electrochemically reduced at −1.24 V vs. Fc/Fc+. This reduction is irreversible, attesting to the instability of the complex. Electrochemical evolution of hydrogen in the presence of the complex has been studied and results are discussed. 相似文献
10.
Trevor J. Davies Craig E. Banks Richard G. Compton 《Journal of Solid State Electrochemistry》2005,9(12):797-808
Recent advances are overviewed which enable simulation of the voltammetric behaviour of surfaces which respond in an electrochemically
spatially heterogeneous fashion. By use of the concept of a “diffusion domain” computationally expensive three-dimensional
simulations may be reduced to tractable two-dimensional equivalents. In this way the electrochemical response of partially
blocked electrodes and microelectrode arrays may be predicted, and are found to be consistent with experimental data. It is,
furthermore, possible to adapt the “blocked” electrode analysis to enable the voltammetric sizing of inert particles present
on an electrode surface. Finally theory of this type predicts the voltammetric behaviour of electrochemically heterogeneous
electrodes—for example composites whose different spatial zones display contrasting electrochemical behaviour toward the same
redox couple. 相似文献
11.
Stuart M. Macdonald Katarzyna Szot Joanna Niedziolka Frank Marken Marcin Opallo 《Journal of Solid State Electrochemistry》2008,12(3):287-293
A hydrophilic carbon nanoparticle–sol-gel electrode with good electrical conductivity within the sol-gel matrix is prepared.
Sulfonated carbon nanoparticles with high hydrophilicity and of 10–20 nm diameter (Emperor 2000) are co-deposited onto tin-doped
indium oxide substrates employing a sol-gel technique. The resulting carbon nanoparticle-sol-gel composite electrodes are
characterized as a function of composition and salt (KCl) additive. Scanning electron microscopy and voltammetry in the absence
and in the presence of a solution redox system suggest that the composite electrode films can be made electrically conducting
and highly porous to promote electron transport and transfer. The effect of the presence of hydrophilic carbon nanoparticles
is explored for the following processes: (1) double layer charging, (2) diffusion and adsorption of the electrochemically
reversible solution redox system 1,1′-ferrocenedimethanol, (3) electron transfer to the electrochemically irreversible redox
system hydrogen peroxide, and (4) electron transfer to the redox liquid tert-butylferrocene deposited into the porous composite
electrode film. The extended electrochemically active hydrophilic surface area is beneficial in particular for surface sensitive
processes (1) and (3), and it provides an extended solid|organic liquid|aqueous solution boundary for reaction (4). The carbon
nanoparticle–sol-gel composite electrodes are optimized to provide good electrical conductivity and to remain stable during
electrochemical investigation. 相似文献
12.
13.
Influence of “direct” and “indirect” pathways of dissociative chemisorption on the form of kinetic dependences ϕ(MCS) and
S(ϕ) was studied by the Monte Carlo method. Langmuir adsorption observed at Sindir/Sdir≤0.1 gradually changes to island-mediated adsorption with an increase of the “indirect” adsorption contribution at 0.1≤Sindir/Sdir≤1.0. At Sindir/Sdir≥1.0 the island-mediated adsorption dominates: large adsorption islands arise and gradually grow. 相似文献
14.
A model of dissociative chemisorption on a surface with a square lattice was studied using the Monte Carlo method. The model
is based on two chemisorption pathways: “direct”—nucleation of adsorption islands, and “indirect”—their growth. The development
of the surface distribution picture of chemisorbed particles was found to depend significantly on the contribution of these
two pathways (Sindir/Sdir). 相似文献
15.
Ab initio optimization of a poly-α-methylstyryl sodium (PMSNa) fragment consisting of two eis units yields a triplet state
energy which is close to the ground state energy. A new mechanism is proposed for depolymerization of “living” polymers, which
implies that an elementary step involves excitation to the low-lying triplet state with a charge transfer and with further
bond cleavage. In the reaction structure, electronic excitation occurs with a minor (≈0.5 ?) displacement of the Na+ cation between the last and the last but one monomer units. The reversible polymerization/depolymerization reaction of PMSNa
in THF was studied experimentally. The experimental (5.6 kcal/mole) and calculated (7.3 kcal/mole) polymerization enthalpies
are in reasonable agreement.
Translated from Zhurnal Strukturnoi Khimii, Vol. 41, No. 4, pp. 701-708, July-August, 2000 相似文献
16.
