Abstract  The ESR and ENDOR spectra and hyperfine coupling constants (hfi) of the tertiary butoxyl spin adduct of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) change markedly, but reversibly, with increase in temperature from 140 to 280 K, and the spectra can be cycled repeatedly between these temperature limits. The variation with temperature of the hfi of nitrogen, beta and gamma protons arises through a change in equilibrium concentrations of the planar and puckered conformers of the adduct pyrroline ring which are interconverting rapidly on the ESR time-scale (10(7) MHz). A simple mathematical model has been derived which successfully simulates the experimental temperature variation of the coupling constants, gives their values in the two conformers and estimates of 4.3 +/- 0.5 kJ mol(-1) for the energy difference between the conformers in three solvents. Accurate hfi measurements by ENDOR spectroscopy across a narrow temperature range (140-160K) corroborated ESR experiments and afforded the assignment of all magnetically inequivalent protons that have previously eluded observation. By using the di-tert-butyl tetroxide equilibrium, (ROOOORt)-R-t reversible arrow 2(t)ROO(.) as a clean source of tert-peroxyl radicals, it was shown that these radicals are not trapped singly by DMPO to give a peroxyl spin adduct but give an ESR-silent complex which decomposes at higher temperatures to the DMPO-OBut spin adduct. These studies highlight the problems of identifying the trapped radical from ESR spectra of spin adducts especially when the spectra are observed at only one temperature. Copyright (C) 2000 John Wiley & Sons, Ltd.

Abstract  Rate constants have been determined at 37-degrees-C for the ring opening of a variety of alkyl-substituted cyclopropylcarbinyl radical ''clocks'' by nitroxide radical trapping (NRT) using TEMPO. Relative yields of unrearranged and rearranged trialkylhydroxylamines were measured at various TEMPO concentrations, and these data were then combined with absolute rate constants for the reactions of structurally appropriate alkyl radicals with TEMPO as determined by laser flash photolysis. Cyclopropylcarbinyl radicals, including the bicyclo[2.1.0]pent-2-yl radical, were generated by H-atom abstraction from the parent hydrocarbon and, in a few cases, also from the appropriate diacyl peroxide. Twelve substrates yielded sixteen clock reactions because some cyclopropylcarbinyls can undergo two different ring-opening reactions. For six methyl-substituted cyclopropylcarbinyls rate constants ranged from 0.8 x 10(8) s-1 for 1-methylcyclopropylcarbinyl to 47 x 10(8) s-1 for pentamethylcyclopropylcarbinyl. Rate constants for the ring opening of cyclopropylcarbinyl, alpha-methyl- and alpha,alpha-dimethylcyclopropylcarbinyl are 1.2, 0.70, and 0.88 x 10(8) s-1, respectively. Rate constants for H-atom ion by tert-butoxyl from various positions in the 12 cyclopropane substrates relative to the rate of H-atom ion from cyclopentane have also been determined by using competitive NRT.

Abstract  Coherent anti-Stokes Raman scattering (CARS) microscopy is demonstrated to be a powerful imaging technique with chemical specificity for studying chemically amplified polymer photoresists. Samples of poly(tertbutyl oxycarbonyloxy styrene) (PTBOCST) resist imprinted by interferometric lithography with a pattern of lines/spaces of 400 nm/400 nm and 200 nm/200 nm were used to test CARS imaging capabilities. Chemical contrast in the image is obtained by probing the carbonyl stretching vibration of the tert-butoxyl carbonyl group of PTBOCST. The experimental images demonstrate high spatial resolution (approximate to270 nm) and strong signal, which allows short acquisition times. Advantages and limitations of CARS in comparison with other imaging techniques with chemical specificity, such as infrared near field scanning optical microscopy (IR NSOM), are discussed.

