Formation of π-excimer or π-exciplex in electrogenerated chemiluminescence involving perylene molecule revealed using a dual-electrolysis stopped-flow method

Electrogenerated chemiluminescence (ECL) reactions involving the perylene cation radical (PE⁺) and perylene anion radical (PE⁻) in acetonitrile have been observed using a dual-electrolysis stopped-flow method. In this method, ECL emission from the systems composed of different kinds of ion radicals can be easily observed by mixing both the electrolyzed solutions directly. Therefore, the ECL spectra were observed systematically in the reactions of PE⁺ and PE⁻ with different ion radicals. Consequently, emission definitely from the singlet state of PE was observed in the reactions between PE⁺ and the 9,10-diphenylanthracene anion radical (DPA⁻), PE⁻ and DPA⁺, and PE⁻ and the thianthrene cation radical (TH⁺). In contrast, emission at long wavelength was obtained in the reaction between PE⁺ and the pyrene anion radical (PY⁻) as well as the reaction between PE⁺ and PE⁻. From this result, for the molecular interactions involving PE and PY, the presence of the π complexing interaction to form the π-excimer and the π-exciplex was strongly suggested in the ECL-emitting reactions. The present approach was thus found to be effective to reveal the molecular aspects of the excited states formed in solution.     

Role of reactive intermediates in the radiolytic degradation of Acid Red 1 in aqueous solution

The role of reactive intermediates of water radiolysis (e_aq⁻, H, HO, O₂⁻/HO₂) in decoloration and mineralization of aqueous solutions of Acid Red 1 dye was investigated. The decoloration is highly effective in the reactions of e_aq⁻ and H, and less effective in HO reactions. The O₂⁻/HO₂ pair does not take part in decoloration. For mineralization, which is an oxidative degradation, HO radicals are needed: the efficiency increases with the dissolved oxygen concentration. The reactions of the O₂⁻/HO₂ radical pair slightly increase the rate of mineralization. Iron and copper ions (possible constituents of waste waters) in low concentration do not influence the reactions.     

Pulse radiolysis studies of etoposide and 4′-demethylepipodophyllotoxin in aqueous solution

The reactions of etoposide (VP 16, 4′-demethyl-epipodophyllotoxin ethylidene-β-d-glucoside) and 4′-demethylepipodophyllotoxin (DMEP) with the primary radiolytic products of water, such as e⁻_aq, H and OH/O⁻ radicals, and the secondary radical (SO⁻₄) in aqueous solution were studied by use of the techniques of pulse radiolysis, respectively. The absorption spectra of reaction products with e⁻_aq, H and OH/O⁻ and SO⁻₄ radicals were observed, and the rate constants of them were determined by following the build-up kinetics of radicals produced or the decay of hydrated electron observed at 600 nm, respectively.     

EPR of gamma irradiation induced radicals in NaHCO3, CsHCO3 and Na2CO3

In this study gamma irradiated NaHCO₃, CsHCO₃ and Na₂CO₃ were investigated at room temperature. The radicals induced by gamma irradiation in NaHCO₃ were found to be CO⁻₃, HCO₃ and CO⁻₂; in CsHCO₃ the species were attributed to HCO₃; and in Na₂CO₃ to CO⁻₃ and CO⁻₂ radicals. The hyperfine parameters for the hydrogen in HCO₃, and the ¹³C nucleus in CO⁻₂ radical have been determined. The results were compared with literature data for similar compounds and the EPR properties of the CO⁻₂ radical were discussed.     

Studies on the reactions of sylvatesmin and lantbeside with oxidizing free radicals

Sylvatesmin (SYL) and lantbeside (LAN) are two lignans, isolated from a Chinese folk medicinal herb, Lancea tibetica. Their abilities of scavenging oxidizing free radical models, OH, SO₄⁻ and N₃, were investigated in aqueous solution by pulse radiolysis techniques. The OH-adduct radicals with small amount of oxidized products were formed by the reaction of SYL or LAN with OH radical. SYL undergoes one-electron oxidation either by SO₄⁻ or N₃ radicals, LAN can only be detected to react with stronger oxidant SO₄⁻ radical anion. No reaction between LAN and N₃ radical was detectable. The relationship of structure with the abilities of scavenging free radicals was discussed. The reaction rate constants were determined by analysis of the build-up trace of the radical products.     