P. Wardman 《Journal of Radioanalytical and Nuclear Chemistry》1998,232(1-2):23-27
Radicals produced in cellular systems are frequently “repaired” by thiols, but the sulphur-centred (thiyl) radical resulting
has to “sink” its unpaired electron in other reactions. It has been suggested that superoxide is the major radical sink, via
thiyl conjugation with thiolate and electron transfer to oxygen. It is argued here, from chemical kinetic data largely obtained
by pulse radiolysis methods, that ascorbate probably provides the major radical sink when radicals are produced in most mammalian
tissues at typical physiological pH. 相似文献
17.
Yu. I. Vesnin 《Journal of Structural Chemistry》1995,36(4):655-660
Published experimental data on the dependence of the properties of crystal substances on crystal size are discussed. The dependence
is of the same form for different properties and different crystal substances. When the crystal diminishes in size, its properties
remain constant until the size decreases to 10−5–10−6 cm; smaller crystals considerably change their properties. It is shown that the currently available experimental data are
sufficient to substantiate the notion of an “elementary unit of a solid crystal” (“crystal quantum”). The elementary unit
of a crystal is regarded as a basis for the secondary structure (macrostructure) of a solid and an elementary carrier of its
“crystal” properties. Due to this notion, we can interpret the vast experimental material on the real structures and properties
of solids from a single viewpoint.
Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 724–730, July–August, 1995.
Translated by L. Smolina 相似文献
18.
René Dybkaer 《Accreditation and quality assurance》2007,12(10):553-557
Laboratory medicine provides results for quantities as well as for properties having no magnitude. The terminology of the
latter is less well established and sources are contradictory. Two recent papers on “protometrology” published in this journal
offer an opportunity to discuss the necessary concept systems. The delineations of “metrology” versus “protometrology”, “observation”
versus “measurement”, and the generic division of “property” are examined with emphasis on avoiding conflict with the International
Vocabulary of Metrology. It is suggested that having “examination” as a top generic concept coupled with systematic modifiers
for division, especially ‘nominal’ and ‘ordinal’, is a preferable terminological solution.
相似文献
René Dybkaer |
19.
Marco Bonechi Walter Giurlani Massimo Innocenti Dario Pasini Suryakant Mishra Roberto Giovanardi Claudio Fontanesi 《Molecules (Basel, Switzerland)》2022,27(14)
This paper studies the mechanism of electrochemically induced carbon–bromine dissociation in 1-Br-2-methylnaphalene in the reduction regime. In particular, the bond dissociation of the relevant radical anion is disassembled at a molecular level, exploiting quantum mechanical calculations including steady-state, equilibrium and dissociation dynamics via dynamic reaction coordinate (DRC) calculations. DRC is a molecular-dynamic-based calculation relying on an ab initio potential surface. This is to achieve a detailed picture of the dissociation process in an elementary molecular detail. From a thermodynamic point of view, all the reaction paths examined are energetically feasible. The obtained results suggest that the carbon halogen bond dissociates following the first electron uptake follow a stepwise mechanism. Indeed, the formation of the bromide anion and an organic radical occurs. The latter reacts to form a binaphthalene intrinsically chiral dimer. This paper is respectfully dedicated to Professors Anny Jutand and Christian Amatore for their outstanding contribution in the field of electrochemical catalysis and electrosynthesis. 相似文献
20.
The phytohormone abscisic acid (ABA) is the major player in mediating the adaptation of plants to stress. Previously developed
phytohormonal biosensors usually employed indirect detection of the products of conjugated oxidase reactions. A label-free
electrochemical impedance immunosensor for ABA detection was developed using an anti-ABA antibody adsorbed directly on a porous
nanogold film. The film was produced electrochemically on a glassy carbon electrode in 0.008 mol/L hydrogen tetrachloroaurate
solution containing 0.004 mol/L lead acetate with an applied potential of −0.5 V (versus Ag/AgCl) for 50 s. The anti-ABA antibody
was immobilized onto the porous nanogold through electrostatic adsorption and covalent conjugation. Electrochemical impedance
spectroscopy was used to characterize the successful construction of the porous nanogold film and the stepwise modification
of the glassy carbon electrode. The concentration increase of the antigen brought about a decrease of the interfacial electron
transfer, which also meant an increase of the impedance signal. The experimental parameters pH, antibody incubation time,
and antibody concentration were optimized. The results showed significant linearity R = 0.9942, with the content of ABA in the range 0.5–5,000 ng/mL with a detection limit at about 0.1 ng/mL. 相似文献