Determination of photoformation rates and scavenging rate constants of hydroxyl radicals in natural waters using an automatic light irradiation and injection system

Photoformation rates and scavenging rate constants of hydroxyl radicals (OH) in natural water samples were determined by an automatic determination system. After addition of benzene as a chemical probe to a water sample in a reaction cell, light irradiation and injection of irradiated water samples into an HPLC as a function of time were performed automatically. Phenol produced by the reaction between OH and the benzene added to the water sample was determined to quantify the OH formation rate. The rate constants of OH formation from the photolysis of nitrate ions, nitrite ions and hydrogen peroxide were comparable with those obtained in previous studies. The percent of expected OH photoformation rate from added nitrate ion were high in drinking water (97.4%) and river water (99.3%). On the other hand, the low percent (65.0%) was observed in seawater due to the reaction of OH with the high concentrations of chloride and bromide ions. For the automatic system, the coefficient of variance for the determination of the OH formation rate was less than 5.0%, which is smaller than that in the previous report. When the complete time sequence of analytical cycle was 40 min for one sample, the detection limit of the photoformation rate and the sample throughput were 8 × 10⁻¹³ M s⁻¹ and 20 samples per day, respectively. The automatic system successfully determined the photoformation rates and scavenging rate constants of OH in commercial drinking water and the major source and sink of OH were identified as nitrate and bicarbonate ions, respectively.     

OH-radical induced degradation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA): A pulse radiolysis and gamma-radiolysis study

The reactions of OH, H and e_aq⁻ with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA) were studied by pulse radiolysis. The site of OH-radicals addition to the aromatic ring of 2,4,5-T was found to be—C1: ∼18%, C2/C4/C5: total ∼28% and C3/C6: total ∼41%. The overall rate constants with OH-radicals were k(OH+2,4,5-T)=6.4 (±0.5)×10⁹ mol dm⁻³ s⁻¹ and k(OH+MCPA)=8.5 (±0.8)×10⁹ mol dm⁻³ s⁻¹. The radiation induced decomposition of the pesticides, chloride- and product formation (phenolic compounds, aliphatic acids) was studied by gamma radiolysis as a function of dose. A mechanism for acetate formation is discussed. The presence of oxygen during irradiation affected the decomposition rate only indiscernibly, however, chloride elimination, ring fragmentation (formation of aliphatic acids), TOC- and toxicity reduction were strongly enhanced. For complete removal of 500 μmol dm⁻³ herbicides a dose of ∼4 kGy was required. Using air saturation during irradiation a reduction of 37-40% of the TOC was observable at 5 kGy, detoxification (luminescence inhibition <20%) was achieved with 10 kGy.     

Formation of dimer radical cation of 1,1-dianisylethylene through two-step hole transfer processes during TiO2 photocatalytic reactions

The formation of the 1,1-dianisylethylene dimer radical cation ((DAE)₂⁺) during TiO₂ photocatalytic reactions was successfully investigated by the transient absorption measurement with time-resolved diffuse reflectance spectroscopy. This reaction was explained in terms of the cascade hole transfer processes from the photo-generated holes in the TiO₂ particles to free DAE (DAE_free) in the bulk solution mediated by the radical cation of p-phenylbenzoic acid adsorbed on the TiO₂ surface (PBA_ads⁺).     

OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1⁺, 3⁺ and 4⁺, respectively) and the α-methyl derivatives 2⁺ and 5⁺ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1⁺-5⁺ under acidic and basic conditions. In acidic solution, the decay of 1⁺-5⁺ proceeds by cleavage of the C-H bond, while in the presence of ⁻OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The ⁻OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H₂O, appears to be affected by frontier orbital interactions.     

Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (³2a) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of ³2* gave reasonable correlations with the substituent constant, σ. The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3*). The results suggest that TL of ³2* and PL of 3* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of ³2 and 3. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2⁺ and 1⁻, which generates ³2*, revealed that substituents not only alter the TL energies but also the TL intensities of ³2*. The observations made in this effort demonstrate that ³2* as well as ³2 and 2⁺ have greatly twisted molecular geometries and highly localized electronic structures.     

Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part I. The case of “pure” radiochemical initiation

As part of a study on the kinetic modelling of polyethylene oxidation under irradiation at low temperature and low dose rate, this first part deals with the kinetic regime in which thermal initiation, linked to hydroperoxide decomposition, is negligible compared to radiochemical initiation due to polymer radiolysis. The kinetic analysis is based on results published 30 years ago by Decker, Mayo and Richardson. A small modification of their mechanistic scheme, consisting in the introduction of a non-terminating bimolecular combination of PO₂ radicals, leads to a more consistent set of radiochemical yield values. The most significant change is a decrease in the radiochemical yield of radicals G_i from 10 to 8. At 45 °C, termination of PO₂ radicals is not very efficient: 35-40% of the PO₂ + PO₂ encounters are non-terminating, 75% of the termination events lead to peroxide bridges, the rest is a disproportionation according to the Russell mechanism.     

Syntheses, crystal structure and properties of two novel coordination polymers with the flexible tetrazole-1-acetic acid (Htza)

Two new coordination polymers, [Ag(tza)]_∞ (1) (Htza=tetrazole-1-acetic acid) and [Cu(tza)₂]_∞ (2) have been prepared at room temperature and characterized by X-ray crystallography, IR, UV-vis, fluorescence spectra and magnetism analysis. Compound 1 exhibits extended helical chains through bridging ligand tza. The AgAg interactions between the adjacent chains form a 3-D framework featuring the extended tza-connected Ag chains that obviously affect the photoluminescent property. Compound 2 features undulated layered structure with hourglass-shaped [Cu₄(tza)₄] as subunits with the weak ferromagnetic interactions between Cu(II) ions, which are further stabilized by inter-lamellar CHO hydrogen bonds in the resulting 3-D supramolecular framework.     

Determination of the hydroxyl radical by its adduct formation with phenol and liquid chromatography/electrochemical detection

An HPLC-ECD method is described for the indirect determination of the hydroxyl (OH) radical. Fenton's reaction is used to produce OH, which simultaneously attacks phenols (phenol or pyrocatechol) to form the adducts, pyrocatechol or pyrogallic acid. Thus, [OH] quantification is based on the separation and detection of pyrogallic acid and/or pyrocatechol by an isocratic HPLC-ECD method. The quantification of OH is also performed alternatively by a chronoamperometric detection in an electrochemical cell, where simultaneously formed Fe^III (Fenton's reaction) combines [Fe^II(CN)₆]⁴⁻ to produce the Prussian blue (PB) molecules (Fe₄^III[Fe^II(CN)₆]₃). Newly formed PB molecules are then immediately converted to colorless Everitts salt (K₄Fe₄^II[Fe^II(CN)₆]₃) with the reduction of the high-spin Fe^III to Fe^II at the surface of a glassy carbon electrode at +0.150 V (versus Ag/AgCl). The calculated concentration of OH during incubation (0.626 ppm) can be detected with negative errors by the HPLC-ECD (0.595 and 0.615 ppm with the errors −5.2 and −1.8%, respectively) and by the chronoamperometric method (0.552 and 0.607 ppm with the errors −11.8 and −3.0%, respectively). For the comparison of the two sets of data, HPLC-ECD method is much more promising.     

Artificial photosynthetic reaction centers with carotenoid antennas

Two reaction center-antenna models based on a purpurin macrocycle linked to a C₆₀ and to a carotenoid polyene have been synthesized. In these systems the C₆₀ moiety is the primary electron acceptor, the purpurin is the primary electron donor and the carotenoid moiety acts both as an antenna and secondary electron donor. Formation of the initial charge separated state, C-Pur⁺-C₆₀⁻, following excitation with light absorbed by either the purpurin or C₆₀ takes place on the 10 ps time scale. The final charge separated state, C⁺-Pur-C₆₀⁻, is formed in one of the compounds with a quantum yield of 32% based upon light absorbed by the carotenoid. In order to function as an antenna, the carotenoid pigment must be electronically coupled to the purpurin. The purpurin C ring provides an excellent framework for locating a carotenoid polyene in partial conjugation with the macrocycle, leading to a relatively strong electronic communication between the chromophores; functionalization of a meso position of the purpurin provides a site for the covalent attachment of C₆₀.     