Abstract  Metabolic activation of peroxides and hydroperoxides to free radicals is associated with the tumor promoting activity of these compounds. tert-Butyl hydroperoxide (t-BOOH) metabolism has been extensively studied as a model of peroxide biotransformation. In vivo studies are limited, and the hemoglobin-thiyl radical was the only species thus far identified in the blood of treated rats. Here we further examine t-BOOH metabolism in vivo with regard to free radical and DNA adduct production. Spin-trapping experiments with phenyl-N-tert-butylnitrone (PBN) led to the detection of EPR signals in the blood, bile, and organic extracts of the liver and stomach of rats treated with t-BOOH. Analysis of these signals demonstrated that t-BOOH metabolism in vivo produces alkyl radicals, detected in the bile and organic extracts of liver and stomach, in addition to the previously identified hemoglobin-thiyl radical. To characterize the produced alkyl radicals, experiments were performed with (13)C-labeled t-BOOH and two spin traps, PBN and alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). The latter was used because the EPR signals obtained with PBN were too weak to be unambiguous. Nevertheless, the EPR signals present in the bile of animals treated with (13)C-labeled t-BOOH and PBN or POBN were consistent with adducts of (13)C-labeled methyl radical and an unidentified alkyl radical. The latter is probably derived from lipids oxidized by the metabolically produced primary radicals, methyl and its precursor, tert-butoxyl. The presence of 8-methylguanine and 7-methylguanine in hydrolysates of DNA from liver and stomach of rats treated with t-BOOH was also examined. 8-Methylguanine, a typical product of methyl radical attack on DNA, was detectable in both the liver and stomach of treated rats. The results may be relevant to the understanding of the genotoxic properties of other peroxides, particularly of cumene hydroperoxide.

Abstract  Poly(ADP-ribose) polymerase (PARP)-1, an enzyme that catalyzes the attachment of ADP ribose to target proteins, acts as a component of enhancer/promoter regulatory complexes. In the present study, we show that pharmacologic inhibition of PARP-1 with 3,4-dihydro-5-[4-(I-piperidinyl)butoxyl]-1(2H)isoquinolinone (DPQ) results in a strong delay in tumor formation and in a dramatic reduction in tumor size and multiplicity during 7,12-dimethylbenz(a)anthracene plus 120-tetradecanoylphorbol-13-acetate-induced skin carcinogenesis. This observation was parallel with a reduction in the skin inflammatory infiltrate in DPQ-treated mice and tumor vasculogenesis. Inhibition of PARP also affected activator protein-1 (AP-1) activation but not nuclear factor-kappa B (NF-kappa B). Using cDNA expression array analysis, a substantial difference in key tumor-related gene expression was found between chemically induced mice treated or not with PARP inhibitor and also between wild-type and parp-.1 knockout mice. Most important differences were found in gene expression for Nfkbiz, S100a9, Hif-1 alpha, and other genes involved in carcinogenesis and inflammation. These results were corroborated by real-time PCR. Moreover, the transcriptional activity of hypoxia-inducible factor-lot (HIF-1 alpha) was compromised by PARP inhibition or in PARP-I-deficient cells, as measured by gene reporter assays and the expression of key target genes for HIF-1 alpha. Tumor vasculature was also strongly inhibited in PARP-1-deficient mice and by DPQ. In summary, this study shows that inhibition of PARP on itself is able to control tumor growth, and PARP inhibition or genetic deletion of PARP-1 prevents from tumor promotion through their ability to cooperate with the activation AP-1, NF-kappa B, and HIF-1 alpha.

Abstract  Although mechanisms of arsenic trioxide (As(2)O(3))-induced cell death have been studied extensively in hematologic cancers, those in solid cancers have yet to be clearly defined. In this study, we showed that the translocation of apoptosis-inducing factor (AIF) from mitochondria to the nucleus is required for As(2)O(3)-induced cell death in human cervical cancer cells. We also showed that reactive oxygen species (ROS)-mediated poly(ADP-ribose) polymerase-1 (PARP-1) activation is necessary for AIF release from mitochondria. The treatment of human cervical cancer cells with As(2)O(3) induces dissipation of mitochondrial membrane potential (Deltapsi(m)), translocation of AIF from mitochondria to the nucleus, and subsequent cell death. Small interfering RNA targeting of AIF effectively protects cervical cancer cells against As(2)O(3)-induced cell death. As(2)O(3) also induces an increase of intracellular ROS level and a marked activation of PARP-1. N-acetyl-l-cystein, a thiol-containing antioxidant, completely blocks As(2)O(3)-induced PARP-1 activation, Deltapsi(m) loss, nuclear translocation of AIF from mitochondria, and the consequent cell death. Furthermore, pretreatment of 1,5-dihydroxyisoquinoline or 3,4-dihydro-5-[4-(1-piperidinyl)butoxy]-1(2H)-isoquinolinone, PARP-1 inhibitors, effectively attenuates the loss of Deltapsi(m), AIF release, and cell death. These data support a notion that ROS-mediated PARP-1 activation signals AIF release from mitochondria, resulting in activation of a caspase-independent pathway of cell death in solid tumor cells by As(2)O(3) treatment.

