Página 1 dos resultados de 437 itens digitais encontrados em 0.016 segundos

## Effective distances of exothermic charge-transfer reactions in the excited state

Costa, Sílvia M. B.; Maçanita, A. L.; Formosinho, S. J.
Fonte: American Chemical Society Publicador: American Chemical Society
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
56.05%
A study of intermolecular, exothermic charge-transfer interactions is described. A distance dependence on the unimolecular charge-transfer rate constant k,,(R) is observed in viscous solvents where transfer at longer distances than the collisional one can compete with dissociation of an encounter complex. A comparative study of the quenching phenomena, occurring in a family of redox partners of aromatic esters and amines, was carried out in nonviscous solvents (hexane and acetonitrile) and viscous solvents (liquid paraffin/heptane mixtures and nonionic micelles of Triton X- 100). In the latter, the experimental reactional distances obtained are discussed in terms of effective distances R,ff, which are related to an average value of kct(Reff) for the reactant pair distribution function. A correlation of these distances with either the free activation energy, Act, or the diffusion coefficient D enables the estimation of an order of magnitude of parameters such as the average radii of donor and acceptor orbitals, L = (2.7 f 0.2) X cm, a preexponential factor at the collisional distance, k’,,(R,) = 3.7 X 10” d, an intrinsic reorganization parameter X = 0.64 eV, and an electronic matrix element V(Rc) N eV for the nonpolar media studied. An equation for kct(R)...

## Low-energy three-body charge transfer reactions with Coulomb interaction in the final state

Sultanov, R. A.; Adhikari, S. K.
Fonte: Iop Publishing Ltd Publicador: Iop Publishing Ltd
Tipo: Artigo de Revista Científica Formato: 5751-5758
ENG
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Three-body charge transfer reactions with Coulomb interaction in the final state are considered within the framework of coordinate-space integro-differential Faddeev-Hahn-type equations within two- and six-state close-coupling approximations. The method is employed to study direct muon transfer in low-energy collisions of the muonic hydrogen H-mu by helium (He2+) and lithium (Li3+) nuclei. The experimentally observed isotopic dependence is reproduced.

## Smooth landscape solvent dynamics in electron transfer reactions

Leite, Vitor B. P.
Tipo: Artigo de Revista Científica Formato: 10067-10075
ENG
Relevância na Pesquisa
55.93%
Solvent effects play a major role in controlling electron-transfer reactions. The solvent dynamics happens on a very high-dimensional surface, and this complex landscape is populated by a large number of minima. A critical problem is to understand the conditions under which the solvent dynamics can be represented by a single collective reaction coordinate. When this unidimensional representation is valid, one recovers the successful Marcus theory. In this study the approach used in a previous work [V. B. P. Leite and J. N. Onuchic; J. Phys. Chem. 100, 7680 (1996)] is extended to treat a more realistic solvent model, which includes energy correlation. The dynamics takes place in a smooth and well behaved landscape. The single shell of solvent molecules around a cavity is described by a two-dimensional system with periodic boundary conditions with nearest neighbor interaction. It is shown how the polarization-dependent effects can be inferred. The existence of phase transitions depends on a factor y proportional to the contribution from the two parameters of the model. For the present model, γ suggests the existence of weak kinetic phase transitions, which are used in the analysis of solvent effects in charge-transfer reactions. © 1999 American Institute of Physics.

## Dynamical calculation of direct muon-transfer rates from thermalized muonic hydrogen to C6+ and O8+

Tipo: Artigo de Revista Científica Formato: 022509-022501
ENG
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56.03%
Moun-transfer reactions from muonic hydrogen to carbon and oxygen nuclei employing a full quantum-mechanical few-body description of rearrangement scattering were studied by solving the Faddeev-Hahn-type equations using close-coupling approximation. The application of a close-coupling-type ansatz led to satisfactory results for direct muon-transfer reactions from muonic hydrogen to C6+ and O8+.

