Gas-phase reactions of halogenated radical carbene anions with sulfur and oxygen containing species

The reactivities of mono- and dihalocarbene anions (CHCl⁻, CHBr⁻, CF₂⁻, CCl₂⁻, and CBrCl⁻) were studied using a tandem flowing afterglow-selected ion flow tube instrument. Reaction rate constants and product branching ratios are reported for the reactions of these carbene anions with six neutral reagents (CS₂, COS, CO₂, O₂, CO, and N₂O). These anions were found to demonstrate diverse chemistry as illustrated by formation of multiple product ions and by the observed reaction trends. The reactions of CHCl⁻ and CHBr⁻ occur with similar efficiencies and reactivity patterns. Substitution of a Cl atom for an H atom to form CCl₂⁻ and CBrCl⁻ decreases the rate constants; these two anions react with similar efficiencies and reactivity trends. The CF₂⁻ anion displays remarkably different reactivity; these differences are discussed in terms of its lower electron binding energy and the effect of the electronegative fluorine substituents. The results presented here are compared to the reactivity of the CH₂⁻ anion, which has previously been reported.     

Effects of some factors during electrochemical degradation of phenol by hydroxyl radicals

Hydroxylation of aromatic compounds has been used extensively as a measure of hydroxyl radicals (OH) formation. In this paper, salicylic acid (2-HBA) was used to trap OH in the process of electrolysis with a couple of Ti-base lead dioxide electrodes in different conditions. Aqueous solution of 2-HBA, a couple of Ti-base lead dioxide electrodes and an AC power were used in the course of OH formation, and then the solution containing 2-HBA and 2,5-dihydroxybenzoic acid (2,5-DHBA) was analyzed by High Performance Liquid Chromatography (HPLC) with fluorescence detector. This method can help us to better understand the reaction mechanism of OH from the viewpoint of quantity.In the same conditions of electrolysis, Total Organic Carbon (TOC) of phenol solutions were detected to identify the effects of some factors during this electrochemical process, because the strong oxidation ability of OH can mineralize the organic pollutants totally and finally achieve the goal of water treatment. The results show that high pH value of electrolyte and high frequency of the AC power are favorable for the generation of OH, however, CO₃²⁻ is opposite to them.     

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.     

Synthesis and chemical oxidation of 3-ferrocenylpyrrole and ferrocenyl-substituted triazoles: Iron versus ligand based oxidation

Copper catalyzed [3+2] cycloaddition reactions between ethynylferrocene and benzylazides yields 1-benzyl-4-ferrocenyl-1,2,3-triazoles (2–5). Reaction between phenylacetylene and azidoferrocene yields 1-ferrocenyl-4-phenyl-1,2,3-triazole (6). Anodic electrochemistry of 2–6 suggests reversible oxidation at potentials more positive than ferrocene. Chemical oxidation of 2 and 3-ferrocenylpyrrole (1) with dichlorodicyanoquinone (DDQ) yields the salts [2⁺] [DDQ⁻] and [1⁺] [DDQ⁻], respectively. ⁵⁷Fe Mössbauer spectroscopy reveals the presence of low-spin Fe^II in [1⁺][DDQ⁻] while Fe^II is oxidized to low-spin Fe^III in [2⁺][DDQ⁻]. Magnetization measurements indicate that [1⁺][DDQ⁻] is paramagnetic and cannot be viewed as a simple neutral charge transfer complex reminiscent of the mixed stack diamagnetic [ferrocene]⁰[TCNE]⁰.     

Radiolysis of rutin in aerated ethanolic solution

Radiolysis of rutin was performed in aerated ethanolic solution. Two major radiolytic products (RP1, RP2) were isolated by HPLC, and their possible structures were deduced from their UV, IR and MS spectra, and elementary analysis as well. The G-values of RP1 and RP2 increase with increasing rutin concentration, and in all cases both of them equal the G-value of rutin consumption. The addition of rutin leads to the decrease of G(H₂O₂) but has little effect on G(CH₃CHO). Therefore the formation of RP1 and RP2 was proposed to be from further addition of HO%s₂ to the phenolic radical (generated from H-abstraction by HO%s₂) followed by fragmentation.     

Determination of hydroxyl radical in Fenton system

Under visible light illumination,2,3-diaminophenazine(DAPN) was generated from the oxidation of o-phenylenediamine (OPDA) in Fe~(3+)/H_2O_2 solution.Hydroxyl radical(~·OH) produced in this system was determined by directly measuring the concentration of DAPN.In comparison with the traditional methods,the determination is more accurate and simple.     

