Iron K-edge X-ray Absorption Research Articles

Organic amendments stimulate co-precipitation of ferrihydrite and dissolved organic matter in soils

Ferrihydrite-organic matter (OM) complexes in soil play a vital role in preserving soil organic carbon (C), either through adsorption (reaction of OM with existing ferrihydrite) or co-precipitation (neo-formation of ferrihydrite in the presence of OM). However, there is still limited understanding of the effects of long-term fertilization on the pathways for the formation of ferrihydrite-OM in soil. In this study, the dissolved OM (DOM) extracted from a Ferralic Cambisol under 25-year inorganic nitrogen (N), phosphorus (P) and potassium (K) (NPK) or NPK plus organic pig manure (NPKM) was used to synthesize adsorption complexes and co-precipitates. Compared with the adsorption, the co-precipitation of ferrihydrite and DOM resulted in higher C/Fe ratios in the solid products, particularly under NPKM. Analysis of the iron K-edge X-ray absorption near-edge fine structure (XANES) showed that there was a preference for DOM from soil under NPKM treatment to co-precipitate with ferrihydrite, whereas adsorption of ferrihydrite on DOM was more likely with the long-term NPK fertilized soil. Fourier-transform infrared (FTIR) spectra suggested that a higher intensity of Si-O functional groups, clay minerals, and aromatic groups in the DOM from the NPKM treatment played important roles in the co-precipitation process. Desorption of co-precipitated OM was lower than that of adsorbed OM in soils, indicating a higher chemical stability of OM in the co-precipitates. In summary, our results demonstrate that 25-year long-term NPK and NPKM fertilizations have contrasting effects on the formation pathway of ferrihydrite-OM complexes. Organic amendments can stimulate the co-precipitation of DOM and ferrihydrite in soil, leading to greater biological C preservation than the adsorption process, and a further enhancement of long-term SOC protection.

Interplays of electron and nuclear motions along CO dissociation trajectory in myoglobin revealed by ultrafast X-rays and quantum dynamics calculations

Ultrafast structural dynamics with different spatial and temporal scales were investigated during photodissociation of carbon monoxide (CO) from iron(II)-heme in bovine myoglobin during the first 3 ps following laser excitation. We used simultaneous X-ray transient absorption (XTA) spectroscopy and X-ray transient solution scattering (XSS) at an X-ray free electron laser source with a time resolution of 80 fs. Kinetic traces at different characteristic X-ray energies were collected to give a global picture of the multistep pathway in the photodissociation of CO from heme. In order to extract the reaction coordinates along different directions of the CO departure, XTA data were collected with parallel and perpendicular relative polarizations of the laser pump and X-ray probe pulse to isolate the contributions of electronic spin state transition, bond breaking, and heme macrocycle nuclear relaxation. The time evolution of the iron K-edge X-ray absorption near edge structure (XANES) features along the two major photochemical reaction coordinates, i.e., the iron(II)-CO bond elongation and the heme macrocycle doming relaxation were modeled by time-dependent density functional theory calculations. Combined results from the experiments and computations reveal insight into interplays between the nuclear and electronic structural dynamics along the CO photodissociation trajectory. Time-resolved small-angle X-ray scattering data during the same process are also simultaneously collected, which show that the local CO dissociation causes a protein quake propagating on different spatial and temporal scales. These studies are important for understanding gas transport and protein deligation processes and shed light on the interplay of active site conformational changes and large-scale protein reorganization.