## Increasing the Negative Charge of a Macroanion in the Gas Phase via Sequential Charge Inversion Reactions

He, Min; McLuckey, Scott A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em 15/07/2004 EN
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Protonated and deprotonated biological molecules in the gas phase play an important role in life sciences research. The structural information accessible from the ions is highly dependent upon their charge states. Therefore, it is desirable to develop means for increasing absolute charge states, particularly for ionization methods, such as MALDI, that yield relatively low charge ions. The work presented here demonstrates the formation of a doubly deprotonated polypeptide or oligonucleotide ion (dianion) from a singly deprotonated analogue via two sequential ion/ion proton-transfer reactions involving charge inversion. The high exoergicity and the large cross section arising from the long-range attractive Coulomb potential of ion/ion reactions make this process plausible. In this example, an overall efficiency of conversion of singly charged ions to doubly charged ions of roughly 8% for polypeptide was noted while lower efficiency (roughly 2%) observed with an oligonucleotide is likely due to a greater degree of neutralization. No other approach to increasing the net negative charge of an anion in the gas phase has as yet been reported.

## Azide as a Probe of Proton Transfer Reactions in Photosynthetic Oxygen Evolution

Cooper, Ian B.; Barry, Bridgette A.
Fonte: The Biophysical Society Publicador: The Biophysical Society
Tipo: Artigo de Revista Científica
EN
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46.05%
In oxygenic photosynthesis, photosystem II (PSII) is the multisubunit membrane protein responsible for the oxidation of water to O2 and the reduction of plastoquinone to plastoquinol. One electron charge separation in the PSII reaction center is coupled to sequential oxidation reactions at the oxygen-evolving complex (OEC), which is composed of four manganese ions and one calcium ion. The sequentially oxidized forms of the OEC are referred to as the Sn states. S1 is the dark-adapted state of the OEC. Flash-induced oxygen production oscillates with period four and occurs during the S3 to S0 transition. Chloride plays an important, but poorly understood role in photosynthetic water oxidation. Chloride removal is known to block manganese oxidation during the S2 to S3 transition. In this work, we have used azide as a probe of proton transfer reactions in PSII. PSII was sulfate-treated to deplete chloride and then treated with azide. Steady state oxygen evolution measurements demonstrate that azide inhibits oxygen evolution in a chloride-dependent manner and that azide is a mixed or noncompetitive inhibitor. This result is consistent with two azide binding sites, one at which azide competes with chloride and one at which azide and chloride do not compete. At pH 7.5...

## Dissecting the Kinetics of the NADP+ - FADH2 Charge Transfer Complex and Flavin Semiquinones in Neuronal Nitric Oxide Synthase

Li, Huiying; Jamal, Joumana; Chreifi, Georges; Venkatesh, Vikram; Abou-Ziab, Hoda; Poulos, Thomas L.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
46.07%
Electron flow within the neuronal nitric oxide synthase reductase domain (nNOSrd) includes hydride transfer from NADPH to FAD followed by two one-electron transfer reactions from FAD to FMN. We have used stopped flow spectrometry to closely monitor these electron transfer steps for both the wild type and the ΔG810 mutant of nNOSrd using a protocol involving both global analyses of the photodiode array spectral scans and curve fittings of single wavelength kinetic traces. The charge transfer complex and interflavin electron transfer events recorded at 750 nm and 600 nm, respectively, show the kinetics in different time frames. All electron transfer events are slow enough at 4 °C to enable measurements of rate constants even for the fast charge transfer event. To our knowledge this is the first time the rate constants for the charge transfer between NADP+ and FADH2 have been determined for NOS. These procedures allow us to conclude that (1) binding of the second NADPH is necessary to drive the full reduction of FMN and; (2) charge transfer and the subsequent interflavin electron transfer have distinct spectral features that can be monitored separately with stopped flow spectroscopy. These studies also enable us to conclude that interflavin electron transfer reported at 600 nm is not limiting in NOS catalysis.

