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N-doped hollow carbon spheres loaded with Cu single atoms (Cu SAs/N-CSs) were prepared via an in situ hard-template method. As an electrode catalyst, the Cu SAs/N-CSs electrochemical-sensing properties to hydroquinone (HQ) and catechol (CC) were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), and the linear detection ranges of HQ and CC were 1–324 and 1–123 μM, respectively; the detection limits are 0.33 and 0.17 μM, respectively. The experimental and DFT calculation results show that Cu SAs/N-CSs have stronger adsorption strength and more effective affinity for CC and HQ due to the synergistic effect of dispersed Cu SAs and N-CSs, thus improving the electrocatalytic and sensing properties of CC and HQ. Furthermore, the prepared electrode has been successfully applied to the simultaneous determination of HQ and CC in practical water samples with satisfactory results. [ABSTRACT FROM AUTHOR]

The development of a green photoelectrochemical (PEC) active material and the manipulation of its carrier migration are of paramount importance for achieving high-performance PEC biosensing. In this study, we engineered a PEC heterojunction involving green InP/ZnS quantum dots (InP/ZnS QDs) with a sulfur-doped Fe–N–C single-atom catalyst (Fe–S/N–C) through electrostatic self-assembly. In the InP/ZnS@Fe–S/N–C heterojunction structure, InP/ZnS QDs significantly enhance the generation of photoinduced carriers due to its exceptional photophysical features, while Fe–S/N–C efficiently manipulates the transfer of interfacial electrons, driving a high-efficiency photoelectric conversion efficiency as demonstrated by a 5.6-fold photocurrent enhancement relative to pure InP/ZnS QDs. Coupling with the efficient peroxide-like activity of Fe–S/N–C, the resultant InP/ZnS@Fe–S/N–C heterojunction was explored to fabricate a PEC biosensing platform for sensitive and selective detection of hydroquinone (HQ) and glucose through synergistic signal amplification. The constructed PEC biosensor reveals outstanding analytical performance, showing a low limit of detection of 9.8 μM for HQ and 50 μM for glucose, respectively. This work provides a promising strategy to enhance the photoelectric response of green semiconductor QDs by coupling them with versatile single-atom catalysts for advancing PEC biosensing applications. [ABSTRACT FROM AUTHOR]

Hydroquinone (HQ) and catechol (CC) are isomers with similar structures, which destroyed ecosystem balance and human health at an alarming rate. Therefore, it is critical to monitor HQ and CC in the environment simultaneously. Herein, a sensitive electrochemical sensor based on multiwalled carbon nanotubes and covalent polyoxometalate-organic frameworks (CPOFs) modified by nanoparticles (PtNPs@CPOFs-MWCNTs) was designed for the simultaneous detection of HQ and CC. Polyoxometalates (POMs) have been extensively studied in the area of electrochemical sensors because of rich reversible multielectron redox behavior. The porous and long stick-shaped CPOFs was synthesized by solvothermal Schiff base reaction, with NH2-POM-NH2 and 2,5-dimethoxy-phenyl-1,4-diformaldehyde as monomers. Using the porosity and large surface area of CPOFs, the obtained PtNPs@CPOFs nanocomposite showed an improved distribution of electroactive sites. At the same time, highly conductive MWCNTs was incorporated to guarantee the conductivity. Consequently, PtNPs@CPOFs-MWCNTs was obtained to improve the electrical conductivity and electrocatalytic activity. The results showed that the PtNPs@CPOFs-MWCNTs/GCE performed a wide range and low detection limit toward the detection of HQ and CC simultaneously. Furthermore, PtNPs@CPOFs-MWCNTs/GCE also had great stability and anti-interference ability. [ABSTRACT FROM AUTHOR]

This article describes a more accurate, reliable, and responsive electrochemical sensor for the specific analysis of major isomers of dihydroxybenzene at binary metal oxide, nickel cobaltite (NiCo2O4) nanosheets. In this sustainable synthesis, we report the obtention of NiCo2O4 (NCO) sheets prepared through the temperature-controlled hydrothermal route which yields NCO I (stacked sheets), NCO II (multilayered sheets), and NCO III (thin sheets). Then, the structural, morphological, and chemical properties were analyzed via, microscopic, diffractive, spectral studies, while the electrocatalytic activity was examined through voltammetric analysis. The electronic structure gained via hydrothermal treatment and analytical findings reveals the appropriate band position of the produced NCO sheets to be used in electrocatalytical methods. Hence, the designed sensor was evaluated to sense the major isomers of dihydroxybenzene, which pollutes the aquatic ecosystems. Therefore, for results the hydrothermally prepared NCO sheets through the temperature-controlled hydrothermal route were employed to sense the isomers of dihydroxybenzene specifically in real-time. [ABSTRACT FROM AUTHOR]

Hydroquinone (HQ) in wastewater is of great concern, as it is harmful to human health and threatens the ecological environment. However, the existing adsorbents have low adsorption capacity for HQ. To improve the removal of HQ, N,S-codoped activated carbon-ZIF-67 (NSAC-ZIF-67@C) was synthesized in this study by in situ growth of ZIF-67 on N,S-codoped activated carbon (NSAC) and carbonization. The influence of pH, contact time, and initial concentration on the adsorption behaviors of NSAC-ZIF-67@C on HQ were investigated. Owing to the synergistic effect of abundant active sites and well-developed pore structure, the NSAC-ZIF-67@C achieved a prominent adsorption capacity of 962 mg·g -1 and can still retain high adsorption performance after 5 cycles for HQ, which is superior to that of reported other adsorbents. HQ adsorption follows the pseudo-second-order kinetics model ( R 2 = 0.99999) and the Freundlich isotherm model. X-ray photoelectron spectroscopy (XPS) analysis before and after adsorption as well as density functional theory (DFT) calculation results showed that pyridinic-N-termini were conducive to the π-π interactions and hydrogen-bonding interactions. Therefore, the adsorption mechanisms of NSAC-ZIF-67@C on HQ involve pore filling, electrostatic attraction, π-π interaction, and hydrogen bonding. This study is expected to provide a reference for designing highly effective adsorbents for wastewater treatment.