Abstract  Nitric oxide (NO) induces cell proliferation or cell death, depending on the cell type involved, the isoform of nitric oxide synthase activated, and its cellular localisation. In neurons, the damaging effect of NO is usually attributed to the highly toxic peroxynitrite, formed by its reaction with superoxide. Peroxynitrite induces DNA damage and consequently the activation of poly (ADP-ribose) polymerase (PARP). This study set out to examine the contribution of peroxynitrite to the damage induced in cerebellar granule neurons (CGNs) by treatment with the NO donor S-nitroso-N-acetylpenicillamine (SNAP), for short (6 h) or prolonged (24 h) exposures. The Alamar blue assay was used to quantify CGN viability, which was also assessed by morphological examination. SNAP (10 microM-1 mM) induced a concentration- and time-dependent reduction of CGN viability, with associated damage to cell bodies and neurite processes evident following 100 microM SNAP treatments. Damage from 6 h exposures was prevented by the presence of haemoglobin (a NO scavenger), uric acid (a peroxynitrite scavenger), melatonin (a non-specific antioxidant), and by cyclosporin A (a permeability transition pore blocker). It was reduced by the PARP-1 inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone (DPQ), whilst superoxide dismutase (SOD) potentiated the effects. Following 24 h exposure to SNAP, damage was only partially blocked by haemoglobin, melatonin, cyclosporin A and DPQ, but was not affected by uric acid or SOD. The data suggest that short exposure to NO induces neuronal damage through peroxynitrite produced by its interaction with superoxide, whereas a longer exposure to NO can induce damage partly by a mechanism which is independent of peroxynitrite formation.

Abstract  Unreported methylated catechin and epicatechin derivatives 5 and 6 were synthesized by an oxa-Pictet-Spengler reaction. Catechin 5 shows the B and C rings coplanar because of the formation of a trans junction between the C ring and the newly generated six-term cycle D, in turn condensed to ring B. In contrast, epicatechin 6 presents a bent geometry due to the establishment of a cis junction between the C ring and the newly formed cycle D. The oxidation of compounds 5 and 6 in the presence of the Trametes villosa laccase/1-hydroxybenzotriazole (HBT) system was investigated under aerobic conditions in both a biphasic system and a reverse micelle. The unexpected different chemoselective oxidation at the benzylic position of catechin and epicatechin derivatives 5 and 6 has been rationalized using a molecular modelling approach. These results demonstrate that the Trametes villosa laccase/HBT system represents a useful tool to functionalize the C-2 or C-4 position of phenolic compounds depending on the structural features.

Abstract  When a p-polarized beam propagates through a high-performance photorefractive polymer composite, poly(N-vinylcarbazole:2,4,7-trinitro-9-fluorenone:1-n-butoxyl-2,5-dimethyl-4-(4'-nitrophenylazo)benzene, its transmission behavior is influenced by three effects: the electroabsorption, the photorefractive coupling with the reflected beam from the rear surface, and the amplified scattering. From the measurements on the incidence angle dependence as well as the applied-electric-field dependence of the three effects, some conclusions are obtained. At a small incidence angle with a low applied electric field, both the amplified scattering and the electroabsorption are small whereas the coupling between the incident beam and the reflected beam plays a principal role. At a large incidence angle or with a high poling electric field, the transmission is influenced mainly by the amplified scattering and the electroabsorption. A poling electric field asymmetric loss to the amplification scattering is also observed.