Bifunctional Oxygen Electrodes with Highly Step-Enriched Surface of Fe–Nx Containing Carbonaceous Thin Film

Carbonaceous air cathodes in zinc-air batteries have attracted attention due to their high electron conductivity and low cost; however, the increment in the catalytic activity for the oxygen reduction and evolution in addition to the durability are demanded. Based on a fundamental study about the substantial effect of a three-dimensional active site at a step on a highly-oriented pyrolytic graphite basal plane on the electrode reactions, we attempted to extend the idea to a graphitic carbon fiber surface as a more applied form for the air electrode. Coating of a carbonaceous thin film derived from iron phthalocyanine on finely-etched graphitic carbon fibers produced a highly step-enriched surface demonstrated by the field-emission scanning electron microscope image and Raman spectra with a developed defect peak. The iron K-edge X-ray absorption fine structure showed a step-oriented iron atom coordinated by two nitrogen atoms and a basal plane oriented iron atom coordinated by 4 nitrogen atoms. The step-enrichment enhanced the oxygen reduction and evolution; in particular, the low overpotential phenomenon observed for the fundamental graphite system was reproduced in the carbon fiber paper, both in a half cell and a full cell of the zinc-air battery, which also showed a favorable cycling performance.

Spectroscopic X-ray and Mössbauer Characterization of M6 and M5 Iron(Molybdenum)-Carbonyl Carbide Clusters: High Carbide-Iron Covalency Enhances Local Iron Site Electron Density Despite Cluster Oxidation

The present study employs a suite of spectroscopic techniques to evaluate the electronic and bonding characteristics of the interstitial carbide in a set of iron-carbonyl-carbide clusters, one of which is substituted with a molybdenum atom. The M6C and M5C clusters are the dianions (Et4N)2[Fe6(μ6-C)(μ2-CO)2(CO)14] (1), [K(benzo-18-crown-6)]2[Fe5(μ5-C)(μ2-CO)1(CO)13] (2), and [K(benzo-18-crown-6)]2[Fe5Mo(μ6-C)(μ2-CO)2(CO)15] (3). Because 1 and 2 have the same overall cluster charge (2−) but different numbers of iron sites (1: 6 sites → 2: 5 sites), the metal atoms of 2 are formally oxidized compared to those in 1. Despite this, Mössbauer studies indicate that the iron sites in 2 possess significantly greater electron density (lower spectroscopic oxidation state) compared with those in 1. Iron K-edge X-ray absorption and valence-to-core X-ray emission spectroscopy measurements, paired with density functional theory spectral calculations, revealed the presence of significant metal-to-metal and carbide 2p-based character in the filled valence and low-lying unfilled electronic manifolds. In all of the above experiments, the presence of the molybdenum atom in 3 (Fe5Mo) results in somewhat unremarkable spectroscopic properties that are essentially a “hybrid” of 1 (Fe6) and 2 (Fe5). The overall electronic portrait that emerges illustrates that the central inorganic carbide ligand is essential for distributing charge and maximizing electronic communication throughout the cluster. It is evident that the carbide coordination environment is quite flexible and adaptive: it can drastically modify the covalency of individual Fe–C bonds based on local structural changes and redox manipulation of the clusters. In light of these findings, our data and calculations suggest a potential role for the central carbon atom in FeMoco, which likely performs a similar function in order to maintain cluster integrity through multiple redox and ligand binding events.

Vanadium, sulfur, and iron valences in melt inclusions as a window into magmatic processes: A case study at Nyamuragira volcano, Africa