Organic clathrates formed by combining hydroquinone (HQ) and CO2 could offer very interesting prospects in the near future, particularly in the field of CO2 capture and storage. However, one of the main limitations hindering the large-scale deployment of this type of clathrate-based technology is the slow enclathration kinetics. Our experiments, performed at different pressures (1.5, 3.0, and 4.5 MPa) and temperatures (298, 323, and 348 K), with HQ in different forms (HQ powder, HQ pellets, and HQ-silica composites, each different in nature and in terms of pore size and HQ content) demonstrated that (i) an increase in both pressure and temperature enhances the enclathration rate, (ii) the textural properties of HQ significantly impact kinetics, and composite materials remain the most efficient for improving HQ clathrate formation kinetics. [ABSTRACT FROM AUTHOR]

A novel synthetic route to polyquinolines with 6-substituted quinoline as the structural unit was developed based on the polymerization of alkyne-aldehyde monomers and aniline derivatives under the catalysis of Lewis acid B(C6F5)3. The polymerization was conducted in dichloroethane at 100 °C for 36 h under air atmosphere, affording polyquinolines with molecular weights up to 13 100 and good solubility in most organic solvents. The substituents in aniline exhibited significant effects on the molecular weight, yield, and solubility of the produced polyquinolines. The structures of prepared polymers were characterized and confirmed by GPC, NMR, and FT-IR. The thermogravimetry (TGA) and differential scanning calorimetry (DSC) analysis suggests that the polyquinolines are highly thermal stable. Further photoluminescence behaviors of the prepared polyquinolines were investigated. Based on the characterization results and small molecule reaction mechanism, the polymerization pathway of the polyquinolines was proposed. Our work has provided a novel simple strategy for the preparation of multifunctional polyquinolines with unique architectures by one-pot synthesis under metal-free catalysis. [ABSTRACT FROM AUTHOR]

Stimuli-responsive self-assembled monolayers (SAMs) are used to confer switchable physical, chemical, or biological properties to surfaces through the application of external stimuli. To obtain spatially and temporally tunable surfaces, we present microcontact printed SAMs of a hydroquinone molecule that are used as a dynamic interface to immobilize different functional molecules either via Diels-Alder or Michael thiol addition reactions upon the application of a low potential. In spite of the use of such reactions and the potential applicability of the resulting surfaces in different fields ranging from sensing to biomedicine through data storage or cleanup, a direct comparison of the two functionalization strategies on a surface has not yet been performed. Although the Michael thiol addition requires molecules that are commercial or easy to synthesize in comparison with the cyclopentadiene derivatives needed for the Diels-Alder reaction, the latter reaction produces more homogeneous coverages under similar experimental conditions. [ABSTRACT FROM AUTHOR]

Conformational isomers of hydroquinone and their 1:1 clusters with water have been spatially separated using a Stark deflector in a supersonic jet. trans-Hydroquinone (HyQ) conformer with zero dipole moment is little influenced by inhomogeneous electric fields, whereas cis conformer with nonzero dipole moment (2.38 D) is significantly deflected from the molecular beam axis into the direction along which the strong field gradient is applied. Resonant two photon ionization carried out by shifting the laser position perpendicular to the molecular beam axis after the Stark deflector then gives an exclusive S1-S0 excitation spectrum of the cis conformer only, making possible immaculate conformer-specific spectroscopy and dynamics. As the spatial separation is apparently proportional to the effective dipole moment strength, conformational assignment could be absolute in the Stark deflector, which contrasts with the hole-burning spectroscopic technique where identification of a conformational isomer is intrinsically not unambiguous. trans- and cis-HyQ-H2O clusters have also been spatially separated according to their distinct effective dipole moment strengths to give absolute spectroscopic identification of each cluster isomer, nailing down the otherwise disputable conformational assignment. This is the first report for the spatial separation of conformational cluster isomers. [ABSTRACT FROM AUTHOR]

The synthesis of a trisaccharide (common to glycoform I and II) and a tetrasaccharide (common to glycoform I) from the outer core domain of Pseudomonas aeruginosa lipopolysaccharide (LPS) using a novel hydroquinone-based reducing-end capping group is reported. This multifunctional capping group was utilized as purification handle and was stable toward many common transformations in oligosaccharide synthesis. The access to outer-core LPS antigens with a TBDPS-protected hydroquinone (TPH) at the reducing end will be useful for glycan array and therapeutic glycoconjugate synthesis. [ABSTRACT FROM AUTHOR]

The wastewater containing polyphenol (hydroquinone, catechol, and resorcinol) is often found in many industrial processes. Liquid-liquid extraction is a commonly used method to separate this style wastewater. The liquid-liquid equilibrium for the quaternary systems of water + polyphenol (hydroquinone, catechol, and resorcinol) + methyl isobutyl ketone (MIBK) + toluene were measured at 298.15 K and 0.1 MPa. The potential for the mixture of MIBK + toluene to extract the polyphenol from its aqueous solution was examined, and the distribution coefficient and selectivity were used to evaluate the extractive ability for the mixed solvent of toluene and MIBK. The results show that mixed solvent has better extraction ability compared with single extractant (MIBK). The nonrandom two-liquid and universal quasi-chemical models were used to correlate experimental data, and the binary interaction parameters were obtained. The values of room-mean-square-deviation (RMSD) were used to evaluate the feasibility of the two models for the systems and all the values of RMSD were less than 2%, which shows that the two models can correlate the experimental data properly. [ABSTRACT FROM AUTHOR]

The molecular cobalt complex, Co- (salophen), and para-hydroquinone (H2Q) serve as effective cocatalysts for the electrochemical reduction of O2 to water. Mechanistic studies reveal redox cooperativity between Co(salophen) and H2Q. H2Q serves as an electron-proton transfer mediator (EPTM) that enables electrochemical O2 reduction at higher potentials and with faster rates than is observed with Co(salophen) alone. Replacement of H2Q with the higher-potential EPTM, 2- chloro-H2Q, allows for faster O2 reduction rates at higher applied potential. These results demonstrate a unique strategy to achieve improved performance with molecular electrocatalyst systems. [ABSTRACT FROM AUTHOR]