## Vers des assemblages de complexes métalliques oligonucléaires, servant d’antenne solaire au niveau moléculaire

Chartrand, Daniel
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
FR
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Ce projet de recherche vise l’élaboration de systèmes métallosupramoléculaires artificiels imitant le processus naturel de la photosynthèse. Idéalement, ces systèmes seraient capables de fournir l’énergie et la séparation de charge nécessaire pour catalyser des réactions à transfert multiélectroniques, tel que l’hydrolyse de l’eau ou la réduction du gaz carbonique. La réalisation d’un tel système catalytique créerait une source d’énergie renouvelable, sous forme d’énergie chimique, crée directement à partir de l’énergie solaire. Le système envisagé, schématisé sous la forme d’une antenne, possède trois parties distinctes. Tout d’abord, des chromophores forment un état excité en captant l’énergie de la lumière visible du soleil. Vient ensuite un centre de liaison qui lie tous les chromophores et qui collecte l’énergie de cet état excité à travers un transfert d’électron. Cet électron est de nouveau transféré vers la dernière partie, un centre réactionnel catalytique. Cet assemblage permet de créer une séparation de charge entre le chromophore et le centre réactionnel qui sont séparés par le centre de liaison, évitant ainsi la recombinaison de charge. Le projet se focalise sur la synthèse...

## Anwendung von Titannitridchlorid als Kathode in einer Lithium-Metall-Batterie und Synthese neuer TiCl4-Additionsverbindungen; Application of titanium nitride chloride as a cathode in a lithium metal battery and synthesis of new TiCl4-charge-transfer complexes

Hoppe, Holger
Fonte: Universität Tübingen Publicador: Universität Tübingen
Tipo: Dissertation; info:eu-repo/semantics/doctoralThesis
DE_DE
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In einem ersten Teil dieser Arbeit wurden die Reaktionen der Lewis-Säure Titantetrachlorid mit den stickstoff- beziehungsweise phosphorhaltigen Donorliganden Acetonitril, Triphenylphosphan, 1,2-Bis-(diphenylphosphino)-ethan und 2,2'-Bipyridyl untersucht und eine Charakterisierung und Kristallstrukturermittlung der jeweils entstehenden Addukt-Komplexe vorgestellt, die als Ausgangssubstanz für Komplexe mit Nitridobrücken am Titanatom dienen sollten. Die Bildung von Komplexen mit Nitridobrücken zwischen Titanatomen konnte durch Reaktion des reinen Metallhalogenids TiCl4 mit N(SiMe3)3 oder unter Verwendung der oben genannten Adduktkomplexe an TiCl4 jedoch nicht beobachtet werden. Ein zweiter Teil dieser Arbeit behandelt die Synthese von Titannitridchlorid und dessen Verwendung als Kathode in einer sekundären Lithium-Metall-Batterie. Titannitridchlorid konnte sowohl durch Ammonolyse von Titantetrachlorid, als auch durch die Umsetzung von TiCl4 mit dem tertiären Amin N(SiMe3)3 dargestellt und charakterisiert werden. Titannitridchlorid wurde als Kathode in einem galvanischen Element Li//LiPF6//TiNCl eingesetzt und deren Eignung zur reversiblen Lithium-Interkalation zu LixTiNCl elektrochemisch untersucht. Die ausgezeichnete Reversibilität der Li+-Einlagerung in TiNCl konnte über 12 vollständige Entlade-Ladezyklen im Spannungsbereich 3.0 V > U > 1.2 V bestätigt werden. Supraleitende Eigenschaften konnten für Li0.19TiNCl nicht festgestellt werden; beobachtet wurde paramagnetisches Verhalten. Die reversible spezifische Kapazität der elektrochemischen Zelle Li//LiPF6//TiNCl beträgt ca. 147 Ah·kg-1 während 12 vollständiger Entlade-Ladezyklen.; In a first part of this work the reactions of titaniumtetrachloride with the nitrogen- or phosphorous-containing ligands acetonitrile...

