Intramolecular electronics description

In a general description of intramolecular electron-transfer (ET) processes one has to differentiate between charge separation in donor/acceptor (D/A) systems via the formation of photoexcited states and a charge-transfer or charge-shift reaction that is thermally activated (Cannon, 1980 Fox and Chanon, 1988 Meyer, 1978). 

Finally, we consider the performance of the MFT method for nonadiabatic dynamics induced by avoided crossings of the respective potential energy surfaces. We start with the discussion of the one-mode model. Model IVa, describing ultrafast intramolecular electron transfer. The comparison of the MFT method (dashed line) with the quantum-mechanical results (full line) shown in Fig. 5 demonstrates that the MFT method gives a rather good description of the short-time dynamics (up to 50 fs) for this model. For longer times, however, the dynamics is reproduced only qualitatively. Also shown is the time evolution of the diabatic electronic coherence which, too, is... 

Electronic and vibrational spectroscopy continues to be important in the characterization of iron complexes of all descriptions. Charge-transfer spectra, particularly of solvatochromic ternary diimine-cyanide complexes, can be useful indicators of solvation, while IR and Raman spectra of certain mixed valence complexes have contributed to the investigation of intramolecular electron transfer. Assignments of metal-ligand vibrations in the far IR for the complexes [Fe(8)3] " " were established by means of Fe/ Fe isotopic substitution. " A review of pressure effects on electronic spectra of coordination complexes includes much information about apparatus and methods and about theoretical aspects, though rather little about specific iron complexes. ... 

The discussed mechanisms represent a form of intramolecular catalysis of the oxidation of the FeII(CN)5 or Run(edta) centers by the Ruii(NH3)5 moiety. The first two moieties react sluggisly and, on the other hand, the electron in RuII(NH3)5 is readily accessible to the external oxidant and is given up. The rapid electronic isomerization processes aid in the consumption of the full oxidation process. This is not truly catalytic because the catalyst is the reactant itself, which, of course, is consumed in the reaction. A better description involves a net oxidation of the FeII(CN)5 or Run(edta) sites through activation by the facile intramolecular electron transfer between the metal centers. The mechanism is described in Fig. 23, bearing some resemblance to the classical chemical mechanism for inner sphere electron... 

In the MO theory, the most reliable approach for the study of reaction pathways perhaps is CASSCF [12, 13], but multi-VBSCF is essentially at the same level with CASSCF [14]. Since a VB wave function can be expanded into the combination of numerous Slater determinants that are used to define configurations in the MO theory, the VB theory provides a very compact, accurate description for chemical reactions. While both MO and VB theories have their own advantages as well as disadvantages, in our opinions, there are some areas where the VB theory is particularly superior to the MO theory 1) the refinement of molecular mechanics force field 2) the development of empirical or semi-empirical VB approaches 3) the impact of intermolecular charge transfer or intramolecular electron delocalization on the structure and properties 4) the validation of classical chemical theories and concepts at the quantitative level 5) the elucidation of chemical reactions and excited states intuitively. 

Abstract Analytical solution of the associative mean spherical approximation (AMSA) and the modified version of the mean spherical approximation - the mass action law (MSA-MAL) approach for ion and ion-dipole models are used to revise the concept of ion association in the theory of electrolyte solutions. In the considered approach in contrast to the traditional one both free and associated ion electrostatic contributions are taken into account and therefore the revised version of ion association concept is correct for weak and strong regimes of ion association. It is shown that AMSA theory is more preferable for the description of thermodynamic properties while the modified version of the MSA-MAL theory is more useful for the description of electrical properties. The capabilities of the developed approaches are illustrated by the description of thermodynamic and transport properties of electrolyte solutions in weakly polar solvents. The proposed theory is applied to explain the anomalous properties of electrical double layer in a low temperature region and for the treatment of the effect of electrolyte on the rate of intramolecular electron transfer. The revised concept of ion association is also used to describe the concentration dependence of dielectric constant in electrolyte solutions. 

