Charge-transfer intermediates

Fluorescence of PDC is also quenched by amines. The ordering of reactivity is tertiary > secondary > primary, which follows inversely the ionization potential (Table 9.13). The results are explained as indicating that PDC undergoes photoreduction by amines, thereby forming triplet charge-transfer intermediates as the primary step in quenching. Therefore, the mechanism of the PDC reaction is not the same as the proposed mode of reaction of PDC, which involves direct formation of an yhde intermediate by electrophilic attack on the lone-pair electrons of the amine (Table 9.13). ... 

This paper is concerned with a special type of charge transfer reactions in which both the precursor and product of charge transfer process are emitting species (dual fluorescence). We will not discuss intramolecular charge transfer between weakly coupled donor and acceptor groups of the type that produces nonemitting charge transfer intermediates. 

Treatment of compound 95 with DDQ produces the hydrolysis of the orthoester when wet acetone is used as solvent and the oxidation of the allylic alcohol when dry benzene is employed. Apparently, the mechanism of the orthoester hydrolysis involves a charge-transfer intermediate, with no influence from the acidity of the generated DDQ hydroquinone. 

The observed deceleration of the rate of transannular cyclization of a series of olefinic substrates 34 > 36 > 37 > 35 has been rationalized by decreasing electron density on the C-atom undergoing protonation. Negative temperature coefficients for the HCl-catalysed transannular hydrochlorination are consistent with the formation of charge-transfer intermediates. An ion-pair mechanism has been proposed80. 

The diradical nature of the intermediate in the copolymerization of monomers through a charge transfer intermediate has been suggested by Zutty et al. (88) as a result of studies on the copolymerization and terpolymerization of monomer systems containing bicycloheptene and sulfur dioxide. The third monomer apparently enters the copolymer chain as a block segment, while the donor-acceptor monomer pair enter the chain in a 1 1 molar ratio, irrespective of the ratio present in the monomer mixture. 

Gindin, Abkin, and Medvedev (21) observed several of the characteristics of copolymerizations involving charge transfer intermediates in the benzoyl peroxide-catalyzed copolymerization of butadiene with acrylonitrile and methacrylonitrile. Irrespective of the initial concentrations, polymerization ceased when one of the components was consumed. The butadiene-acrylonitrile copolymer had a 67% alternating structure, while the butadiene-methacrylonitrile copolymer had an 80% alternating structure. 

In our work on copolymerizations involving charge transfer intermediates, it has been noted that when a mixture of styrene and maleic anhydride is heated to 80 °C. in the presence of benzoyl peroxide, an extremely exothermic reaction occurs, and in a sealed system the temperature rises from 80° to 250°C. within three minutes, and the conversion is quantitative. 

In consideration of the above experimental and theoretical evidence, it is concluded that the reaction, at least the ones that have been investigated, be viewed as proceeding through a weakly solvated 17-electron Re(CO)5 radical instead of a 19-electron or a charge-transfer intermediate. The fact that no other intermediates are detected prior to the product formation suggests that the reaction involve only the rate-limiting Cl atom transfer step. 

Fig. 6. A diagram showing the dynamic interconveision of solvent-separated ion pairs (SSIP, exdplexes, contact ion pairs (CIP), and free ions in solution. Electron transfer takes place within a cage of solvent molecules to generate a SSIP or more intimate charge-transfer complex, the latter being an exciplex or CIP. The nature of the charge-transfer intermediate generated may depend on the distance separating the reactants. The distance depends on the molecular structures of the reactants, i.e., their sizes, shapes, and steric features. Free ions are produced by ion dissociation from the solvent cage...

In medium polarity solvents, recent spectroscopic measurements have been able to distinguish between SSIP and CIP in studies on [S-naphtholate [22]. This work confirms the general picture expounded by Winstein on the existence of various charge-transfer intermediates ranging from tight CIP s to loose SSIP s [23]. 

As the preceding examples demonstrate, magnetic field effects can be useful in identifying the nature of charge-transfer intermediates in photoelectron transfer. In particular, with a better understanding of the relationships between AEhf and J, and between J and the separation distance, we can predict that magnetic-field experiments, similar to those pioneered by Weller, will assist in differentiating exciplexes and CIP s from SSIP s. 

In PET, the rate can be markedly affected by the solvent polarity. With the formation of each new charge-transfer intermediate, solvent dipoles undergo reorientation in response to the new charge distribution on the reactants [49]. The solvent response influences the free-energy barrier of the reaction by altering the potential energy surface of the electron transfer. We consider this facet of solvent motion in this section. In a later section, we examine dynamical solvent effects. 

Other studies have exploited medium effects to achieve the generation of long-lived intermediates. As pointed out earlier, choice of solvent can have a dramatic effect on the character of the charge-transfer intermediates. In exciplexes and contact ions, electron return usually takes place at the expense of ion dissociation, i.e., kret > kdis. However, in more polar solvents, ion dissociation into free ions becomes energetically more favorable, and kre, kdis. 

Schlenk equilibrium [13] the simpler diorganomagnesium compounds are often easier to study [14,78,79] because of the absence of complications from additional halide ligands. As the organolithium compounds, organomagnesium species often react rapidly at ambient temperatures with substrates so that the charge transfer intermediates are not observed or reported. 