This study describes microscale sulfur (S), vanadium (V), and iron (Fe) K-edge X-ray absorption near edge structure (µ-XANES) spectroscopy measurements on olivine-hosted melt inclusions (MI) preserved in tephras (1986 and 2006) and lavas (1938 and 1948) erupted from Nyamuragira volcano (D.R. Congo, Africa). The S, V, and Fe spectroscopic data are used to constrain the evolution of oxygen fugacity (fO2) and sulfur speciation for the entrapped melts. Melt inclusions from lavas show evidence of post-entrapment crystallization and were thus reheated prior to µ-XANES analysis. The MI from tephra show no evidence of post-entrapment crystallization and were, therefore, not reheated. Sulfur, V, and Fe µ-XANES results from 1938, 1948, and 2006 eruptive materials are all similar within analytical uncertainty and provide similar average calculated melt fO2’s based on XANES oxybarometry. However, olivine-hosted MI from the 1986 tephras yield significantly different S, V, and Fe XANES spectra when compared to MI from the other eruptions, with disagreement between calculated fO2’s from the three valence state oxybarometers beyond the uncertainty of the calibration models. Their V µ-XANES spectra are also significantly more ordered and yield more reduced average V valence. The S µ-XANES spectra display a significantly more intense low-energy spectral resonance, which indicates differences in Fe-S bonding character, and greater variability in their measured sulfate content. These V and S spectroscopic features are best explained by crystallization of sub-micrometer magnetite and sulfide crystallites within the 1986 inclusions. The sensitivity of XANES spectroscopy to short-range order allows these crystallites to be recognized even though they are not easily detected by imaging analysis. This shows that V and S µ-XANES are potentially highly sensitive tools for identifying the presence of volumetrically minor amounts of spinel and sulfide within inclusions extracted from rapidly-cooled samples of tephra. Additionally, the observation that rehomogenized 1938 and 1948 inclusions from lava yield similar S, V, and Fe XANES spectra to the 2006 inclusions from tephra may be an encouraging indication that rehomogenization appears to have enabled the successful recovery of their pre-eruptive fO2, despite their complex post-eruptive histories.

Decreased Electron Transfer between Cr(VI) and AH2DS in the Presence of Goethite.

9,10-Anthraquinone-2,6-disulfonic acid (AQDS) is commonly used as a model species to examine the influence of quinones on different biogeochemical cycles. The reduced form of this quinone, AHDS, can donate electrons to the toxic metal species Cr(VI), leading to the precipitation of less soluble Cr(III) phases. Due to the environmental abundance of Fe(III) (oxyhydr)oxides, such as goethite (α-FeOOH), it is important to study the role of these mineral phases on the electron transfer reaction between AHDS and Cr(VI). In this study, this electron transfer reaction is examined in the presence and absence of goethite at three different ratios of AHDS/Cr(VI). Ultraviolet-visible spectroscopy is used to qualitatively assess the oxidation state of AQDS during reactions with goethite. Iron K-edge and Cr K-edge X-ray absorption spectroscopy are used to examine the role of goethite in electron transfer and identify Cr(III) phases that form. Goethite inhibits the extent of Cr(VI) reduction to Cr(III), most notably at the highest ratio of AHDS/Cr(VI) investigated. Production of semiquinone radical species may limit electron transfer and decrease the yields of Fe(II) and Cr(III), both in the presence and absence of goethite. Understanding abiotic electron transfer reactions that occur in systems with multiple redox active species is important to determine the contribution of abiotic redox reactions to Fe biogeochemical cycling in natural soils.

Preservation of organic matter in marine sediments by inner-sphere interactions with reactive iron

Interactions between organic matter and mineral matrices are critical to the preservation of soil and sediment organic matter. In addition to clay minerals, Fe(III) oxides particles have recently been shown to be responsible for the protection and burial of a large fraction of sedimentary organic carbon (OC). Through a combination of synchrotron X-ray techniques and high-resolution images of intact sediment particles, we assessed the mechanism of interaction between OC and iron, as well as the composition of organic matter co-localized with ferric iron. We present scanning transmission x-ray microscopy images at the Fe L3 and C K1 edges showing that the organic matter co-localized with Fe(III) consists primarily of C=C, C=O and C-OH functional groups. Coupling the co-localization results to iron K-edge X-ray absorption spectroscopy fitting results allowed to quantify the relative contribution of OC-complexed Fe to the total sediment iron and reactive iron pools, showing that 25–62% of total reactive iron is directly associated to OC through inner-sphere complexation in coastal sediments, as much as four times more than in low OC deep sea sediments. Direct inner-sphere complexation between OC and iron oxides (Fe-O-C) is responsible for transferring a large quantity of reduced OC to the sedimentary sink, which could otherwise be oxidized back to CO2.