The concerted mechanisms of excited state intramolecular proton transfer (ESIPT) of bis-2,4-(2-benzoxazolyl)-hydroquinone (BBHQ') and its derivatives (BBHQ'- and DHBO') have been investigated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). The calculated absorption and emission spectra of BBHQ' and its derivatives are in good agreement with the experimental results. The calculated bond lengths, bond angles, and IR vibrational spectra linked with hydrogen bond of molecular BBHQ' in the S0 and S1 states demonstrate that the hydrogen bond is strengthened in the S1 state. Compared to BBHQ', BBHQ'- has a weak change of hydrogen bond between the S1 and S0 states. The calculation results show that there are three stable structures of BBHQ' in the S1 state. We find that the structure corresponding to the 481 nm fluorescence spectrum corresponds to BBHQ'-A rather than BBHQ'--K (Tetrahedron Lett., 2016, 57, 3518). The calculated frontier molecular orbitals (MOs) indicate the nature of the charge distribution and the trend of proton transfer of BBHQ'-A. The constructed potential energy surfaces of BBHQ' and DBHO' further elucidate the proposed mechanism that one-proton or two-proton transfer can happen (stepwise or synchronous) in the S1 states. The proposed ESIPT mechanism can provide a good explanation of the phenomenon of fluorescence quenching of BBHQ' and its derivatives. Finally, the weak interaction types are discriminated through the reduced density gradient (RDG) analyses of BBHQ' and BBHQ'-. [ABSTRACT FROM AUTHOR]

The excited-state quenching of [Ru- (TAP)2(HAT)]2+ (TAP = 1,4,5,8-tetraazaphenanthrene, HAT= 1,4,5,8,9,12-hexaazatriphenylene) by hydroquinone (H2Q), N-acetyl-tyrosine (N-Ac-Tyr) or guanosine-5'- monophosphate (GMP) was investigated at various pH values. The quenching occurs via electron/proton transfer, as evidenced by transient absorption spectroscopy and confirmed by 1H photochemically induced dynamic nuclear polarization (photo-CIDNP). Reductive quenching also occurs in strongly acidic solution despite a much shorter lifetime of the protonated excited-state complex. Photo-CIDNP revealed a different mechanism at low pH, involving protonation before electron transfer and yielding a distinct protonated monoreduced complex. The experimental photo-CIDNP patterns are consistent with density functional theory calculations. This work highlights the power of 1H photo-CIDNP for characterizing, at the atomic level, transient species involved in electron-transfer processes. [ABSTRACT FROM AUTHOR]

Since the discovery of molecular chirality, nonsuperimposable mirror-image organic molecules have been found to be essential across biological and chemical processes and increasingly in materials science. Generally, carbon centers containing four different substituents are configurationally stable, unless bonds to the stereogenic carbon atom are broken and re-formed. Herein, we describe sp3-stereogenic carbon-bearing molecules that dynamically isomerize, interconverting between enantiomers without cleavage of a constituent bond, nor through remote functional group migration. The stereodynamic molecules were designed to contain a pair of redox-active substituents, quinone and hydroquinone groups, which allow the enantiomerization to occur via redox-interconversion. In the presence of an enantiopure host, these molecules undergo a deracemization process that allows observation of enantiomerically enriched compounds. This work reveals a fundamentally distinct enantiomerization pathway available to chiral compounds, coupling redox-interconversion to chirality. [ABSTRACT FROM AUTHOR]

The effect of seed loading and batch trajectory on the growth of crystals with two size coordinates is studied using a simple model for two crystallization systems known to exhibit shape evolution: hen egg white lysozyme and hydroquinone. For moderate seed loading, batch trajectories that lead to a high supersaturation near the beginning of the batch result in a larger nucleated mass and less growth of the seed crystals than other alternatives, while trajectories that keep the supersaturation almost constant give good performance. Increasing seed mass and decreasing seed size also improves performance. Critical seed loading analysis shows that the two-dimensional systems considered in this work behave similarly to one-dimensional crystallization systems considered previously. [ABSTRACT FROM AUTHOR]

Carbonaceous materials are commonly used in agronomic and environmental applications primarily as geosorbents, but their redox properties that may affect biogeochemical reactions are rarely documented. Herein, the role of activated carbon (AC) mediating microbial reduction of ferrihydrite is studied. Our batch experiment results show that AC facilitated the reduction of ferrihydrite by Shewanella oneidensis MR-1, but the pretreatment of AC with HNO3 further increased the rate of reduction. The redox-active oxygen-containing functional groups in AC were found to be responsible for the enhancement of the microbial reduction of ferrihydrite. This conclusion was supported by the electrochemical evidence that showed that the electron exchange capacity (EEC) of AC was facilitated due to the presence of quinone/hydroquinone groups and strongly positively correlated with the content of CO groups. Moreover, the coprecipitation of vivianite and siderite was found in the products in the presence of AC, but siderite only was present in the absence of AC. The proper identification of potential functional groups in AC-mediating electron transfer during microbial reduction of ferrihydrite provides insights into the mechanism of reaction and potential roles carbonaceous materials may play in biogeochemical redox processes and, consequently, the fate of contaminants in the environment. [ABSTRACT FROM AUTHOR]

Flavin has long been known to function as a single electron reductant in biological settings, but this reactivity has rarely been observed with flavoproteins used in organic synthesis. Here we describe the discovery of an enantioselective radical dehalogenation pathway for α-bromoesters using flavin-dependent ‘ene’-reductases. Mechanistic experiments support the role of flavin hydroquinone as a single electron reductant, flavin semiquinone as the hydrogen atom source, and the enzyme as the source of chirality. [ABSTRACT FROM AUTHOR]

Hydroquinones are important mediators of electron transfer reactions in soils with a capability to reduce Fe(III) minerals and molecular oxygen, and thereby generating Fenton chemistry reagents. This study focused on 2,6-dimethoxy hydroquinone (2,6-DMHQ), an analogue to a common fungal metabolite, and its reaction with ferrihydrite and goethite under variable pH and oxygen concentrations. Combined wet-chemical and spectroscopic analyses showed that both minerals effectively oxidized 2,6-DMHQ in the presence of oxygen. Under anaerobic conditions the first-order oxidation rate constants decreased by one to several orders of magnitude depending on pH and mineral. Comparison between aerobic and anaerobic results showed that ferrihydrite promoted 2,6-DMHQ oxidation both via reductive dissolution and heterogeneous catalysis while goethite mainly caused catalytic oxidation. These results were in agreement with changes in the reduction potential (EH) of the Fe(III) oxide/Fe(II)aq redox couple as a function of dissolved Fe(II) where EH of goethite was lower than ferrihydrite at any given Fe(II) concentration, which makes ferrihydrite more prone to reductive dissolution by the 2,6-DMBQ/2,6-DMHQ redox couple. This study showed that reactions between hydroquinones and iron oxides could produce favorable conditions for formation of reactive oxygen species, which are required for nonenzymatic Fenton-based decomposition of soil organic matter. [ABSTRACT FROM AUTHOR]