## Towards the nanomechanical actuation and controlled assembly of nanomaterials using charge-transfer reactions in electroactive self-assembled monolayers

Norman, Lana
Fonte: Université de Montréal Publicador: Université de Montréal
Tipo: Thèse ou Mémoire numérique / Electronic Thesis or Dissertation
EN
Relevância na Pesquisa
56.05%
Les microcantileviers fonctionnalisés offrent une plateforme idéale pour la nano- et micro-mécanique et pour le développement de (bio-) capteurs tres sensible. Le principe d’opération consiste dans des évènements physicochimiques qui se passent du côté fonctionnalisé du microcantilevier induisant une différence de stress de surface entre les deux côtés du cantilevier qui cause une déflexion verticale du levier. Par contre, les facteurs et les phénomènes interfacials qui régissent la nature et l'intensité du stress de surface sont encore méconnus. Pour éclaircir ce phénomène, la première partie de cette thèse porte sur l'étude des réactions de microcantileviers qui sont recouverts d'or et fonctionnalisés par une monocouche auto-assemblée (MAA) électroactive. La formation d'une MAA de ferrocènylundécanethiol (FcC11SH) à la surface d'or d'un microcantilevier est le modèle utilisé pour mieux comprendre le stress de surface induit par l’électrochimie. Les résultats obtenus démontrent qu'une transformation rédox de la MAA de FcC11SH crée un stress de surface qui résulte dans une déflexion verticale du microcantilevier. Dépendamment de la flexibilité du microcantilevier, cette déflexion peut varier de quelques nanomètres à quelques micromètres. L’oxydation de cette MAA de FcC11SH dans un environnement d'ions perchlorate génère un changement de stress de surface compressive. Les résultats indiquent que la déflexion du microcantilevier est due à une tension latérale provenant d'une réorientation et d'une expansion moléculaire lors du transfért de charge et de pairage d’anions. Pour vérifier cette hypothèse...

## Use of the Charge Transfer Reactions for the Spectrophotometric Determination of Risperidone in Pure and in Dosage Forms

Deepakumari, Hemavathi Nagaraju; Revanasiddappa, Hosakere Doddarevanna
Fonte: Hindawi Publishing Corporation Publicador: Hindawi Publishing Corporation
Tipo: Artigo de Revista Científica
EN
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The aim of study was to develop and validate two simple, sensitive, and extraction-free spectrophotometric methods for the estimation of risperidone in both pure and pharmaceutical preparations. They are based on the charge transfer complexation reactions between risperidone (RSP) as n-electron donor and p-chloranilic acid (p-CA) in method A and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in method B as π-acceptors. In method A, RSP reacts with p-CA in methanol to produce a bright pink-colored chromogen measured at 530 nm whereas, in method B, RSP reacts with DDQ in dichloromethane to form orange-colored complex with a maximum absorption at 460 nm. Beer's law was obeyed in the concentration range of 0–25 and 0–50 μg/mL with molar absorptivity of 1.29 × 104 and 0.48 × 104 L/moL/cm for RSP in methods A and B, respectively. The effects of variables such as reagents, time, and stability of the charge transfer complexes were investigated to optimize the procedures. The proposed methods have been successfully applied to the determination of RSP in pharmaceutical formulations. Results indicate that the methods are accurate, precise, and reproducible (relative standard deviation <2 %).

## Localization of positive charge in DNA induced by its interaction with environment

Uskov, Dmitry B.; Burin, Alexander L.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Microscopic mechanisms of positive charge transfer in DNA remain unclear. A quantum state of electron hole in DNA is determined by the competition of the pi-stacking interaction $b$ sharing a charge between different base pairs and the interaction $\lambda$ with the local environment which attempts to trap charge. To determine which interaction dominates we investigated charge quantum states in various $(GC)_{n}$ sequences choosing DNA parameters satisfying experimental data for the balance of charge transfer rates $G^{+} \leftrightarrow G_{n}^{+}$, $n=2,3$ \cite{FredMain}. We show that experimental data can be consistent with theory only assuming $b\ll \lambda$ meaning that charge is typically localized within the single $G$ site. Consequently any DNA sequence including the one consisting of identical base pairs behaves more like an insulating material then a molecular conductor. Our theory can be verified experimentally, for instance measuring balance of charge transfer reactions $G^{+} \leftrightarrow G_{n}^{+}$, $n \geq 4$ and comparing the experimental results with our predictions.; Comment: Submitted to the Journal of Chemical Physics