A comparative study on the ionic liquid [bmim][BF4] and its solution with transient absorption spectroscopy

A detailed study was explored to compare the transient absorption spectra of the neat 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) with its solution in water or acetonitrile. It was concluded that the excited triplet state ³[bmim]^+* was produced after 266 nm laser irradiation, and then the neutral radical [bmim] and the cation radical [bmim] ²⁺ were formed through two possible paths. The transient absorption spectra of the neat [bmim][BF₄] and its solution were similar but the reaction kinetics were different due to their different local structures such as dimeric or cluster. The energy transfer between excited [bmim][BF₄] and β-carotene further affirmed the existence of ³[bmim]^+*. And the reaction that the hydrated electron captured by [bmim]⁺ to produce [bmim] in solution was observed.     

A new cataluminescence sensor for carbon tetrachloride using its catalytic reduction by hydrogen on palladium/carbon surface

A new cataluminescence (CTL) sensor was developed based on the chemiluminescence (CL) emission from the catalytic hydrodechlorination of carbon tetrachloride on the surface of palladium/carbon catalyst. The factors influencing the CTL signal, such as the catalyst, carrier gas, gas flow rate, temperature and the CL wavelength, were investigated in detail. Under the optimal conditions, the linear range of the CTL intensity versus concentration of carbon tetrachloride was 4.7–235 μg/mL (R = 0.9944, n = 7), with a limit of detection of 0.7 μg/mL (σ = 3). GC/MS results suggest that the possible CTL mechanism of the reduction is the formation of CCl₃ radicals. The CCl₃ radicals combine with H free atoms or capture hydrogen atoms from H₂ molecules to form excited CHCl₃ intermediates, which decay from the excited-state to the ground giving CTL emission for the detection. It is also found that some benzene derivatives with α-H of branched-chain, such as toluene, ethylbenzene and xylenecan, can play a role of catalyst in the reaction.     

Photocycle dynamics of the E149A mutant of cryptochrome 3 from Arabidopsis thaliana

The E149A mutant of the cryDASH member cryptochrome 3 (cry3) from Arabidopsis thaliana was characterized in vitro by optical absorption and emission spectroscopic studies. The mutant protein non-covalently binds the chromophore flavin adenine dinucleotide (FAD). In contrast to the wild-type protein it does not bind N5,N10-methenyl-5,6,7,8-tetrahydrofolate (MTHF). Thus, the photo-dynamics caused by FAD is accessible without the intervening coupling with MTHF. In dark adapted cry3-E149A, FAD is present in the oxidized form (FAD_ox), semiquinone form (FADH), and anionic hydroquinone form (FAD_redH⁻). Blue-light photo-excitation of previously unexposed cry3-E149A transfers FAD_ox to the anionic semiquinone form (FAD⁻) with a quantum efficiency of about 2% and a back recovery time of about 10 s (photocycle I). Prolonged photo-excitation leads to an irreversible protein re-conformation with structure modification of the U-shaped FAD and enabling proton transfer. Thus, a change in the photocycle dynamics occurs with photo-conversion of FAD_ox to FADH, FADH to FAD_redH⁻, and thermal back equilibration in the dark (photocycle II). The photocycle dynamics of cry3-E149A is compared with the photocycle behaviour of wild-type cry3 and other photo-sensory cryptochromes.     

Evaluation of lipid peroxidation inhibition and free radical scavenging abilities of 5,6,7-trimethoxy dihydroflavonols

Four naturally rare 5,6,7-trimethoxy-2,3-cis-dihydroflavonols(3-6) and two 5,6,7-trimethoxy-2,3-trans-dihydroflavonols(7-8) were designed and synthesized.Their antioxidative properties were evaluated by way of examining their scavenging capacities towards DPPH and O_2~(·-) free radicals,as well as by measuring their inhibitory ability against LPO.Both the 2,3-trans and the 2,3- cis conformers exhibited certain quenching abilities to DPPH and O_2~(·-) radicals,while most of the synthetic dihydroflavonols ...     

Degradation efficiencies and mechanisms of the ZnO-mediated photocatalytic degradation of Basic Blue 11 under visible light irradiation

A ZnO-mediated photocatalysis process was used to successfully degrade Basic Blue 11 (BB-11) under visible light irradiation. The effects of influential factors like initial dye concentration, catalyst dosage, and initial pH were studied. To obtain a better understanding the mechanistic details of ZnO-assisted photodegradation of the BB-11 dye with low watt visible light irradiation, a large number of the intermediates resulting from the photodegradation were separated, identified, and characterized by high-performance liquid chromatography–photodiode array-mass spectrometry (HPLC–PDA-MS) techniques. The results indicated that the N-de-alkylation and oxidative degradation of BB-11 dye took place and that N-hydroxyalkylated intermediates were generated during the process. From the same identified intermediates we got under UV or visible light irradiation, it is proposed that the major oxidant under visible light irradiation was OH radical, not O₂⁻. HPLC–PDA-MS analysis verified the identity of intermediates, and a reaction mechanism based on them was proposed.     