Hydroquinone (HQ) is known to form organic clathrates with different gaseous species over a wide range of pressures and temperatures. However, the enclathration reaction involving HQ is not fully understood. This work offers new elements of understanding HQ clathrate formation and dissociation mechanisms. The kinetics and selectivity of the enclathration reaction were also investigated. The focus was placed on HQ clathrates formed with CO2 and CH4 as guest molecules for potential use in practical applications for the separation of a CO2/CH4 gas mixture. The structural transition from the native form (α-HQ) to the clathrate form (β-HQ), as well as the reverse process, were tracked using in situ Raman spectroscopy. The clathrate formation was conducted at 323 K and 3.0 MPa, and the dissociation was conducted at 343 K and 1.0 kPa. The experiments with CH4 confirmed that a small amount of gas can fill the α-HQ before the phase transition from α- to β-HQ begins. The dissociation of the CO2-HQ clathrates highlighted the presence of a clathrate structure with no guest molecules. We can therefore conclude that HQ clathrate formation and dissociation are two-step reactions that pass through two distinct reaction intermediates: guest-loaded α-HQ and guest-free β-HQ. When an equimolar CO2/CH4 gas mixture is put in contact with either the α-HQ or the guest-free β-HQ, the CO2 is preferentially captured. Moreover, the guest-free β-HQ can retain the CO2 quicker and more selectively. [ABSTRACT FROM AUTHOR]

The formation and guest enclathration of hydroquinone (HQ) clathrates with gas mixtures were investigated for potential applications to a clathrate-based removal process for sulfides. The HQ clathrate samples were prepared with hydrogen sulfide (H2S)-carbonyl sulfide (COS)-nitrogen (N2) gas mixtures at pressures from 2.0 to 8.0 MPa and ambient temperature. Solid-state 13C NMR and Raman spectroscopy methods were used to identify the crystal structure of the samples and guest enclathration into the clathrate framework. Quantitative information regarding the amount of captured gases was obtained by a combination of the elemental analysis and the numerical integration of the NMR spectra. The results show that H2S can be more concentrated in the solid HQ clathrates than in the gas phase, depending on the formation pressure. In particular, COS did not participate in the enclathration under all conditions, indicating that the molecular size of COS is too large to fit into the cages of the HQ clathrate. The recovery efficiency of H2S was described in terms of the enhancement of the concentration and selectivity of H2S in the HQ clathrates. These results open up the potential for clathrate-based gas separation for sulfur compounds, especially under dilute conditions. [ABSTRACT FROM AUTHOR]

The energy landscape and its evolution with pressure have been studied for two hydroquinone clathrates in combination with the apohost β-hydroquinone structure. Pressure is used as a means for probing interatomic, or intermolecular, potentials. The forces in organic clathrates are generally dominated by strong intermolecular interactions in a host framework, with smaller contributions from subtle interactions between the guest molecules and the surrounding framework. Pairwise intermolecular interactions energies have been quantified for the hydroquinone-methanol and hydroquinone-acetonitrile clathrate up to pressures of 8.6(2) and 14.1(4) GPa, respectively. In both cases, reversible pressure induced phase transitions are observed, where the host framework tilts to form skewed cavity channels and break the 3-fold rotation symmetry. The compression of the hydroquinone-acetonitrile structure is found to be isotropic, whereas it is anisotropic for the hydroquinone-methanol compound, emphasizing the implication of different guest molecules. Host-host interactions especially through the O-H---O bonding network have an order of magnitude larger contribution to the lattice energy than the host-guest interactions, but these host-host interactions become increasingly less favorable with pressure ultimately leading the phase transitions. When small pressure is applied to the β-hydroquinone apohost, it transforms to the denser α-hydroquinone structure. However, when guest atoms are enclathrated, a templating effect is observed leading to new and different high-pressure host structures. Thus, even though the host-guest interactions are comparably modest in magnitude, the guest atoms direct the high-pressure structural rearrangements. The present study illustrates the complexity of the numerous superimposed interactions in even the simplest supramolecular aggregates, highlighting the need to reliably quantify intermolecular interaction energies before aspiring to predict the formation of more complex supramolecular structures. [ABSTRACT FROM AUTHOR]

The chemical confinement of a pH sensitive fluorophore to a thin-reaction layer adjacent to an electrode surface is explored as a potentially innovative route to improving the spatial resolution of fluorescence electrochemical microscopy. A thin layer opto-electrochemical cell is designed, facilitating the visualization of a carbon fiber (diameter 7.0 µm) electrochemical interface. Proton consumption is driven at the interface by the reduction of benzoquinone to hydroquinone and the resulting interfacial pH change is revealed using the fluorophore 8-hydoxypyrene-1,3,6-trisulfonic acid. It is demonstrated that the proton depletion zone may be constrained and controlled by the addition of a finite acid concentration to the system. Simulation of the resulting fluorescence intensity profiles is achieved on the basis of a finite difference model, with excellent agreement between the theoretical and experimental results. [ABSTRACT FROM AUTHOR]

A highly efficient N-trifluoromethylation of nitrosoarenes is reported. The inexpensive and convenient Langlois reagent (sodium triflinate) is employed as a CF3-radical source in combination with a copper catalyst and an oxidant. N-Trifluoromethylated hydroxylamines are obtained in high yields within 1 h at room temperature. The addition of hydroquinone was found to be instrumental to prevent the formation of side products. The method is high-yielding, is scalable, and displays a high functional group tolerance. [ABSTRACT FROM AUTHOR]