Abstract  Organic luminescent materials with aggregation-induced emission (AIE) characteristics have attracted much attention among the scientists in the fields of optoelectronic devices and fluorescence biotechnology. ATE materials overcomes the aggregation-caused quenching problem of traditional organic fluorescent compounds, which possess high fluorescence quantum efficiency in the aggregated states. Thanks to the great research effort of worldwide scientists, a large variety of AIE materials have been developed and the underlying mechanism has been rapidly explored. The deep understanding of the structure-property relationship of AIE compounds is still in an urgent demand for the design of new materials. In this work, based on the classical propeller-shaped AIEgen, tetraphenylethene (TPE), we designed and synthesized a series of electron donor/acceptor-containing alkynone derivatives with AIE feature such as cyano, nitro, butyl and butoxyl groups-substituted alkynone derivatives. Their chemical structures have been fully characterized by H-1 NMR, C-13 NMR, IR, and HRMS spectra, providing satisfactory analysis results. Their photophysical properties are systematically studied and the effect of substitution groups on the emission maximum, emission efficiency, as well as AIR property are discussed, respectively. Their emission maxima are located at 511 similar to 565 nm with the fluorescence quantum yields of up to 31% in the aggregated states in THF/water mixtures with high water content. The fluorescence intensity of the unsubstituted TPE-containing alkynone derivative in THF/H2O with phi(w)=90% water content is 157 times higher than that in THE solution. It is suggested that both electron-donating and electron-withdrawing substitution groups on the terminal phenyl ring decrease the emission efficiency of the the aggregated state and the introduction of nitro group dramatically quenches the emission while redshifts the emission maximum. Most importantly, the alkynone groups in these compounds can selectively coordinate with Pd2+ among a large variety of metal ions, which quench the emission of the nanoaggregates and possess high sensitivity towards Pd2+, demonstrating the potential application as an efficient Pd2+ fluorescent sensor.

Abstract  The quenching of the n,pi*-excitedazoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene by 19 heteroatom-containing electron and hydrogen donors, that is, amines, sulfides, ethers, and alcohols, was investigated in the gas phase. Deuterium isotope effects were measured for 9 selectively deuterated derivatives. The data support the involvement of an excited charge-transfer complex, that is, an exciplex, for tertiary amines and sulfides, and a competitive direct hydrogen transfer from the C-H bonds of ethers or from the N-H or O-H bonds of secondary and primary amines or alcohols. The recently observed "inverted" solvent effect for the fluorescence quenching of azoalkanes by amines and sulfides in solution is supported by the observed rate constants in the gas phase, which are substantially larger than those in solution. A more pronounced inverted solvent effect for the weaker electron-donating sulfides and a presumably faster exciplex deactivation result in a switch-over in absolute reactivity relative to tertiary amines in the gas phase. Most importantly, the kinetic data demonstrate that the reactivity of the strongly dipolar O-H and N-H bonds in photoinduced hydrogen abstraction reactions shows a larger decrease upon solvation than that of the less polar C-H bonds. The azoalkane data are compared with previous studies on quenching of n,pi*-triplet-excited ketones in the gas phase.

Abstract  A thermolysis mechanism with the concerted dissociation of two bonds, -OO- and -C-O-, and formation of carboxylic acid and two radicals (tert-butoxyl and acetyl) is suggested on the basis of kinetic measurements, studies of the decomposition products, and quantum-chemical modeling of the thermolysis of per oxides of the general formula (CH3)(3)C-OO-CH(CH3)OC(O)R. Concerted dissociation is proved by low thermolysis activation energies, negative activation entropies, the independence of the decomposition rate constant from medium viscosity, carboxylic acid formation (one mole per mole of reacted peroxide), and the special features of the structure of peroxides. The formation of the tert-butoxyl and acetyl radicals and carboxylic acid molecule is more favorable energetically than the other possible reaction directions.

Abstract  Two modified poly(N-carbazole), poly(2-carbazolylethyl methacrylate) (PCEMA) and poly(3-carbazolylpropyl methacrylate) (PCPMA), were synthesized. Doped with electro-optic (EO) chromophore 1-n-butoxyl-2,5-dimethyl-4-(4'-nitrophenylazo)benzene and the photosensitizer 2,4,7-trinitro-9-fluorenone, the EO response and photorefractive (PR) effect were studied experimentally. Due to the shorter spacer between the carbazolyl and the main-chain and thus a higher glass-transition-temperature T-g, PCEMA-based composite exhibited an effective EO coefficient as only a. quarter as that in PCPMA-based composite. However, a slight higher two-beam coupling coefficient and much faster PR response were observed in the former. It was suggested that both the spacer length and the polymerization degree changed the T-g and the number density of traps, which decided the EO and PR responses.