On the mechanism of the photoinduced reduction of an adduct of ferricytochrome C with a poly(4-vinylpyridine) polymer containing –Re (CO)3(3,4,7,8-tetramethyl-1,10-phenanthroline) pendants

Changes in the UV–vis absorption spectrum revealed the formation of adducts between the Re^I polymer and ferricytochrome C, Fe^III-Cyt c. Different morphologies for the Re^I polymer and the adducts formed between the Re^I polymer and Fe^III-Cyt c were observed by TEM. The reduction of the Re^I chromophores in the polymer, achieved by the reductive quenching of the MLCT excited state of the Re^I polymer by triethylamine (TEA) and/or by the reaction between e⁻_solv and {[(vpy)₂vpyRe^I(CO)₃(tmphen)⁺]}_n200 in pulse radiolysis experiments, produces –Re^I(CO)₃(tmphen) and –Re^I(CO)₃(tmphenH)⁺ as the main species. The reductive quenching of the MLCT of the Re^I polymer by TEA was followed by a rapid electron transfer from the –Re^I(CO)₃(tmphen) to the Fe^III center in the heme to produce ferrocytochrome C, Fe^II-Cyt c.     

Production of reactive oxygen species induced by a new [60]fullerene derivative bearing a tetrazole unit and its possible biological applications

The wide range of physical and chemical properties of modified fullerenes has drawn increasing attention in the past few years. As part of this research, this paper describes the preparation, characterization, and photophysical properties of a new fullerene derivative chemically modified with a tetrazole. The photophysical properties were studied by EPR radical spin-trapping technique and showed that reactive oxygen species (ROS) can be produced through UVA photosensitization. EPR spin-trapping experiments with singlet oxygen (¹O₂) and superoxide (O₂⁻) inhibitors (β-carotene and superoxide dismutase, respectively) revealed also that: (i) the main ROS produced is ¹O₂ and (ii) ¹O₂ is being partially dismutated to O₂⁻. The results suggest that this derivative can be used in biological applications, as for example, in topic photodynamic therapy (PDT) as a photosensitizer.     

Recycling of the used automotive lubricating oil by ionizing radiation process

The recycling process of the used mineral oils has been gaining a very important gap in the context of environmental protection. Among mineral oils from petroleum, the lubricating oils are not entirely consumed during their use; therefore, it is necessary to apply a treatment for recuperation seeking their reuse. Moreover, the environmental legislation of countries does not allow their discard in any type of soils, rivers, lakes, oceans or sewerage systems.The conventional treatment has shown certain difficulties in the recuperation process for used oils. The ionizing radiation process is renowned in the industrial effluents treatments due to its high efficiency in the degradation of organic compounds and in the removal of metals by the action of OH, H and e_aq radicals.In this work, used automotive lubricating oil was treated by the ionizing radiation process for metal removal and degradation of organic compounds. The samples were irradiated with 100 and 200 kGy irradiation doses. Determination of the elements Mg, Al, P, S, Cl, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Se, Mo, Nb, Cd, Sn, Ba, Bi and Pb, before and after the irradiation, was done by X-ray fluorescence technique and the organic profile was obtained by infrared spectroscopy.     

Photoactivity of mechanochemically prepared nanoparticulate titanium dioxide investigated by EPR spectroscopy

Titanium dioxide nanopowders were prepared by mechanochemical synthesis in a high-energy ball mill using TiOSO₄·xH₂O and Na₂CO₃ followed by annealing in the temperature range 200–800 °C. The UVA photonic efficiency of radical processes on synthesized TiO₂ powders was determined by in situ EPR spectroscopy, using, as indicators, the N-oxide spin trapping agents (5,5-dimethyl-1-pyrroline N-oxide and 5-(diisopropoxy-phosphoryl)-5-methyl-1-pyrroline N-oxide) or the radical cation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate, ABTS⁺). The results obtained by monitoring the photoinduced generation of hydroxyl radical spin adducts correlated with those found by the investigation of the photoreduction of ABTS⁺. The presence of iron and chromium ions, which were evidenced in samples milled in steel, decreased the photonic efficiency of radical processes. The presence of a sulfate salt matrix during the annealing process distinctly inhibits the transformation of anatase to rutile. The highest photocatalytic activity was shown by anatase samples which were prepared by milling in corundum and annealed at 700 °C. They were composed of crystallites with a mean size of 25–30 nm and well developed crystal faces.     