Typical poly-p-phenylene wires are characterized by strong interchromophoric electronic coupling with redox and optical properties being highly length-dependent. Herein we show that an incorporation of a pair of para-methoxy groups at each p-phenylene unit in poly-p-phenylene wires (i.e., PHEn) changes the nodal structure of HOMO that leads to length-invariant oxidation potentials and cation radical excitation energies. As such, PHEn represents a unique class of isoenergetic wires where hole delocalization mainly occurs via dynamic hopping and thus may serve as an efficient medium for long-range charge transfer. Availability of these wires will allow demonstration of long-range electron transfer via incoherent hopping using donor-bridge-acceptor systems with isoenergetic PHEn-based wires as bridges. [ABSTRACT FROM AUTHOR]

1,4-Hydroquinone and several other phenolic compounds generate (M - 2) radical-anions, rather than deprotonated molecules, under certain negative-ion mass spectrometric conditions. In fact, spectra generated under helium-plasma ionization (HePI) conditions from 1,4-hydroquinone and 1,4-benzoquinone (by electron capture) were practically indistinguishable. Because this process involves a net loss of H and H, it can be termed oxidative ionization. The superoxide radical-anion (O ), known to be present in many atmospheric-pressure plasma ion sources operated in the negative mode, plays a critical role in the oxidative ionization process. The presence of a small peak at m/z 142 in the spectrum of 1,4-hydroquinone, but not in that of 1,4-benzoquinone, indicated that the initial step in the oxidative ionization process is the formation of an O adduct. On the other hand, under bona fide electrospray ionization (ESI) conditions, 1,4-hydroquinone generates predominantly an (M - 1) ion. It is known that at sufficiently high capillary voltages, corona discharges begin to occur even in an ESI source. At lower ESI capillary voltages, deprotonation predominates; as the capillary voltage is raised, the abundance of O present in the plasma increases, and the source in turn increasingly behaves as a composite ESI/APCI source. While maintaining post-ionization ion activation to a minimum (to prevent fragmentation), and monitoring the relative intensities of the m/z 109 (due to deprotonation) and 108 (oxidative ionization) peaks recorded from 1,4-hydroquinone, a semiquantitative estimation of the APCI contribution to the overall ion-generation process can be obtained. [ABSTRACT FROM AUTHOR]

Little information is available on the roles of natural phenolic compounds in polycyclic aromatic hydrocarbons (PAHs) attenuation at dry soil-air interfaces. The purpose of this study was to determine the roles of model phenolic constituents of soil organic matter (SOM) on the abiotic attenuation of PAHs. The phenolic compounds can significantly change the attenuation rates of PAHs, among which hydroquinone was the most effective in promoting anthracene and benzo[ a ]anthracene attenuation. Product identification and sequential extraction experiments revealed hydroquinone enhanced the formation of oxidative coupling products and promoted the incorporation of PAHs into humic analogues, thereby reducing potential risks to humans and ecosystems. Electron paramagnetic resonance spectroscopy analyses showed both PAHs and phenolic compounds could donate electrons to Lewis acid sites of soil minerals, resulting in the generation of persistent free radicals (PFRs). PFRs could promote the generation of ·OH to enhance PAH oxidation and could cross-couple with PAHs, resulting in high-molecular-weight oxidative coupling products. This study revealed for the first time the reaction mechanism between PAHs and phenolic components of SOM under relatively dry conditions and provided new insights into promoting PAHs detoxification in soils but also a potential strategy to increase the organic carbon sequestration.

The vapor pressure (p sat) of methyl oleate was measured with and without the addition of 0.2 mass % of the antioxidant stabilizer tert-butylhydroquinone (TBHQ). The measurements were made by the gas saturation method with a temperature range of 303.15-343.15 K. In the absence of TBHQ, oxidative decomposition severely compromised the measurements, as evidenced by dramatic decreases in the measured p sat for repeat measurements at 323.15 K. When combined with a room-temperature N2 flush of the apparatus, the addition of 0.2 mass % TBHQ limited the decomposition to insignificant levels and resulted in repeatable measurements of p sat. Simultaneous measurements on the control sample n-eicosane (C20H42) yielded values of p sat that were in excellent agreement with reference correlations. [ABSTRACT FROM AUTHOR]

We report eight new cocrystals with an analgesic drug, propyphenazone, which belongs to a family of compounds that possess limited or no hydrogen bonding functionality. Such molecules present difficulties in predicting and selecting appropriate coformers for potential multicomponent systems, and current prediction methodologies are unsuitable due to their focus around hydrogen bonding. This study used a knowledge-based strategy to identify appropriate coformers as well as testing a wide variety of molecules to confirm the initial predictions. All new cocrystals were characterized using X-ray diffraction techniques as well as measuring their physicochemical properties. These included the thermal behavior, stability under slurry and accelerated conditions, and solubility and dissolution rate and were compared with the parent propyphenazone. It was found that while the stability of most of the cocrystals and propyphenazone were comparable, a cocrystal with hydroquinone showed an increase in both the solubility and dissolution rate of propyphenazone. Propyphenazone possesses limited or no hydrogen bonding functionality and, along with other similar molecules, presents difficulties in selection of coformers for cocrystal design. A knowledge-based strategy was used to identify appropriate coformers, and eight new cocrystals were discovered. All the cocrystals showed comparable stability to propyphenazone, and a cocrystal with hydroquinone improved the solubility and dissolution rate of propyphenazone. [ABSTRACT FROM AUTHOR]

This article demonstrates how a student research project could RESEARCH PROBLEMS connect classroom and community. Using local citizens' concerns about the adulteration of cosmetics by prohibited substances as a research problem, fifthyear pharmaceutical chemistry students were challenged to use their knowledge to create cost-effective and environmentally friendly chromatographic devices from easily obtainable materials. After validation of the procedure, the devices were utilized for screening skin-whitening and antiwrinkle cosmetic samples submitted to the faculty through the community service program for the screening of hydroquinone and retinoic acid. Coactivities such as delivery of health education to the community and program evaluation were also conducted by students to promote their community engagement and work experience in a real-world context. Furthermore, the knowledge gained from the project was passed on to younger students to illustrate in lectures and stimulate their interest in chemistry. The project was wellreceived and considered useful by the community since it addressed the real needs of the people, thereby helping to cross the line between the academic world and the real world and foster relationships between the university and the community. [ABSTRACT FROM AUTHOR]