Metal ion-coupled and decoupled electron transfer

Effects of metal ions on thermal and photoinduced electron-transfer reactions from electron donors (D) to electron acceptors (A) are reviewed in terms of metal ion-coupled electron transfer (MCET) vs. metal ion-decoupled electron transfer (MDET). When electron transfer from D to A is coupled with binding of metal ions to A^?, such an electron transfer is defined as MCET in which metal ions accelerate the rates of electron transfer. A number of examples of electron-transfer reactions from D to A, which are energetically impossible to occur, are made possible by strong binding of metal ions to A^? in MCET. The structures of metal ion complexes with A^? are also discussed in relation with the MCET reactivity. The MCET reactivity of metal ions is shown to be enhanced with an increase in the Lewis acidity of metal ions. In contrast to MCET, strong binding of metal ions to A^? results in deceleration of back electron transfer from metal ion complexes of A^? to D⁺ in the radical ion pair, which is produced by photoinduced electron transfer from D to A in the presence of metal ions, as compared with back electron transfer without metal ions. The deceleration of back electron transfer in the presence of metal ions results from no binding of metal ions to A. This type of electron transfer is defined as metal ion-decoupled electron transfer (MDET). The lifetimes of CS state (D⁺–A^?) produced by photoinduced electron transfer from D to A in the D–A linked systems are also elongated by adding metal ions to the D–A systems because of the stabilization of the CS states by strong binding of metal ions to A^? and the resulting slow MDET processes.     

Hydroxyl radical,sulfate radical and nitrate radical reactivity towards crown ethers in aqueous solutions

Reaction rate constants of crown ethers (12-crown-4, 15-crown-5, 18-crown-6) and their analogs 1,4-dioxane (6C2) with some important oxidative radicals, hydroxyl radical (OH), sulfate radical (SO₄^?) and nitrate radical (NO₃), were determined in various aqueous solutions by pulse radiolysis and laser photolysis techniques. The reaction rate constants for 6C2 and crown ethers with OH and SO₄^? increase with the number of hydrogen atoms in the ethers, indicating that the hydrogen-atom abstraction is a dominant reaction between crown ethers and these two radicals. The presence of cations in solution has negligible effect on the rate constants of crown ether towards OH and SO₄^?. However, for the NO₃, the rate constants are not proportional to the number of hydrogen atoms in ethers, and 12-crown-4 (12C4) is the most reactive compared with other crown ethers. Except 12C4 and 6C2, the cations in the aqueous solution affect the reactivities of 15-crown-5 (15C5) and 18-crown-6 (18C6). The cations with high binding stability for crown ether would improve the reactivity of 15C5. For the studied crown ethers, the reaction rate constants of these oxidative radicals have the order OH>SO₄^?>NO₃. Furthermore, the formation of radicals after the reaction of crown ethers with sulfate radical could be observed in the range of 260–280 nm using laser photolysis and pulse radiolysis. This is the first report on the kinetic behavior of crown ethers with NO₃, and it would be helpful for the understanding of stability of crown ethers in the processing of spent nuclear fuel.     

Radical intermediates generated in the reactions of l-arginine with hydroxyl radical and sulfate radical anion: A pulse radiolysis study

Reactions of l-arginine (Arg) with hydroxyl radical (OH) and sulfate radical anion (SO₄⁻) were kinetically investigated by the pulse radiolysis technique. Hydrogen abstraction from Arg by OH afforded redox chemically oxidizing, neutral, and reducing carbon-centered Arg radicals. Kinetic properties of the radicals indicated that the reducing species might include the δ-C-centered Arg radical and CO₂ radical anion. Similar transient spectra were observed in the SO₄⁻ reaction with Arg, suggesting direct oxidation at the guanidino group is less likely.     

Mechanistic analysis and thermochemical kinetic simulation of the products from pyrolysis of poly(α-methylstyrene), especially the unrecognized role of phenyl shift

Mechanisms for pyrolysis of poly(α-methylstyrene) must rationalize high selectivity for monomer formation, negligible formation of volatile oligomers, and notably slow decrease in molecular weight compared with the rate of weight loss, i.e., unzipping dominates both back-biting and transfer. Backbone homolysis should form both a tert-benzylic radical R_tb and a prim radical R_p, with formation of the latter potentially supplemented in chain propagation steps emanating from the former. Hence product-forming pathways characteristic of each are expected to compete. Simulations of initial product distributions based on assigned rate constants for chain propagation steps indicate that R_tb is indeed predicted to efficiently unzip with minimal transfer or back-biting. However, R_p is predicted to give comparable amounts of transfer and back-biting with minimal unzipping, behavior inconsistent with experimental data. The proposed escape from this impasse is a previously unrecognized pathway, 1,2-phenyl shift in R_p to form a tert radical. If it undergoes β-scission, the net result is an inter-conversion of R_p to R_tb. Quantitative simulations suggest that this sequence is indeed highly competitive with other reactions of R_p and thus efficiently subverts the otherwise expected propagation of chains emanating from R_p.     