## Proton-${}^3$H scattering calculation: Elastic and charge-exchange reactions up to 30 MeV

Deltuva, A.; Fonseca, A. C.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.01%
Proton-${}^3$H elastic scattering and charge-exchange reaction ${}^3$H$(p,n){}^3$He in the energy regime above four-nucleon breakup threshold are described in the momentum-space transition operator framework. Fully converged results are obtained using realistic two-nucleon potentials and two-proton Coulomb force as dynamic input. Differential cross section, proton analyzing power, outgoing neutron polarization, and proton-to-neutron polarization transfer coefficients are calculated between 6 and 30 MeV proton beam energy. Good agreement with the experimental data is found for the differential cross section both in elastic and charge-exchange reactions; the latter shows a complicated energy and angular dependence. The most sizable discrepancies between predictions and data are found for the proton analyzing power and outgoing neutron polarization in the charge-exchange reaction, while the respective proton-to-neutron polarization transfer coefficients are well described by the calculations.; Comment: 6 figures, to be published in Phys. Rev. C

## Low-energy three-body charge transfer reactions with Coulomb interaction in the final state

Tipo: Artigo de Revista Científica
Relevância na Pesquisa
65.97%
Three-body charge transfer reactions with Coulomb interaction in the final state are considered in the framework of coordinate-space integro-differential Faddeev-Hahn-type equations within two- and six-state close coupling approximations. The method is employed to study direct muon transfer in low-energy collisions of the muonic hydrogen H$_\mu$ by helium (He$^{++}$) and lithium (Li$^{+++}$) nuclei. The experimentally observed isotopic dependence is reproduced.; Comment: 14 pages REVTeX, accepted for publication in Journal of Physics B

## Charge constrained density functional molecular dynamics for simulation of condensed phase electron transfer reactions

Oberhofer, H.; Blumberger, J.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.07%
We present a plane-wave basis set implementation of charge constrained density functional molecular dynamics (CDFT-MD) for simulation of electron transfer reactions in condensed phase systems. Following earlier work of Wu et al. Phys. Rev. A 72, 024502 (2005), the density functional is minimized under the constraint that the charge difference between donor and acceptor is equal to a given value. The classical ion dynamics is propagated on the Born-Oppenheimer surface of the charge constrained state. We investigate the dependence of the constrained energy and of the energy gap on the definition of the charge, and present expressions for the constraint forces. The method is applied to the Ru2+-Ru3+ electron self-exchange reaction in aqueous solution. Sampling the vertical energy gap along CDFT-MD trajectories, and correcting for finite size effects, a reorganization free energy of 1.6 eV is obtained. This is 0.1-0.2 eV lower than a previous estimate based on a continuum model for solvation. smaller value for reorganization free energy can be explained by fact that the Ru-O distances of the divalent and trivalent Ru-hexahydrates are predicted to be more similar in the electron transfer complex than for the separated aqua-ions.; Comment: 12 pages...