UV-visible spectroscopic studies of aqueous hydroquinone (HQ) and 1,4-benzoquinone (BQ) have been carried out along with classical molecular dynamics (MD) and quantum calculations. The experimental results confirmed that HQ is stable in hot compressed water up to at least 523 K at 70 bar, but BQ decomposes at temperatures lower than 373 K, leading to the formation of HQ and other nonabsorbing products. Even though benzoquinone is not stable, our study significantly extended the temperature range of other spectroscopic studies, and the spectra of HQ up to 523 K can still be useful for other studies, particularly those related to organic species in deep ocean hydrothermal vents. Classical MD simulations at high temperatures show, as expected, a weakening of the solute-solvent H-bonding interactions. The dependence of the maximum absorption of BQ on temperature was also analyzed, although a significant degree of decomposition was observed in the time frame of our experiments. The shift of the maximum absorption peak of BQ with temperature was consistent with time-dependent density functional theory calculations. [ABSTRACT FROM AUTHOR]

We report herein a systematic investigation on the effect of the size of silver (Ag) nanoparticles employed as starting materials over the morphological features and catalytic performances of AgPt nanoshells produced by a combination of galvanic replacement between Ag and PtCl62- and PtCl62- reduction by hydroquinone. More specifically, we focused on Ag nanoparticles of four different sizes as starting materials, and found that the outer diameter, shell thickness, and the number of Pt surface atoms of the produced nanoshells increased with the size of the starting Ag nanoparticles. The produced AgPt nanoshells were supported into SiO2, and the catalytic performances of the AgPt/SiO2 nanocatalysts toward the gas-phase oxidation of benzene, toluene, and o-xylene (BTX oxidation) followed the order: AgPt 163 nm/SiO2 > AgPt 133 nm/SiO2 > AgPt 105 nm/SiO2 > AgPt 95 nm/SiO2. Interestingly, bigger AgPt nanoshell sizes lead to better catalytic performances in contrast to the intuitive prediction that particles having larger outer diameters tend to present poorer catalytic activities due to their lower surface to volume ratios as compared to smaller particles. This is in agreement with the H2 chemisorption results, and can be assigned to the increase in the Pt surface area with size due to the presence of smaller NPs islands at the surface of the nanoshells having larger outer diameters. This result indicates that, in addition to the overall diameters, the optimization of the surface morphology may play an important role over the optimization of catalytic activities in metal-based nanocatalysts, which can be even more pronounced that the size effect. Our data demonstrate that the control over surface morphology play a very important role relative to the effect of size to the optimization of catalytic performances in catalysts based on noble-metal nanostructures. [ABSTRACT FROM AUTHOR]

Organic clathrate compounds, particularly those formed between hydroquinone (HQ) and gases, are supramolecular entities recently highlighted as promising alternatives for applications such as gas storage and separation processes. This study provides new insights into CO2-HQ clathrate, which is a key structure in some of the proposed future applications of these compounds. We present a novel synthesis and purification of CO2-HQ clathrate monocrystals. Clathrate crystals obtained from a single synthesis and native HQ are characterized and compared using Raman/Fourier transform infrared/NMR spectroscopies, optical microscopy, and thermogravimetric analysis coupled to mass spectrometry. The molecular structure of the clathrate has been resolved by X-ray diffraction analysis, and detailed crystallographic information is presented for the first time. [ABSTRACT FROM AUTHOR]

The article presents the results of spectroscopic studies of 4-(5-methyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C1) and 4-(5-heptyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C7) in organic solvent solutions. Depending on the concentration of the compound used, three bands were observed in the fluorescence emission spectra of the compounds in DMSO solutions. A single band was observed in methanol, propan-2-ol, or ethanol. The significantly shortened fluorescence lifetimes and the different shapes of circular dichroism (CD) spectra clearly indicate association of the fluorescence effects with the aggregation processes in the analyzed compounds. The differences in the course of the CD spectra also imply an effect of the substituent group structure on the molecule aggregation interactions. Therefore, it has been postulated that the occurrence of the different spectral forms induced by changes in the compound concentration may be related to the aggregation effects of C1 and C7 molecules, which are also induced by differences in the alkyl substituent structure. [ABSTRACT FROM AUTHOR]

Template-directed protocols provide a routine approach to the synthesis of mechanically interlocked molecules (MIMs), in which the mechanical bonds are stabilized by a wide variety of weak interactions. In this Article, we describe a strategy for the preparation of neutral [2]catenanes with sliding interlocked electron-rich rings, starting from two degenerate donor-acceptor [2]catenanes, consisting of a tetracationic cyclobis(paraquat-p-phenylene) cyclophane (CBPQT4+) and crown ethers containing either (i) hydroquinone (HQ) or (ii) 1,5-dioxynaphthalene (DNP) recognition units and carrying out four-electron reductions of the cyclophane components to their neutral forms. The donor-acceptor interactions between the CBPQT4+ ring and both HQ and DNP units present in the crown ethers that stabilize the [2]catenanes are weakened upon reduction of the cyclophane components to their radical cationic states and are all but absent in their fully reduced states. Characterization in solution performed by UV-vis, EPR, and NMR spectroscopic probes reveals that changes in the redox properties of the [2]catenanes result in a substantial decrease of the energy barriers for the circumrotation and pirouetting motions of the interlocked rings, which glide freely through one another in the neutral states. The solid-state structures of the fully reduced catenanes reveal profound changes in the relative dispositions of the interlocked rings, with the glycol chains of the crown ethers residing in the cavities of the neutral CBPQT0 rings. Quantum mechanical investigations of the energy levels associated with the four different oxidation states of the catenanes support this interpretation. Catenanes and rotaxanes with sliding rings are expected to display unique properties. [ABSTRACT FROM AUTHOR]