DFT studies of unique stereoelectronic effects of substituents on divergent reaction pathways of methylenecyclobutanone radical cations

The results of DFT investigation suggest that C2–C3 bond cleavage of the 2,2-dianisyl-3,3-dimethyl-4-methylenecyclobutanone radical cation (2b⁺) is preferred from both a thermodynamic and a kinetic perspective while C1–C2 bond cleavage is both thermodynamically and kinetically favored in the parent methylenecyclobutanone radical cation (MCB⁺) and the 2,2-diphenyl- and 2,2-dianisyl-4-isopropylidenecyclobutanone radical cations (1a-b⁺). The DFT calculations also suggest that a bonding character exists in C2–C3 bond of the 2,2-diphenyl-3,3-dimethyl-4-methylenecyclobutanone radical cation (2a⁺) but not in that of 2b⁺. Consequently, the unique reactivity of 2b⁺ can be accounted for by the existence of the steric hindrance between methyl and anisyl substituents, which favors C2–C3 bond cleavage, and the absence of C2–C3 bonding character. Those results support that the previous experimental results of photoinduced electron-transfer reactions of 1 and 2. The combined factors comprise stereoelectronic substituent effects that lead to a drastic change in the reaction pathways followed by 2b⁺ relative to that of other methylenecyclobutanone-type radical cations.     

Electrochemical oxidation of xanthosine

The electrochemical oxidation of xanthosine in aqueous solution at pH 2.0 at the pyrolytic graphite electrode has been studied. The primary electrochemical oxidation reaction is an irreversible 1 e⁻, 1 H⁺ reaction giving the C(8) free radical. In order to account for the ultimate products formed, the latter primary radical reacts with xanthosine to give at least one N free radical and with water to give an 8-hydroxylated free radical. The C(8) and N radicals couple to give a xanthosylxanthosine dimer which rapidly loses one ribosyl residue to give a xanthosylxanthine dimer. The 8-hydroxylated radical reacts with the C(8) and N radicals to give two isomeric hydroxylated xanthosylxanthosines. The 8-hydroxylated radical can also undergo further electrochemical oxidation (1 e⁻, 1 H⁺) to 9-β-D-ribofuranosyluric acid which is immediately oxidized (2 e⁻, 2 H⁺) to a very reactive quinonoid. Attack by water on the quinonoid gives two isomeric tertiary alcohol intermediates which have been isolated and characterized by their UV and mass spectra and by their reaction with water to give a diol. The latter diol decomposes to 5-hydroxyhydantoin-5-carboxamide-3-riboside.     

Studies on the interaction of peroxy radicals with transition metals complexes: I. Effect of 2-cyano-2-propyl and 2-cyano-2-propyl peroxy radicals on cobaloximes

Reactions of CH₃Co(DH)₂py (1) and [Co(DH)₂py]₂ (2) with (CH₃)₂(CN)C (r) and (CH₃)₂(CN)COO (rO₂) radicals were investigated. At 60°C, reaction or r with (1) results in non-homogeneous ligand decomposition, whereas for 2, complex (CH₃)₂CNCCo(DH)₂py (6) and a precipitate are formed. Ligand decomposition also took place at 60°C when the reaction of rO₂ radicals with 1 and 2 was investigated. However, the same reaction with rO₂ radicals at −10°C, yielded two complexes, CH₃OOCo(DH)₂py (3) and Co(DH)₂py (4) with 1, and complex 6 for the reaction of rO₂ with 2.     

Effect of N+ ion implantation on antioxidase activity in Blakeslea trispora

The effect of N⁺ implantation on the activities of CAT, POD, SOD, T-AOC and the capacities of scavenging O₂⁻ and OH in Blakeslea trispora (−) were studied. Results showed that N⁺ implantation caused different changes of CAT, POD, SOD, T-AOC activities and cell scavenging O₂⁻ and OH capacities. With the implantation dose increasing CAT activity was lower than the control sample, while POD, SOD activities and the scavenging O₂⁻ and OH capacities all decreased at the beginning, and then increased lately. At the dose of 6.0×10¹⁵ N⁺ cm⁻² T-AOC activity was lowest, while at the dose of 1.2×10¹⁵ N⁺ cm⁻² its activity was highest, and this change trend was same to the B. trispora (−) survival rate curve. So we speculated that the changes of these antioxidases activity of B. trispora (−) induced by low-energy N⁺ probably have some relationship with its “saddle shape” survival rate curve.     