Evaluation of chemiluminescence reagents for selective detection of reactive oxygen species

In order to evaluate the chemiluminescence (CL) reagents for selective detection of reactive oxygen species (ROS), we comprehensively measured the CL responses of 20 CL reagents (three luminol derivatives, two imidazopyrazinone derivatives, eight lophine derivatives, six acridinium ester derivatives and lucigenin) against six types of ROS (superoxide anion: O₂⁻, hydroxyl radical: OH, hydrogen peroxide: H₂O₂, hypochlorite anion: ClO⁻, singlet oxygen: ¹O₂, and nitric oxide: NO). As a result of the screening, it was found that nine CL reagents selectively detected O₂⁻ while one CL reagent selectively detected OH. However, no CL reagent had selectivity on the detection of H₂O₂, ClO⁻, ¹O₂ and NO. Our screening results could help to select the most suitable CL reagent for selective determination of different ROS.As an application study, 4-methoxyphenyl-10-methylacridinium-9-carboxylate (MMAC), one of the acridinium ester derivatives, showed high selectivity on the detection of O₂⁻, and thus was applied to the assay of superoxide dismutase (SOD) activity. The dynamic range and detection limit of the developed CL assay were 0.1-10 and 0.06 U mL⁻¹, respectively. Significant correlation (r = 0.997) was observed between the results by the CL assay using MMAC and the spectrophotometric assay using 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium monosodium salt.     

Structures and EPR spectra of binary sulfur-nitrogen radicals from DFT calculations

The scattered electron paramagnetic resonance (EPR) spectroscopic data for binary sulfur-nitrogen (S,N) radicals have been compiled and critically assessed.Many of these are inorganic rings or cages.For each species, possible equilibrium structures in the gas phase and the EPR hyperfine coupling (hfc) constants have been calculated with DFT using the B3LYP functional and basis sets of triple-ζ (or better) quality.Good agreement is obtained between calculated and measured values for the well characterized [S₃N₂]⁺, a planar π-radical for which the s-component of the orbitals is likely to be reasonably independent of minor geometrical changes between gas-phase and condensed-phase states.The cage compounds [S₄N₄]⁻ and [S₄N₅]⁻², for which reliable experimental EPR spectra have been reported, show larger variation between calculated and measured hfc, as a consequence of the dependence of the s orbital content of the molecular orbitals on small structural changes.The very large disagreements between the DFT calculated and experimentally claimed hfc constants for [NS], [SNS] and [S₄N₄]⁻³ in condensed phases lead us to question their assignment.Among binary S,N radicals, ³³S hfc data has only been reported for [S₃N₂]⁺ (through isotopic enrichment).These values were essential for the correct identification of the EPR spectra of this important radical, which previously was misassigned to other species.Our results suggest that ³³S data will be equally important for the correct identification of the EPR spectra of other binary S,N species, many of which are cyclic systems, e.g.[S₃N₃], [S₄N₃] and[S₄N₅].     

Degradation of γ-irradiated linear perfluoroalkanes at high dosage

The effects of the carbon backbone chain length on the EPR spectra of linear perfluoro-n-alkanes (PFAs) γ-irradiated at 77 K was studied for the short chain n-C₆F₁₄, n-C₈F₁₆, n-C₁₂F₂₆, and n-C₁₆F₃₄ molecules as well as the polymer polytetrafluoroethylene (PTFE). The experimental data show that the processes occurring during radiolysis of perfluoro-n-alkanes and polytetrafluoroethylene are very similar. EPR spectra of irradiated perfluoro-n-alkanes at low radiation dose show superimposed signals from three radicals: -F₂CCFCF₂-, -CF₂CF₂ and F₃C. The signal intensity decreases with perfluoro-n-alkanes chain length. At doses above 2.0 MGy, a constant increase in concentration of the radicals -F₂CCFCF₂- and -CF₂CF₂ is observed with decreasing chain length. The concentration of these radicals formed during radiolysis of PFA is described by the ratio: [-CF₂CF₂]/[-F₂CCFCF₂-] ≈3/(n − 2), where n is the number of carbon atoms in the linear perfluoroalkanes. Density functional theory was used to calculate the structures of the radicals and C-F bond energies in model perfluoro-n-alkanes as well as the EPR spectra of the associated radicals. This data is used to provide further insight into the radiation stability of PTFE. Four topographical structures of polytetrafluoroethylene, one amorphous and three crystalline, were identified by thermomechanical analysis. In the crystal phase, γ-irradiation results in their transformation to the amorphous form. The helical structure of individual perfluroalkanes readily distorts on removal of a fluorine and this will have an impact on the overall structure of the material. Such structural reorganization can lead to loss of the mechanical stability of polytetrafluoroethylene.