Hydroquinone (HQ) forms organic clathrates in the presence of various gas molecules in specific thermodynamic conditions. For some systems, clathrate phase equilibrium and occupancy data are very scarce or inexistent in literature to date. This work presents experimental results obtained for the CO2-HQ, CH4-HQ, and N2-HQ clathrates, in an extended range of temperature from about 288 to 354 K. Formation/dissociation pressures, and occupancies at the equilibrium clathrate forming conditions, were determined for these systems. Experiments showing the influence of the crystallization solvent, and the effect of the gas pressure on HQ solubility, were also presented and discussed. A good agreement is obtained between our experimental results and the already published experimental and modeling data. Our results show a clear dependency of the clathrate occupancy with temperature. The equilibrium curves obtained for CO2-HQ and CH4-HQ clathrates were found to be very close to each other. The results presented in this study, obtained in a relatively large temperature range, are new and important to the field of organic clathrates with potential impact on gas separation, energy storage, and transport. [ABSTRACT FROM AUTHOR]

Gold nanostars have unique plasmonic properties that are related to the highly branched nanostructures. However, it is challenging to precisely control these branches. Here we studied the reaction kinetics on the seed-mediated growth process of gold nanostars using in situ UV-vis spectroscopy. The impact of hydroquinone ligands on the formation and evolution of active intermediates was systematically explored. In addition, we improved the classical seed-mediated method to achieve a much better control on the final morphology of gold nanostars by a sudden addition of a high concentration ligand solution. Our method can significantly advance the syntheses of gold nanostars and provide numerous opportunities to prepare nanomaterials with unique morphology and plasmonic properties. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and fullerene complexation property of novel and functionalized On-corona[n]arenes were reported. Based on the fragment coupling strategy, ester-containing On-corona[n]arenes (n = 6, 8) were obtained readily starting from 1,4-hydroquinone and diethyl 2,5-difluoroterephthalate. Reduction of esters with LiAlH4 produced almost quantitatively hydroxymethylated On-corona[n]arenes, which underwent etherification with MeI to afford methoxymethyl-substituted On-corona[n]arenes (n = 6, 8) in good yields. The macrocycles adopt unique corona-type conformation with a large cylindroid cavity. They are strong macrocyclic host molecules to form 1:1 complexes with fullerenes C60 and C70 in toluene with an associate constant up to (1.59 ± 0.04) × 105 M-1. [ABSTRACT FROM AUTHOR]

Lewis acid catalyzed Friedel-Crafts alkylation of indoles has been achieved in high yields and selectivities using p-quinols as electrophiles. (S)-Binol-3,3'-(9-anthracenyl)-phosphoric acid was able to catalyze the enantioselective formation of 5-(3-indole)-2-cyclohexenone derivatives. Experimental results and theoretical calculations explained the enantioselectivity based on a transition state where two water molecules act as a tether joining the p-quinol with the phosphoric acid and the NH of indole, thus facilitating the desymmetrization of the prochiral cyclohexadienone framework. The toc/abstract graphic was corrected on April 19, 2016. On May 6, 2016, a missing assignment of the configuration of the catalyst 12h and a mistake in the configuration of compounds in the Supporting Information were corrected. [ABSTRACT FROM AUTHOR]

Palladium(II) carboxylate salts have been shown to catalyze the oxidation of various hydroquinones to benzoquinones in the presence of t-BuOOH. This new catalytic system has been integrated into the oxidative 1,4-functionalization of cyclic 1,3-dienes where the palladium plays a remarkable dual role, catalyzing both the diene oxidation itself and the regeneration of the active quinone oxidant, which is required for diene functionalization. These new conditions offer considerable increases in reaction rate over prior art and allow a significant decrease in the equivalents of the nucleophilic carboxylate required for full conversion. [ABSTRACT FROM AUTHOR]

A Lewis acid catalyzed Nazarov-type cyclization of arylvinylcarbinol has been developed for the asymmetric synthesis of carbotetracyclic core of merosesquiterpenes. The reaction works only in the presence of 2 mol % of Sn(OTf)2 and Bi(OTf)3 in dichloroethane under elevated temperature. The methodology offers the synthesis of a variety of enantioenriched arylvinylcarbinols from commercially available (3aR)-sclareolide 9 in six steps with an eventual concise total synthesis of marine sesquiterpene quinol, akaol A (1a). [ABSTRACT FROM AUTHOR]

Liquid-liquid equilibria (LLE) data of the quaternary system {methyl butyl ketone + phenol + hydroquinone + water} and two relevant ternary systems, {methyl butyl ketone + phenol + water} and {methyl butyl ketone + hydroquinone + water}, were measured at T = 313.2 K and p = 101.3 kPa. The experimental data were correlated with the nonrandom two-liquid (NRTL) and universal quasichemical (UNIQUAC) models. The root-mean-square deviation of the NRTL and UNIQUAC models were 0.144 and 0.0476%, respectively. Good agreement was found between experimental LLE data and those calculated from the NRTL or UNIQUAC equations for all systems studied in this work. [ABSTRACT FROM AUTHOR]

Macrocyclic metal complexes and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions. In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a CoII(salophen) complex, a CoIII-superoxide adduct, and a hydrogen-bonded adduct between the hydroquinone and the CoIII-O2 species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a CoIII-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H2O2 intermediate oxidizes a second equivalent of hydroquinone. Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O2 reduction and generate the reactive p-benzoquinone co-catalyst. [ABSTRACT FROM AUTHOR]

Two metabolites from the ascomycete fungus Septofusidium berolinense were recently identified as having antineoplastic activity [Ekiz et al. (2015) J. Antibiot., DOI: 10.1038/ja.2015.84]. However, the basis for this activity is not known. One of the compounds [3,6-dihydroxy-2-propylbenzaldehyde (GE-1)] is a hydroquinone, and the other [2-hydroxymethyl-3-propylcyclohexa-2,5-diene-1,4-dione (GE-2)] is a quinone. Because some hydroquinones and quinones act as topoisomerase II poisons, the effects of GE-1 and GE-2 on DNA cleavage mediated by human topoisomerase IIα were assessed. GE-2 enhanced DNA cleavage-4-fold and induced scission with a site specificity similar to that of the anticancer drug etoposide. Similar to other quinone-based topoisomerase II poisons, GE-2 displayed several hallmark characteristics of covalent topoisomerase II poisons, including (1) the inability to poison a topoisomerase IIα construct that lacks the N-terminal domain, (2) the inhibition of DNA cleavage when the compound was incubated with the enzyme prior to the addition of plasmid, and (3) the loss of poisoning activity in the presence of a reducing agent. In contrast to GE-2, GE-1 did not enhance DNA cleavage mediated by topoisomerase IIα except at very high concentrations. However, the activity and potency of the metabolite were dramatically enhanced under oxidizing conditions. These results suggest that topoisomerase IIα may play a role in mediating the cytotoxic effects of these fungal metabolites. [ABSTRACT FROM AUTHOR]

