Radicals, reduction triplet

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

The persistent radical anion II was obtained by chemical or electrochemical reduction of the parent neutral compound. The EPR spectrum of II is composed of a triplet of triplet (ap(2P)=3.50 mT and ap(2P)=0.89 mT) characteristic of a planar conjugated structure (Fig. 9) [87]. Amazingly, the dianion III was found to be paramagnetic exhibiting an EPR spectrum composed of a distorted dou-... 

According to Hercules 5> a measure of the relationship between direct excitation of the first excited singlet state by radical-ion recombination and triplet-triplet annihilation is the entropy factor FAS, estimated to be on average 0.2 eV. The enthalpy of the radical cation-radical anion recombination can be measured as the difference between the redox potentials 1/2 Ar—Ar (oxidation) and 1/2 Ar—Ar<7> (reduction). This difference has to be corrected by the entropy term. If this corrected radical-ion recombination enthalpy is equal to or larger... 

A flash photolysis method has been developed that prepares the MoVI-Fe11 state and thus allows the rate constants k3 and k 3 to be measured. Solutions containing 5-deazariboflavin, semicarbazide, and sulfite oxidase are subjected to 555 nm flash photolysis. The deazariboflavin is excited to a triplet state, which is then reduced by semicarbazide to form the 5-deazariboflavin semiquinone radical. This radical is then rapidly oxidized back to its parent species through the one-electron reduction of sulfite oxidase. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach," the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

The triplet ground state (OkJ/mol) for the O2 molecule is represented by the term symbol Eg and has two unpaired electrons, one each in the rr or the K level. The first excited state (92kJ/mol above the ground state) is a singlet (electrons spin paired with both electrons in either the irf or the Kf level). The singlet state with paired spin electrons, one each in the rr and K levels, is the next excited level 155 kJ/mol above the ground state. Reduction of O2 by one electron yields the superoxide ion (O2) a radical anion. Reduction by two electrons yields the peroxide ion, (02 ). As noted in equation 7.1, the reduction potential for O2 in the presence of protons is thermodynamically favorable. Therefore reversible binding of O2 to a metal can only be achieved if competition with protons and further reduction to superoxide and peroxide are both controlled. ... 

As described earlier, the reactivity of photoinduced electron transfer is remarkably enhanced by the complexation of excited states with metal ions. Even if there is no direct interaction between excited states and metal ions, however, metal ions can enhance the reactivity of photoinduced electron transfer when the radical anion produced in photoinduced electron transfer binds with metal ions [11,12,25]. For example, although there is no direct interaction between the triplet excited state of Ceo ( Ceo ) and Sc(OTf)3, an efficient electron transfer occurs from Ceo to p-chloranil (CI4Q) to produce Ceo" and the p-chloranil radical anion CUQ -Sc complex [135]. In contrast to the facile reduction of Ceo,... 

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

Typical aromatic donors and acceptors undergo only minor geometry changes upon oxidation or reduction or upon population of the triplet state for these compounds, the reaction sequence ET followed by BET has no effect on the structure. If the triplet state or biradical belongs to a different stmcture type than radical ion and ground-state precursor, as is the case for cis- or fraui-1,2-diphenylcyclopropane (65) or norbornadiene (16) BET may occur with cleavage or for-mation of one or more C—C bonds. In such cases, the sequence ET-BET may... 

Oxygen has two possible interactions during the polymerization process [94], and these reactions are illustrated in Fig. 2. The first of these is a quenching of the excited triplet state of the initiator. When this quenching occurs the initiator will absorb the light and move to its excited state, but it will not form the radical or radicals that initiate the polymerization. A reduction in the quantum yield of the photoinitiator will be observed. The second interaction is the reaction with carbon based polymerizing radicals to form less reactive peroxy radicals. The rate constant for the formation of peroxy radicals has been found to be of the order of 109 1/mol-s [94], Peroxy radicals are known to have rate constants for reaction with methyl methacrylate of 0.241/mol-s [100], while polymer radicals react with monomeric methyl methacrylate with a rate constant of 5151/mol-s [100], This difference implies that peroxy radicals are nearly 2000 time less reactive. Obviously, this indicates that even a small concentration of oxygen in the system can severely reduce the polymerization rate. 

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