In this chapter some aspects of the present state of the concept of ion association in the theory of electrolyte solutions will be reviewed. For simplification our consideration will be restricted to a symmetrical electrolyte. It will be demonstrated that the concept of ion association is useful not only to describe such properties as osmotic and activity coefficients, electroconductivity and dielectric constant of nonaqueous electrolyte solutions, which traditionally are explained using the ion association ideas, but also for the treatment of electrolyte contributions to the intramolecular electron transfer in weakly polar solvents [21, 22] and for the interpretation of specific anomalous properties of electrical double layer in low temperature region [23, 24], The majority of these properties can be described within the McMillan-Mayer or ion approach when the solvent is considered as a dielectric continuum and only ions are treated explicitly. However, the description of dielectric properties also requires the solvent molecules being explicitly taken into account which can be done at the Born-Oppenheimer or ion-molecular approach. This approach also leads to the correct description of different solvation effects. We should also note that effects of ion association require a different treatment of the thermodynamic and electrical properties. For the thermodynamic properties such as the osmotic and activity coefficients or the adsorption coefficient of electrical double layer, the ion pairs give a direct contribution and these properties are described correctly in the framework of AMSA theory. Since the ion pairs have no free electric charges, they give polarization effects only for such electrical properties as electroconductivity, dielectric constant or capacitance of electrical double layer. Hence, to describe the electrical properties, it is more convenient to modify MSA-MAL approach by including the ion pairs as new polar entities. 

The nontraditional example of applying the AMSA theory is connected with the treatment of electrolyte effects in intramolecular electron transfer (ET) reactions [21, 22], Usually the process of the transfer of the electron from donor (D) to acceptor (A) in solutions is strongly nonadiabatic. The standard description of this process in connected with semiclassical Marcus theory [35], which reduces a complex dynamical problem of ET to a simple expression of electron... 

When interfacial electron exchange rate(s) are sufficiently high and the response is free from mass transport hmitations, the catalytic current will be determined by the inherent activity of the enzyme. Variation of current (activity) with potential can be explained by an extension of the Michaelis-Menten description of enzyme kinetics that relates activity to oxidation state through incorporation of the Nemst equation." " The resulting expressions describe the catalytic cycle, and include rates of intramolecular electron exchange, chemical events, substrate binding and product release, together with the reduction potentials of centres in the enzyme, and the influence of... 

In 1932 Kistiakowsky and Nelles showed that intense resonance fluorescence could be excited in low pressures of benzene with the 2536 A Hg line. This revealed an opportunity to study isolated molecule relaxation, which was not used until 1964. Since that time, Kistiakowsky and co-workers as well as others have completed extensive studies of isolated molecule relaxation from the several levels populated by absorption of 2536 A radiation. Although the experiments are now being superseded by those which explore relaxation from single vibronic levels, the 2536 A experiments play an important role in answering fundamental questions about isolated molecule properties. They formed the first extensive body of kinetic data on large-molecule intramolecular electronic relaxation, and they still remain one of the most secure experimental descriptions of the intramolecular nature of intersystem crossing. 

In this monograph, the concept of acid-base function is based on the inevitable change in intramolecular electron density pattern as molecules approach each other, a change giving rise to intermolecular forces. It is appreciated that there is a wide range of intensities, but I contend that any attempt to draw a line of demarcation between what certain persons call physical as distinct from chemical interactions is not merely arbitrary but unnecessary and undesirable. Descriptions involving such a distinction carry ad hoc reservations and excuses which tend to cancel out on passing from one specialist to another. 

Although there are many components in a mechanistic description of a chemiluminescent reaction, the heart of the matter is the actual excitation step. Several such steps have been identified. Some are molecular in character e. g. the decomposition of dioxetans and some are intermolecular electron transfer steps. There is an intermediate class in which the step can be formulated as an /n ramolecular electron transfer. Many luminescent reactions have been ascribed to this category with varying degrees of confidence. Cyclic hydrazides such as luminol belong rather uncertainly here. Electron rich dioxetans and dioxetanones and the luciferins with such intermediates on the pathway are a little more reasonably assigned to an intramolecular electron transfer mechanism. Even here however caution is required in that direct evidence for discrete electron transfer will by its very nature be almost impossible to obtain and will probably remain circumstantial. 

While being very similar in the general description, the RLT and electron-transfer processes differ in the vibration types they involve. In the first case, those are the high-frequency intramolecular modes, while in the second case the major role is played by the continuous spectrum of polarization phonons in condensed 3D media [Dogonadze and Kuznetsov 1975]. The localization effects mentioned in the previous section, connected with the low-frequency part of the phonon spectrum, still do not show up in electron-transfer reactions because of the asymmetry of the potential. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

The brief review of the newest results in the theory of elementary chemical processes in the condensed phase given in this chapter shows that great progress has been achieved in this field during recent years, concerning the description of both the interaction of electrons with the polar medium and with the intramolecular vibrations and the interaction of the intramolecular vibrations and other reactive modes with each other and with the dissipative subsystem (thermal bath). The rapid development of the theory of the adiabatic reactions of the transfer of heavy particles with due account of the fluctuational character of the motion of the medium in the framework of both dynamic and stochastic approaches should be mentioned. The stochastic approach is described only briefly in this chapter. The number of papers in this field is so great that their detailed review would require a separate article. 

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