Two basic schemes have been used for initiating intracluster processes. In the first the reaction is induced by electronic excitation of an atom inside the complex. Jouvet and Soep used this scheme for the study of several reactions of electronically excited mercury atoms Hg( Pi). In the reaction with CI2 they obtained conclusive evidence of the formation of a charge transfer intermediate in the process. In the reaction with Hj a strong dependence of the reactivity on the geometry was observed. It was found that the reaction occurs on the n surface with a C2 symmetry. 

In [13a], a triplet-triplet exchange interaction requiring orbital overlap is the proposed quenching process. The very efficient quenching observed in [13c] occurs via formation of a charge transfer Intermediate, in which the zinc moiety is reduced by the cobalt system. The lifetime of this intermediate is estimated to... 

Duncan, D. C. Netzel, T. L. Hill, C. L. Early-time dynamics and reactivity of polyoxometalate excited states - identification of a short-lived lmct excited-state and a reactive long-lived charge-transfer intermediate following picosecond flash excitation of W10O32 (4-) in acetonitrile. Inorg. Chem. 1995, 34, 4640-4646. 

An important feature of the LH form of the hydride is that it contains a Co111 bearing a very open coordination site for reaction with nucleophiles. There are several publications on the unusual reaction of naked cobalt111 in porphyrins with acetylenes and olefins.25611 J-258 This could also explain the trans addition of the hydrides to acetylenes. A charge-transfer intermediate in the reaction of the LH isomer with an acetylene or alkene could explain the difference in behavior of substituted versus unsubstituted olefins. If the above explanation is correct, then it would require essential equality between the energies of the Co—H and L—H complexes. Parameters such as solvent properties could shift the equilibrium concentrations of the two isomers of the hydrides, leading to apparently different results in the same reaction and explain poor reproducibility in stereoselectivities.232,233... 

The mechanism of metallation varies with the reactants. Both electrophilic and nucleophilic metallations are knownand charge-transfer intermediates are likely present in some metallations. 

Catalytic quantities of HF or FSO3H decrease the temperature required for fiuorination of graphite -250°C is sufficient at a HF partial pressure of 200 torr . Catalytic fiuorination of graphite proceeds via ionic or n charge-transfer intermediates which are characterized by high mobility of the intercalant. In contrast, there is no evidence for mobile fluorine in (CFJ Silver fluoride , LiF and CuFj have also been used as fiuorination catalysts. 

These properties were utilized in the study of energy transfer from NO to a variety of accepting molecules. Acceptors whose one-photon absorption spectrum strongly overlaps that of NO were investigated for the first time. Efficiency of resonant and nonresonant processes could thus be empirically compared. A unified mechanism, involving a charge transfer intermediate, was found to account reasonably well for all the observed rate constants. 

Similar behavior is observed with some of the flavoprotein monooxygenases, which also do not use a metal co-factor. The reduced flavin (68) has a structure which resembles that of luciferin (62) and reacts readily with molecular oxygen, through the intermediacy of its carbanion which forms charge-transfer intermediates leading to the hydroperoxide ion rather than to superoxide radical ion and pyrazyl radicals (158). Although the precise point of attachment of oxygen is controversial, the principle remains the same, namely the formation of a non-delocalized carbanion (69 or 70) (159—161). 

Initial Charge Transfer Intermediate State with... 

NAD(P)H. 32. Stereoselective reduction of camphoroquinone by a chiral NAD(P)H model. Bull Chem Soc Jpn 54 3478-3481 Ohno A, Nakai J, Nakamura K, Goto T, Oka S (1981c) Reduction by a model of NAD(P)H. 33. Steric and electronic effects on asymmetric reduction of 2-acylpyridines. Bull Chem Soc Jpn 54 3482-3485 Ohno A, Yamamoto H, Oka S 0 981d) Reduction by a model of NAD(P)H. 35. Spectroscopic detection of charge-transfer intermediate. Bull Chem Soc Jpn 54 3489-3491... 

The u.v. spectra of thiolan and thian were satisfactorily accounted for by SCF (CNDO/2) theory only if rf-orbitals were included on the sulphur atoms. The photolysis of thiolan vapour gave mainly ethylene, together with numerous other hydrocarbons, hydrogen, thiols, and thiiran, all in minor amounts. No sulphur atoms were formed, and the results were interpreted in terms of the intermediacy of two excited states, and the initial homolysis of a C—S bond, a conclusion supported by other spectroscopic data. The photochemical rearrangement of 9-thiabicyclo-[3,3,l]non-6-ene (30) proceeded via a charge-transfer intermediate (31), since photolysis in deuteriomethanol gave mainly (32) together with a little... 

A similar mean diameter i.e., 170 nm) was found for Ceo-aniline clusters in toluene - acetonitrile (1 3 v/v). This cluster formation exerted an improving impact on the performance of Ceo-based donor-acceptor dyads. For example, in aggregates of a Ceo-aniline dyad the radical pairs have a lifetime of 60 ps, while no detectable charge-transfer intermediates were noted for the isolated dyad. The close and condensed network in the cluster composites fecilitates the hopping of electrons from the parent fullerene to an adjacent one etc., progressively increasing the spatial separation between radical ions of the charge-separated state. 

Duncan, D., Netzel, T. and Hdl, C. (1995). Early-Time Dynamics and Reactivity of Polyoxomet-alate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [Wio032] in Acetonitrile, Inorg. Chem., 34, pp. 4640-4646. 

When rationalizing a reaction as involving charge-transfer intermediates, caution should be exercised. Complex formation and its slow disappearance during the reactions may result from side reactions and/or disappearance of one of the components, leading to unwarranted conclusions. 

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