Unimolecular chemistry of metastable dimethyl isophthalate radical cations

The MS/MS spectrum of the metastable molecular ions of dimethyl isophthalate 1 differs from that of the isomeric dimethyl terephthalate 2 by the observation of, inter alia, a quite intense loss of C,H₂,O ascribed to formaldehyde. Results obtained using a combination of mass spectrometry techniques suggest that this process could consist of an isomerization reaction of the molecular ion into an ion–neutral complex (INC) linking a benzoyl radical and neutral formaldehyde to a proton [ArCOHOCH₂]⁺. Within the complex, a proton transfer catalyzed by formaldehyde occurs resulting in the production of an ionized cyclohexadienylidene methanone (ketene) structure.     

Characterization of the transient species generated by the photoionization of Berberine: A laser flash photolysis study

Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO₄⁻ radical oxidation method, the assignment of radical cation of BBR was further confirmed, the pK_a value of it was calculated, and the related set up rate constant was also determined.     

Effects of hydroxyl radicals and oxygen species on the 4-chlorophenol degradation by photoelectrocatalytic reactions with TiO2-film electrodes

The supported nano-TiO₂ electrode was prepared by sol–gel and hydrothermal method, and the photoelectrocatalytic degradation of 4-chlorophenol (4-CP) under UV irradiation has been investigated to reveal the roles of hydroxyl radicals and dissolved oxygen species for TiO₂-assisted photocatalytic reactions. The degradation kinetics, the formation and decay of intermediates, the isotopic tracer experiments with H₂O¹⁸, the removal yield of total organic carbon and the formation of active radical species in the presence of oxygen or not were examined by HPLC, GC–MS, TOC and spin-trap ESR spectrometry. It was found that most of OH radicals in the primary hydroxylated intermediates derived from the oxidation of adsorbed H₂O or HO⁻ by photo-holes in the electrochemically assisted TiO₂ photocatalytic system. It also indicates that the enhancement in the separation efficiency of photogenerated charges by applying a positive bias (+0.5 V vs SCE) has little role in the following decomposition and mineralization of these hydroxylated intermediates in the absence of oxygen. According to above experimental results, the pathway of 4-CP photocatalytic degradation was deduced initially. Due to the combined effect of OH radicals and dissolved oxygen species, the hydroxylated 4-chlorphenol, via cis, cis-3-chloromuconic acid, was decomposed into low molecular weight acid and CO₂.     

DFT study of the structure of hydroxybenzoic acids and their reactions with OH and radicals

B3LYP/6-31++G(d) method was used for the structural study of 3,4,5-trihydroxybenzoic acid (3,4,5-THBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), and 4-hydroxybenzoic acid (4-HBA). Calculated structures agree with available X-ray experimental data within 2%. The phenolic OH bond dissociation enthalpy (BDE) of all sites in each benzoic acid were determined and compared with those of phenol (for 4-HBA) and catechol (for 3,4-DHBA). The consistency between the calculated values and experimental ones are within 5.4% and 9.2%, respectively, for 4-HBA and 3,4-DHBA. The reactions of benzoic acids with and OH radicals were studied and it turns out that benzoic acids react differently with both radicals. We have shown that the reaction of hydroxybenzoic acids with the hydroxyl radical was governed by a phenolic hydrogen (H⁺ + e⁻) transfer from the acid to the radical, while for the superoxide radical, the reaction is governed by a proton (H⁺) transfer from the acid to the radical. The first reaction is evidenced by the delocalization of the radical on the entire quinone moiety, and the second reaction is evidenced by the negative NBO charge on the phenoxide moiety as well as the localization of the radical on the hydroperoxy (O₂H) moiety.     

Redox catalysis versus heterogeneous one-electron catalysis. Mixed reduction of arenes and alkyl halides at specific palladized electrodes

Reduction of aromatic compounds (A) is achieved in the presence of alkyl halides RX (X = I or Br) at Ag–Pd electrodes in organic solvents. Those electrodes allow the one-electron reduction of RX with the selective formation of free radicals R. This new process (heterogeneous one-electron catalysis, H1EC) was used to alkylate in situ arenes. This mode of alkylation leads to re-visit previous results concerning redox catalysis published by Henning Lund when more conventional electrodes (like glassy carbon or mercury) were used and afforded similar results within a totally different potential scale. These preliminary results underline the reactivity differences between the molecular electrode (A⁻) and the almost ideal catalysis process (facile and specific liberation of free alkyl radicals).