Two disparate polyketide families, the benzenediol lactones and the azaphilones, are produced by fungi using iterative polyketide synthase (iPKS) enzymes consisting of collaborating partner subunits. Exploitation of this common biosynthetic logic using iPKS subunit shuffling allowed the diversity-oriented combinatorial biosynthesis of unprecedented polyketide scaffolds new to nature, bearing structural motifs from both of these orthogonal natural product families. Starter unit acyltransferase domain replacements proved necessary but not sufficient to guarantee communication between iPKS subunits. [ABSTRACT FROM AUTHOR]

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni0 and Pd0-quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M(OAc)2 precursors. O2 does not react with the Pd0-quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd0-hydroquinone complex (4-H) was prepared using a one-pot metalation with PdII followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η²-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a AgI center incapable of O2 binding (7-H) or a cationic Pd-H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O2, at higher temperatures, by H+/e- transfers. [ABSTRACT FROM AUTHOR]

Despite the biogeochemical significance of the interactions between natural organic matter (NOM) and iron species, considerable uncertainty still remains as to the exact processes contributing to the rates and extents of complexation and redox reactions between these important and complex environmental components. Investigations on the reactivity of low-molecular-weight quinones, which are believed to be key redox active compounds within NOM, toward iron species, could provide considerable insight into the kinetics and mechanisms of reactions involving NOM and iron. In this study, the oxidation of 2-methoxyhydroquinone (MH2Q) by ferric iron (Fe(III)) under dark conditions in the absence and presence of oxygen was investigated within a pH range of 4-6. Although Fe(III) was capable of stoichiometrically oxidizing MH2Q under anaerobic conditions, catalytic oxidation of MH2Q was observed in the presence of O2 due to further cycling between oxygen, semiquinone radicals, and iron species. A detailed kinetic model was developed to describe the predominant mechanisms, which indicated that both the undissociated and monodissociated anions of MH2Q were kinetically active species toward Fe(III) reduction, with the monodissociated anion being the key species accounting for the pH dependence of the oxidation. The generated radical intermediates, namely semiquinone and superoxide, are of great importance in reaction-chain propagation. The kinetic model may provide critical insight into the underlying mechanisms of the thermodynamic and kinetic characteristics of metal-organic interactions and assist in understanding and predicting the factors controlling iron and organic matter transformation and bioavailability in aquatic systems. [ABSTRACT FROM AUTHOR]

No single linear free energy relationship (LFER) exists that can predict reduction rate constants of all munition constituents (MCs). To address this knowledge gap, we measured the reduction rates of MCs and their surrogates including nitroaromatics [NACs; 2,4,6-trinitrotoluene (TNT), 2,4-dinitroanisole (DNAN), 2-amino-4,6-dinitrotoluene (2-A-DNT), 4-amino-2,6-dinitrotoluene (4-A-DNT), and 2,4-dinitrotoluene (DNT)], nitramines [hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and nitroguanidine (NQ)], and azoles [3-nitro-1,2,4-triazol-5-one (NTO) and 3,4-dinitropyrazole (DNP)] by three dithionite-reduced quinones (lawsone, AQDS, and AQS). All MCs/NACs were reduced by the hydroquinones except NQ. Hydroquinone and MC speciations were varied by controlling pH, permitting the application of a speciation model to determine second-order rate constants ( k ) from observed pseudo-first-order rate constants. The intrinsic reactivity of MCs (oxidants) decreased upon deprotonation, while the opposite was true for hydroquinones (reductants). The rate constants spanned ∼6 orders of magnitude in the order NTO ≈ TNT > DNP > DNT ≈ DNAN ≈ 2-A-DNT > DNP - > 4-A-DNT > NTO - > RDX. LFERs developed using density functional theory-calculated electron transfer and hydrogen atom transfer energies and reported one-electron reduction potentials successfully predicted k , suggesting that these structurally diverse MCs/NACs are all reduced by hydroquinones through the same mechanism and rate-limiting step. These results increase the applicability of LFER models for predicting the fate and half-lives of MCs and related nitro compounds in reducing environments.

Interactions of 1,4-hydroquinone with soluble iron species over a pH range of 3-5 in the air-saturated and partially deoxygenated solution are examined here. Our results show that 1,4-hydroquinone reduces Fe(III) in acidic conditions, generating semiquinone radicals (Q•-) that can oxidize Fe(II) back to Fe(III). The oxidation rate of Fe(II) by Q•-increases with increase in pH due to the speciation change of Q•- with its deprotonated form (Q•-) oxidizing Fe(II) more rapidly than the protonated form (HQ•). Although the oxygenation of Fe(II) is negligible at pH < 5, O2 still plays an important role in iron redox transformation by rapidly oxidizing Q•- to form benzoquinone (Q). A kinetic model is developed to describe the transformation of quinone and iron under all experimental conditions. The results obtained here are compared with those obtained in our previous studies of iron-Suwannee River fulvic acid (SRFA) interactions in acidic solutions and support the hypothesis that hydroquinone moieties can reduce Fe(III) in natural waters. However, the semiquinone radicals generated in pure hydroquinone solution are rapidly oxidized by dioxygen, while the semiquinone radicals generated in SRFA solution are resistant to oxidation by dioxygen, with the result that steady-state semiquinone concentrations in SRFA solutions are 2-3 orders of magnitude greater than in solutions of 1,4-hydroquinone. As a result, semiquinone moieties in SRFA play a much more important role in iron redox transformations than is the case in solutions of simple quinones such as 1,4-hydroquinone. This difference in the steady-state concentration of semiquinone species has a dramatic effect on the cycling of iron between the +II and +III oxidation states, with iron turnover frequencies in solutions containing SRFA being 10-20 times higher than those observed in solutions of 1,4-hydroquinone. [ABSTRACT FROM AUTHOR]