Electronic medium effects

Existence of the end user was recognized in the 1980s it was not until the 1990s that vendors made concerted efforts to accommodate end user needs by providing faster, easier, and more powerhil ways to retrieve relevant information. In electronic information retrieval, the term accessibiUty implies that data exist in electronic form, that data retrieval is cost effective, intuitive, and easy, and that the electronic medium contributes to the quaUty and usabihty of the information (27). 

The effects of the modulation of electron density by local vibrations and polarization fluctuations are most pronounced for reactions involving transfer of weakly bound electrons. These effects were investigated in Ref. 16 for the transfer of weakly bound electrons from a donor Az to an acceptor BZ2 in a polar medium. 

Radiation techniques, application to the study of organic radicals, 12, 223 Radical addition reactions, gas-phase, directive effects in, 16, 51 Radicals, cation in solution, formation, properties and reactions of, 13, 155 Radicals, organic application of radiation techniques, 12,223 Radicals, organic cation, in solution kinetics and mechanisms of reaction of, 20, 55 Radicals, organic free, identification by electron spin resonance, 1,284 Radicals, short-lived organic, electron spin resonance studies of, 5, 53 Rates and mechanisms of solvolytic reactions, medium effects on, 14, 1 Reaction kinetics, polarography and, 5, 1... 

The separation of a reactant system (solute) from its environment with the consequent concept of solvent or surrounding medium effect on the electronic properties of a given subsystem of interest as general as the quantum separability theorem can be. With its intrinsic limitations, the approach applies to the description of specific reacting subsystems in their particular active sites as they can be found in condensed phase and in media including the rather specific environments provided by enzymes, catalytic antibodies, zeolites, clusters or the less structured ones found in non-aqueous and mixed solvents [1,3,6,8,11,12,14-30],... 

Zeng, J., Craw, J. S., Hush, N. S. and Reimers, J. R. Medium effects on molecular and ionic electronic spectra. Application to the lowest l(n,Tt ) state of dilute pyridine in water, Chem.Phys.Lett., 206 (1993), 323-328... 

Ion-radicals have a dual character. They contain an unpaired electron and are, therefore, close to radicals. At the same time, they bear a charge and are, naturally, close to ions. This is why the words ion and radicaf are connected by a hyphen. Being radicals, ion-radicals are ready to react with strange radicals. Like all other radicals, they can dismutate and recombine. Being ions, ion-radicals are able to react with particles of the opposite charge, and are prone to form ionic aggregates. In contrast to radicals, the ion-radicals are specially sensitive to medium effects. 

Studies of medium effects on hexacyanoferrate(II) reductions have included those of dioxygen,iodate, peroxodisulfate, - [Co(NH3)5(DMSO)] +, and [Co(en)2Br2]+. Rate constants for reaction with dioxygen depended strongly on the electron-donor properties of the organic cosolvent. Rate constants for reduction of peroxodisulfate in several binary aqueous media were analyzed into their ion association and subsequent electron transfer components. Rate constants for reduction of [Co(en)2Br2] in methanol water and dioxan water mixtures were analyzed by a variety of correlatory equations (dielectric constant Grunwald-Winstein Swain Kamlet-Taft). 

Medium effects on the linewidths of electronic transitions are quite pronounced in certain instances. For example, it is quite general that the 0-0 band of an electronic transition will be the sharpest line in the vibronic spectrum of that state. The additional line broadening over that of the 0-0 band usually amounts to 1-5 cm-1, and increases in certain progressions with increasing vibrational separation from the origin. These line broadenings must gauge the vibrational relaxation rates, which are clearly much smaller than the usual rates of electronic relaxation. The assistance... 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

Diketones are easily reduced electrochemically because of the mutual electron-withdrawing effect of one carbonyl group on the other. Two-electron reduction usually takes place to afford an enediolate (62), which can then react with an electrophile E present in the medium (equation 31 see also Schemes 10 and 13) an alpha-hydroxy ketone is formed by subsequent tautomerization when E = H1,85,86. An unusual exception to the generalization that these substances undergo two-electron reduction is the electrochemical behavior of acenaphthenequinone (63), which is converted to the expected enediol dibenzoate (64) in the presence of aroyl chlorides but affords the dimeric substance 65 (shown to be the meso diastereomer by X-ray crystallography) in the presence of acetic... 

Recently, Siehl and co-workers have made computational studies of the C NMR shift of a series of a-vinyl-substituted vinyl cations (l,3-dienyl-2-cations, Figure 3.14). They found that inclusion of electron correlation effects are important to get reliable data. The small differences (A = 1-2 ppm) between calculated [CCSD(T)/ tzp/dz] and observed NMR chemical shifts suggest that the geometry of the cations is not significantly affected by the medium. 

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