## Collisional and photoinitiated reaction dynamics in the ground electronic state of Ca–HCl

Sanz, Cristina; Avoird, Ad van der; Roncero, Octavio
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Artículo Formato: 203135 bytes; application/pdf
ENG
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46.14%
11 pages, 14 figures, 1 table.-- PACS nrs.: 82.30.Cf; 82.50.-m; 82.20.Kh; 82.30.Fi; 82.20.Rp; 82.20.Fd; 33.20.Ea; 33.15.Ry; 33.15.Dj; 82.30.Hk.; Ca+HCl(ν,j) reactive collisions were studied for different rovibrational states of the HCl reactant using wave-packet calculations in reactant Jacobi coordinates. A recently proposed potential-energy surface was used with a barrier of ≈ 0.4 eV followed by a deep well. The possibility of an insertion mechanism due to this last well has been analyzed and it was found that once the wave packet passes over the barrier most of it goes directly to CaCl+H products, which shows that the reaction dynamics is essentially direct. It was also found that there is no significant change in the reaction efficiency as a function of the initial HCl rovibrational state, because CaHCl at the barrier has an only little elongated HCl bond. Near the threshold for reaction with HCl(ν=0), however, the reaction shows significant steric effects for j>0. In a complementary study, the infrared excitation from the Ca–HCl van der Waals well was simulated. The spectrum thus obtained shows several series of resonances which correspond to quasibound states correlating to excited HCl(ν) vibrations. The Ca–HCl binding energies of these quasibound states increase dramatically with ν...

## Fast charge-transfer reactions of binuclear complexes undergoing large changes in metal---metal bond lengths

Gennett, Thomas; Geiger, William; Willet, Brian; Anson, Fred
Fonte: Elsevier: Journal of Electroanalytical Chemistry Publicador: Elsevier: Journal of Electroanalytical Chemistry
Tipo: Abstract Formato: 37365 bytes; application/pdf
EN_US
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Heterogeneous electron-transfer rates have been measured for three binuclear complexes, Cp2Fe2(CO)2(μ-SMe)2, 1, Cp2Fe2(CO)2(μ-PPh2)2, 2, and Cp2Mo2(μ-SMe)4, 3. Dahl and co-workers had shown previously that the cations of the two iron complexes display significant shortening of the metal---metal distance (47 pm and 36 pm, respectively, for 1 and 2) due to removal of an electron from an antibonding M---M orbital from the neutral compounds. By comparison, the structure of 3 is virtually unchanged upon one-electron oxidation. Surprisingly, the greater structural changes of 1 and 2 do not lead to lower heterogeneous electron-transfer rates than 3. In acetonitrile at Pt or mercury electrodes all three complexes have ks values, measured by cyclic voltammetry, of ca. 0.2 cm s−1. Several possible reasons for this behavior are discussed.

## Factors controlling the addition of carbon-centered radicals to alkenes-an experimental and theoretical perspective

Fischer, Hanns; Radom, Leo
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.01%
The successful exploitation of syntheses involving the generation of new carbon - carbon bonds by radical reactions rests on some prior knowledge of the rate constants for the addition of carbon-centered radicals to alkenes and other unsaturated molecules, and of the factors controlling them. Two former classical reviews in Angewandte Chemie by Tedder (1982) and by Giese (1983) provided mechanistic insight and led to various qualitative rules on the complex interplay of enthalpic, polar, and steric effects. In the meantime, the field has experienced very rapid progress: many more experimental absolute rate constants have become available, and there have been major advances in the efficiency and reliability of quantum-chemical methods for the accurate calculation of transition structures, reaction barriers, and reaction enthalpies. Herein we review this progress, recommend suitable experimental and theoretical procedures, and display representative data series for radical additions to alkenes. On this basis, and guided by the pictorial tool of the state-correlation diagram for radical additions, we then offer a new and more stringent quantification of the controlling factors. Our analysis leads to a partial revision of the previous qualitative rules...

## A New Formulation of the Frequency Factor in the Expression for the Rate Constant of Interfacial Charge Transfers

Velasco,Jaime González
Fonte: Sociedade Portuguesa de Electroquímica Publicador: Sociedade Portuguesa de Electroquímica
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2007 EN
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65.89%
A new way for deriving the fundamental equation of the electrochemistry has been developed in the potential range in which Butler-Volmer behaviour is found. In this approach the frequency factor is considered to be a function of the potential instead of the activation energy. The frequency factor is calculated by means of a classical statistical mechanics treatment giving rise to a new definition for the symmetry factor. A new expression for the standard rate constant in adiabatic heterogeneous charge transfer reactions has been derived.