Spin-density distribution of the high Tcp-O2N·C6F4·CNSSN free radical studied by polarised neutron diffraction

Knowledge of the spin-density distribution in the dithiadiazolyl radical ring (DTDA) constitutes a major step towards the understanding of the magnetic and electronic properties of the rich magnetism of DTDA derivatives. The p-O₂N·C₆F₄·CNSSN radical was chosen as the most favourable CNSSN derivative to study the spin distribution in this kind of free radical by polarised neutron diffraction. Spin-density maps obtained for the p-O₂N·C₆F₄·CNSSN radical show that almost all the spin density is localised on the sulphur and nitrogen atoms of the CNSSN ring. A small negative spin density on the carbon atom of the CNSSN ring and a negligible spin density over the rest of the radical are observed, in good agreement with ab initio calculations.     

Fluorescence detection of hydroxyl radicals

The hydroxyl radical (OH), a product of water radiolysis, reacts to hydroxylate aromatic organic compounds. In some cases, these hydroxylated products are fluorescent. Examples include the benzoate, coumarin, and phenoxazinone systems. For representative members of these systems, we have determined both the rate constants for reaction with OH and the yields of the fluorescent products. The rate constants all fall in the range 2×10⁹ to 2×10¹⁰ L mol⁻¹ s⁻¹, and the yields 5–11% per OH. These results suggest that it may prove feasible to construct a probe consisting of two groups both of which must react with OH to become fluorescent. The efficient process of fluorescence resonance energy transfer implies that such a probe might be able to detect OH clusters, which are generally assumed to be a characteristic feature of energy deposition by ionizing radiation.     

A combined experimental and model analysis on the effect of pH and O2(aq) on γ-radiolytically produced H2 and H2O2

The effects of pH and dissolved O₂ on the γ-radiolysis of water were studied at an absorbed dose rate of 2.5 Gy s⁻¹. Argon- or air-saturated water with no headspace was irradiated and the aqueous samples were analyzed for molecular radiolysis products (H₂ and H₂O₂) as a function of irradiation time. The experimental results were compared with computer simulation results using a comprehensive water-radiolysis kinetic model, consisting of the primary radiolysis production, subsequent reactions and related acid–base equilibria. Both the experimental and computer model results were discussed based on the steady-state kinetic analysis of smaller reaction sets consisting of key production and removal reactions. While the main production path for a water decomposition product is the primary radiolysis, the main removal path varies. For H₂O₂ the main removal path is the reactions with e_aq⁻ and OH, whereas for H₂ it is the reaction with OH. As a result, the presence of a dissolved species, or a change in chemical environment, affects the concentrations of H₂O₂ and H₂ through interaction with radicals e_aq⁻ and OH. Over a wide range of conditions, there exist quantitative but simple relationships between the radical and the molecular product concentrations. The experimental and model analyses show that dissolved oxygen increases the steady-state concentrations of H₂O₂ and H₂ by reacting with OH and e_aq^–, and the impact of oxygen is more noticeable at pH below 8. The steady-state concentrations of water decomposition products are nearly independent of pH in the range 5–8. However, raising pH above the pKa value of the acid–base equilibrium of H (⇆e_aq⁻+H⁺) significantly increases [H₂O₂] and [H₂] at the expenses of [OH] and [e_aq^–]. At pH >10, the radiolytical production of O₂ becomes significant, but at a finite rate. This considerably increases the time for the irradiated system to reach a steady state, and is responsible for different impacts on [H₂O₂] and [H₂] due to radically produced O₂, compared to impacts due to initially dissolved O₂. Model sensitivity analysis has shown that at higher pHs (pH >10) transient species such as O₂⁻ and O₃⁻ play a major role in determining the steady-state concentration of molecular products H₂ and H₂O₂. Further validation of the water radiolysis model, particularly at higher pHs, is also discussed.     