Abstract  A catalyzed synthesis of cyclopropanes and cyclobutanes via radical chemistry is described. The method that generally proceeds in high yields uses epoxides as radical precursors and titanocene(III) complexes as the electron transfer catalysts (see scheme). The key to the success of the transformation is constituted by the chemoselectivity of radical reduction. Electrophilic enol radicals generated through cyclization are reduced rapidly whereas their precursors, nucleophilic alkyl radicals, remain unaffected.

Abstract  A new prenylated coumarin, S-6-[2-(hydroxymethyl)butoxy]-7-hydroxy-4-methyl-2 H-chromen-2-one ( 1), named pavietin, has been isolated from the leaves of an Aesculus pavia genotype along with three known flavonol glycosides, quercetin 3- O-alpha-rhamnoside (quercitrin, 2), quercetin 3- O-alpha-arabinoside ( 3), and isorhamnetin 3- O-alpha-arabinoside (distichin, 4). The chemical structure of compound 1 was determined by chemical and spectroscopic methods, inclusive of UV, MS, and 1D and 2D NMR experiments. It showed appreciable antimicrobial properties against several pathogens, displaying a significant antifungal activity toward one of the main fungal parasites of Aesculus species, Guignardia aesculi. The same biological tests performed with a mixture of flavonoids 2- 4 resulted in weak or no activity. Compound 1 was undetectable in Aesculus hippocastanum, a closely related species lacking resistance to fungal pathogens. The possible role of 1 in plant resistance is discussed.

Abstract  Several techniques have been used to probe polymer end groups. The nitroxide radical trapping technique has been used (i) to show that initiator-derived unsaturated end groups in polymethyl methacrylate can be minimized by using t-hexyl peroxypivalate as the initiator (ii) to predict the end and penultimate groups in acrylonitrile/ethyl vinyl ether copolymer produced by t-butoxyl initiation by analogy with the initiation mechanism (iii) to predict probable end groups in polyacrylonitrile and polystyrene produced by cyanoisopropyl initiation in the presence of adventitious oxygen. NMR techniques have been used to show that the end groups of functionalized oligomers, made from styrene and methacrylonitrile by the addition-fragmentation chain transfer technique with allylic sulphides, conform to the expected structures. (C) 1998 John Wiley & Sons, Ltd.

Abstract  In this paper two hypotheses are tested: (i) the active oxygen species is similar in energetics for all cytochrome P450 (CYP) enzymes and (ii) linear free-energy relationships can be used to evaluate the mechanism of the reaction of these enzymes. A series of intramolecular isotope effects were determined and compared for CYPs 1A2, 2B1, 2C9, 2E1, and P450cam. The results indicate that the isotope effects are very similar for each of these isoforms of P450 and that the first hypothesis is likely to be true. Attempts to establish a linear free-energy relationship were only moderately successful: log Vmax = 0.11sigma+p + 1.73; r2 = 0.588. It was determined, through the use of intermolecular isotope effects, that the rates of hydrogen atom abstraction are masked. Thus, the second hypothesis is found to be false. This is likely to be a general result for CYP reactions, and linear free-energy relationships can only be used to determine the mechanism under very special circumstances. In all cases, the rate-limiting step should be evaluated with isotope effect experiments before any mechanistic conclusions can be drawn. If the intermolecular isotope effects are found to be masked, no mechanistic conclusion can be drawn from the linear free-energy relationship study.

Abstract  Phytochemical studies of the roots of Dorstenia excentrica afforded a diastereoisomer of prandiol having the 2 ' S,1 " S configuration, 4-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxy]-7H-furo[3,2-g][1] 1 benzopyran-7-one, psoralen and 7-hydroxycoumarin. The furocoumarins 5-[3-(4,5-dihydro-5,5-dimethyl-4-oxo-2-furanyl)-butoxyl]-7H-[3,2-g][1]benzopyran-7-one and bergapten were also present in the roots of D. drakena, while 7-hydroxycoumarin, psoralen and the psoralen dimer were isolated from D. lindeniana. The structure of the psoralen dimer was established by X-ray diffraction analysis. (C) 1999 Elsevier Science Ltd. All rights reserved.

