Charge transfer in solution

Charge Transport in Device Configuration Versus Charge Transfer in Solution Chemistry Experiments... 

Electrochemistry, to distinguish it from the topics discussed in previous sections, is concerned with low-energy charge transfer in solution. The electron transfer typically occurs on the surface of a charged (usually metal) electrode. Possible chemical reactions that may occur, and that may be of importance in chemical synthesis, are the generation or annihilation of gases (in an electrolysis or a fuel cell, respectively) and the generation or neutralization of ions, which may be accompanied with the dissolution or deposition of a solid material. 

In favorable cases, the rate of charge transfer in solutions can be measured directly by following changes in absorption due to the formation of separated ions [38]. Furthermore, the distance dependence of the electron-transfer rate may be... 

The partial charge on the ion in vacuum, qi decreases substantially at small distances from the surface (Fig. 37). Ion hydration stabilizes the ionic charge and consequently the partial charge transfer from the ion to the surface is reduced. Substantial charge transfer in solution occurs only at distances smaller than 4.5 A from the surface. At larger distances, qh is almost equal to -1. At 4.5 A, however, the... 

In metals, current flow is due to electron motion. This is not the case in electrolytic solution here a potential difference leads to net ionic migration. It is the motion of the ions which accounts for charge transfer in solution. 

Ameer-Beg S, Ormson SM, Poteau X et al (2004) Ultrafast measurements of charge and excited-state intramolecular proton transfer in solutions of 4 -(N, N-dimethylamino) derivatives of 3-hydroxyflavone. J Phys Chem A 108 6938-6943... 

However, a detailed model for the defect structure is probably considerably more complex than that predicted by the ideal, dilute solution model. For higher-defect concentration (e.g., more than 1%) the defect structure would involve association of defects with formation of defect complexes and clusters and formation of shear structures or microdomains with ordered defect. The thermodynamics, defect structure, and charge transfer in doped LaCo03 have been reviewed recently [84],... 

The increased importance of charge transfer in proceeding up the series of NO+ complexes with the enhanced donor strength of arenes that vary from benzene with IP = 9.23 eV to the electron-rich hexamethylbenzene (IP - 7.85 eV) has its chemical consequences with respect to thermal (adiabatic) electron transfer. Thus the benzene complex with Z = 0.52 is persistent in acetonitrile solution for long periods, provided the solution is protected from adventitious moisture and light. By contrast, the hexamethylbenzene complex with Z = 0.97 slowly liberates nitric oxide under... 

Intramolecular charge transfer in p-anthracene-(CH2)3-p-Ar,Af-dimethylaniline (61) has been observed174 in non-polar solvents. Measurements of fluorescence-decay (by the picosecond laser method) allow some conclusions about charge-transfer dynamics in solution internal rotation is required to reach a favourable geometry for the formation of intramolecular charge-transfer between the donor (aniline) and the acceptor (anthracene). 

This shows that the number of hydrogen ions used in cathodic reactions is equal to the number of charges transferred in the anodic reaction. The pH value in the solution can then be maintained constant by a pH stat, controlling the addition of acid to the solution at such a rate that the loss of hydrogen ions is compensated. Then the following condition is fulfilled ... 

Kinetic Scheme. Generally, metal ions in a solution for electroless metal deposition have to be complexed with a ligand. Complexing is necessary to prevent formation of metal hydroxide, such as Cu(OH)2, in electroless copper deposition. One of the fundamental problems in electrochemical deposition of metals from complexed ions is the presence of electroactive (charged) species. The electroactive species may be complexed or noncomplexed metal ion. In the first case, the kinetic scheme for the process of metal deposition is one of simple charge transfer. In the second case the kinetic scheme is that of charge transfer preceded by dissociation of the complex. The mechanism of the second case involves a sequence of at least two basic elementary steps ... 