Pulse radiolysis studies on reactions of α-hydroxyalkyl radicals with nicotinamide and 6-methyl nicotinic acid

Reactions of α-hydroxyalkyl radicals derived from 2-propanol, ethanol and methanol with nicotinamide (NICAM) and 6-methyl nicotinic acid (6-MNA) were studied at various pHs using pulse radiolysis technique. It is found that α-hydroxyalkyl radicals react with NICAM and 6-MNA at pHs when nitrogen is in the protonated state. In these reactions, radical adducts of NICAM/6-MNA with α-hydroxyalkyl radicals are formed which have absorption maxima at about 340–350 nm which subsequently decay to give pyridinyl type of radicals of NICAM and 6-MNA having λ_max at 410 nm. Rate constants for the reactions of (CH₃)₂COH, CH₃CHOH and CH₂OH radicals with NICAM and 6-MNA were found to have linear dependence on reduction potentials of corresponding α-hydroxyalkyl radicals. Adducts formed in the reactions of CH₃CHOH and CH₂OH radicals with both NICAM and 6-MNA decayed slowly compared to the decay of adduct formed in reactions with (CH₃)₂COH radicals.     

Characterization,stability, and origin of natural radiation-induced defects in the biogenic calcite of Belemnitella americana from the Upper Cretaceous: An electron paramagnetic resonance study

The rostrum of Belemnitella americana (Morton) from the Marshalltown formation (Kmt, Upper Cretaceous) of the Chesapeake and Delaware Canal was investigated by electron paramagnetic resonance (EPR) spectroscopy. The rostrum composed of biogenic calcite possessed inorganic radical centers CO₂⁻, SO₂⁻, and SO₃⁻ with isotropic resonances with g values of 2.0007, 2.0057, and 2.0031, respectively. SO₃⁻ was found to also display an axially symmetric resonance typical of that seen in calcite of geologic origin with g_⊥=2.0036 and g_∥=2.0021. Mn²⁺ signals of orthorhombic symmetry and very narrow line width (∼0.1 mT) were also noted (|D|=9.3 mT (∼0.009 cm⁻¹), |E|=3.1 mT (∼0.003 cm⁻¹)). Isochronal annealing studies reveal that these inorganic radical species reside in energy traps that are significantly deeper than previously determined as revealed by their annealing temperatures: SO₂⁻ (isotropic), T^*∼340 °C; SO₃⁻ (isotropic), T^*∼230 °C; SO₃⁻ (axial), T^*∼190 °C. These data suggest that these spin centers may be used to extend the upper limit for dating purposes to times on the order of 1 Ma for SO₃⁻ (axial) and 200–300 Ma for SO₃⁻ (isotropic). Spin–spin and spin–lattice relaxation studies employing progressive microwave saturation were determined for all sulfur-based radical species and found to be consistent with the supposition of the isotropic signals existing in environments that are conducive to dynamic averaging of the g-anisotropy.     

Estimation of the standard reduction potentials of some 1-arylethyl radicals in acetonitrile

The redox properties of some arylethyl radicals, which are involved in the electrochemical synthesis of important anti-inflammatory agents, have been investigated in CH₃CN by an indirect electrochemical method based on the catalytic reduction of the corresponding arylethyl halides (RX) by electrogenerated aromatic or heteroaromatic anion radicals (D⁻). The reaction between RX and D⁻ leads to a radical R, which can react with D⁻ either by radical coupling (k₃) or by electron transfer (k₄). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q=k₄/(k₃+k₄), as a function of E⁰_D/D⁻ allows estimation of the reduction potentials of the arylethyl radicals. The standard reduction potentials obtained for the radicals 1-(4-isobutylphenyl)ethyl, 1-(6-methoxy-2-naphthyl)ethyl, and 1-(4-biphenyl)ethyl are −1.64, −1.62, and −1.15 V vs. SCE, respectively.     

A theoretical study on the alkene insertion step in Rh-Yanphos catalyzed hydroformylation

This paper studied the mechanism of the alkene insertion elementary step in the asymmetric hydroformylation （AHF） catalyzed by RhH（CO）2[（R,S）-Yanphos] using four alkene substrates （CH2=CH- Ph, CH2=CH-Ph-（p）-Me, CH2=CH-C（==O）OCH3 and CH2=CH-OC（=O）-Ph, abbreviated as A1-A4）. Interestingly, the equatorial vertical coordination mode （A mode） with respect to the Rh center was found for AI and A2 but not for A3 and A4, although the equatorial in-plane coordination mode （E mode） was found for A1 -A4. The relative energy of the E mode of the -q2-intermediates is lower than that of the A mode. In the alkene insertion step, Path 1 is more favorable than Path 2 for this system. As for AI and A2, there could be a transformation between 2eq and 2ax.