Abstract  The photolysis of the pyridinethiones 1a-d releases oxyl (hydroxyl, isopropyloxyl, tert-butoxyl, and benzoyloxyl) radicals, which damage DNA through strand cleavage and guanine oxidation. Unexpectedly, the N-hydroxy derivative 1a is significantly less effective in the oxidation of cell-free DNA than the alkoxy and benzoyloxy ones 1b-d. A similar trend was observed for the photooxidative consumption of 2'-deoxyguanosine (dG) by these pyridinethiones. Detailed mechanistic investigations have revealed that the guanine oxidation by the pyridinethiones 1b-d is not caused by oxyl radicals but by a type-I photooxidation process through the novel betain intermediate 8, which is formed in small amounts (ca. 5%) during the photolysis of these thiones from the intermediary photoproduct disulfide 3. This photosensitized guanine oxidation is effectively inhibited by the N,N'-dioxide 7, which is produced only in the photolysis of the N-hydroxypyridine-2-thione (1a) and not from the N-alkoxy and N-benzoyloxy derivatives 1b-d. Thus, for the N-hydroxy derivative 1a hydroxyl radicals are the main DNA-damaging species. The N-alkoxy and N-benzoyloxy derivatives 1b-d are more effective DNA-photooxidizing reagents than the N-hydroxypyridinethione 1a, because 1b-d oxidize DNA photocatalytically through sensitization by the betain, while 1a oxidizes DNA mainly through the stoichiometrically photogenerated hydroxyl radicals.

Abstract  A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(•))/k(H)(CumO(•)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(•))/k(H)(CumO(•)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(•) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(•), BnO(•)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.

Abstract  The C-H bond dissociation enthalpies (BDEs) of molecules related to naphthalene radical chemistry were investigated using quantum chemistry calculations (CBS-QB3 and CCSD(T*)-F12), and the C-H BDE in 1,2-dihydronaphthalene was determined using time-resolved photoacoustic calorimetry. A thermodynamically consistent data set of the energetics (C-H BDEs and enthalpies of formation) of the molecules involved (naphthalene, 1- and 2-hydronaphthyl radicals, 1,2- and 1,4-dihydronaphthalene, tetralyl radical, and tetralin) is presented. It was constructed using a thermodynamic cycle based on the simple notion that BDEs represent the difference between two states, bridging accurate experimental enthalpies of formation of the parent compounds with theoretical and experimental C-H BDEs leading to, and from, the radicals. (C) 2013 Elsevier Ltd. All rights reserved.

Abstract  A time-resolved kinetic study on the reactions of the tert-butoxyl (t-BuO*), cumyloxyl (CumO*), and benzyloxyl (BnO*) radicals with alkylferrocenes has been carried out in MeCN solution. With all radicals, clear evidence for an electron transfer (ET) process has been obtained, and with the same ferrocene donor, the reactivity has been observed to increase in the order t-BuO* < CumO* < BnO*, with the difference in reactivity approaching 3 orders of magnitude on going from t-BuO* to BnO*. With BnO*, an excellent fit to the Marcus equation has been obtained, from which a value of the reduction potential of BnO* (E degrees(BnO*/BnO(-)) = 0.54 V/SCE) has been derived. The latter value appears, however, to be significantly higher than the previously determined reduction potential values for alkoxyl radicals and in contrast with the differences in the computed solution-phase electron affinities determined for t-BuO*, CumO*, and BnO*, indicating that the reaction of BnO* with ferrocene donors may not be described in terms of a straightforward outer sphere ET mechanism. From these data, and taking into account the available value of the reduction potential for CumO*, a value of E degrees (BnO*/BnO(-)) = -0.10 V/SCE has been estimated. On the basis of computational evidence for the formation of a pi-stacked prereaction complex in the reaction between BnO* and DcMFc, an alternative ET mechanism is proposed for the reactions of both CumO* and BnO*. In these cases, the delocalized nature of the unpaired electron allows for the aromatic ring to act as an electron relay by mediating the ET from the ferrocene donor to the formal oxygen radical center. This hypothesis is also in line with the observation that both BnO* and CumO* react with the ferrocene donors with rate constants that are in all cases at least 2 orders of magnitude higher than those measured for t-BuO*, wherein the radical is well-localized.