The total quantum yield [4>cs(total)] for CS is decreased to 0.17 in dimethyl-formamide (DMF) due to the competition of the CSH from Fc-ZnP-H2F+-C6o (1.63 eV) to Fc-ZnP- -HzP-Cso (1.34 eV) versus the decay of Fc-ZnP-Fl2P -C6o to the triplet states of the freebase porphyrin (1.40 eV) and the Ceo (1.50 eV) [47]. In contrast to the case of most donor-acceptor-linked systems, the decay dynamics of the charge-separated radical pair (Fc -ZnP-H2P-C6o ) does not obey first-order kinetics, but, instead, obeys second-order kinetics [47]. This indicates that the mframolecular electron transfer in Fc -ZnP-H2P-C6o" is too slow to compete with the diffusion-limited inter-molecular electron transfer in solution. 

The electrochemical rate constants of the Zn(II)/Zn(Hg) system obtained in propylene carbonate (PC), acetonitrile (AN), and HMPA with different concentrations of tetraethylammonium perchlorate (TEAP) decreased with increasing concentration of the electrolyte and were always lower in AN than in PC solution [72]. The mechanism of Zn(II) electroreduction was proposed in PC and AN the electroreduction process proceeds in one step. In HMPA, the Zn(II) electroreduction on the mercury electrode is very slow and proceeds according to the mechanism in which a chemical reaction was followed by charge transfer in two steps (CEE). The linear dependence of logarithm of heterogeneous standard rate constant on solvent DN was observed only for values corrected for the double-layer effect. 

It is possible to solve this model analytically. The model is useful for calculating the qualitative potential distribution in high-surface-area solar cells and how it is influenced by the relative facility of different charge-transfer pathways. Solutions for a 100-resistor network are shown in Fig. 2, where Vapp = 1.0 V, Rs = 0.001 11, and RTi02 + Rct = 104fl. 

At the early stages the photoconductivity of solid solutions of the leucobase of malachite green in various organic media was investigated [285]. In these systems, carrier transport occurs by direct interaction between the leucobase molecules. No direct participation of the organic matrix in the charge transfer was observed. A model was proposed which links charge transfer in these systems with impurity conduction in semiconductors. 

Thus hole or electron transfer can follow a number of pathways across the semiconductor/electrolyte interface. First, one can have direct oxidative or reductive charge transfer to solution species resulting in desired product formation. Second, one can have direct charge transfer resulting in surface modification, such as oxide film growth on GaP or CdS in aqueous PECs. Finally, one can have photoemission of electrons or holes directly into the electrolyte. All of these processes provide some information about the electronic structure of the interface. 

The knowledge of the two-minima energy surface is sufficient theoretically to determine the microscopic and static rate of reaction of a charge transfer in relation to a geometric variation of the molecule. In practice, the experimental study of the charge-transfer reactions in solution leads to a macroscopic reaction rate that characterizes the dynamics of the intramolecular motion of the solute molecule within the environment of the solvent molecules. Stochastic chemical reaction models restricted to the one-dimensional case are commonly used to establish the dynamical description. Therefore, it is of importance to recall (1) the fundamental properties of the stochastic processes under the Markov assumption that found the analysis of the unimolecular reaction dynamics and the Langevin-Fokker-Planck method, (2) the conditions of validity of the well-known Kramers results and their extension to the non-Markovian effects, and (3) the situation of a reaction in the absence of a potential barrier. 

Mattay et al., having discovered exciplex emission from solutions of benzene and 1,3-dioxole [122], continued their investigations with a study on selectivity and charge transfer in photoreactions of a,a,a-trifluorotoluene with 1,3-dioxole and some of its derivatives, and with vinylene carbonate and dimethylvinylene carbonate [15,143,144], a,a,a-Trifluorotoluene and 1,3-dioxole upon irradiation yield three types of products ortho cycloadducts, meta cycloadducts, and so-called substitution products (Scheme 44). The products are formed in the ratio ortho adductsimeta adducts substitution products = 0.8 1.7 0.3. The substitution reaction (which is really an addition of a C—F bond to the double bond of 1,3-dioxole, but named substitution in order to distinguish it from the ortho addition [186] is supposed to start with electron transfer from 1,3-dioxole to excited a,a,a-trifluorotoluene. The radical anion then releases a fluoride ion, which adds to the 1,3-dioxole radical cation. Radical combination then leads to the product. 

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