Cu2O-promoted degradation of sulfamethoxazole by α-Fe2O3-catalyzed peroxymonosulfate under circumneutral conditions: synergistic effect, Cu/Fe ratios, and mechanisms

ABSTRACTTo promote the application of iron oxides in sulfate radical-based advanced oxidation processes, a convenient approach using Cu2O as a catalyst additive was proposed. Composite catalysts based on α-Fe2O3 (CTX%Cu2O, X = 1, 2.5, 5, and 10) were prepared for peroxymonosulfate (PMS) activation, and sulfamethoxazole was used as a model pollutant to probe the catalytic reactivity. The results show that a synergistic catalytic effect exists between Cu2O and α-Fe2O3, which was explained by the promoted reduction of Fe(III) by Cu(I). Iron K-edge X-ray absorption spectroscopy investigations indicated that the promoted reduction probably occurred with PMS acting as a ligand that bridges the redox centers of Cu(I) and Fe(III). The weight ratio between Cu2O and α-Fe2O3 influenced the degradation of sulfamethoxazole, and the optimal ratio depended on the dosage of PMS and catalysts. With 40 mg L–1 PMS and 0.6 g L–1 catalyst, a pseudo-first-order constant of ∼0.019 min–1 was achieved for CT2.5%Cu2O, whereas only 0.004 min–1 was realized for α-Fe2O3. Nearly complete degradation of the sulfamethoxazole was achieved within 180 min under the conditions of 40 mg L–1 PMS, 0.4 g L–1 CT2.5%Cu2O, and pH 6.8. In contrast, less than 20% degradation was realized with α-Fe2O3 under similar conditions. The CT2.5%Cu2O catalyst had the best stoichiometric efficiency of PMS (0.317), which was 4.5 and 5.8 times higher than those of Cu2O (0.070) and α-Fe2O3 (0.054), respectively. On the basis of the products identified, the cleavage of the S–N bond was proposed as a major pathway for the degradation of sulfamethoxazole.

Iron K-edge X-ray absorption near-edge structure spectroscopy of aerodynamically levitated silicate melts and glasses

The local structure about Fe(II) and Fe(III) in silicate melts was investigated in-situ using iron K-edge X-ray absorption near-edge structure (XANES) spectroscopy. An aerodynamic levitation and laser heating system was used to allow access to high temperatures without contamination, and was combined with a chamber and gas mixing system to allow the iron oxidation state, Fe3+/ΣFe, to be varied by systematic control of the atmospheric oxygen fugacity. Eleven alkali-free, mostly iron-rich and depolymerized base compositions were chosen for the experiments, including pure oxide FeO, olivines (Fe,Mg)2SiO4, pyroxenes (Fe,Mg)SiO3, calcic FeO-CaSiO3, and a calcium aluminosilicate composition, where total iron content is denoted by FeO for convenience. Melt temperatures varied between 1410 and 2160K and oxygen fugacities between FMQ – 2.3(3) to FMQ+9.1(3) log units (uncertainties in parentheses) relative to the fayalite-magnetite-β-quartz (FMQ) buffer. Remarkably, XANES pre-edge peak areas imply mean Fe-O coordination numbers (nFeO) close to 5 in all cases, with only a slight tendency toward higher values in the most iron rich melts, suggesting an intermediate role for both Fe(II) and Fe(III) in terms of network formation. End member coordination numbers for Fe(II)-O and Fe(III)-O are estimated to be similar, having means (and standard deviations) of 5.0(2) and 4.9(1), respectively. As such, the preference for ferric iron to occupy lower coordination sites than ferrous is weak, in contrast to published behavior in some alkali-rich systems, which may explain the larger published viscosity variations with Fe3+/ΣFe in alkali-, compared to alkaline earth-iron silicates. Temperature effects on nFeO are inferred to be small based on the melt data, as well as by comparison to glasses formed on quenching. Positive shifts of the pre-edge peak centroids observed in many cases on quenching are attributed to rapid oxidation enabled by the stirring of the melt droplets by the levitation gas jet. Fe3+/ΣFe values were estimated from XANES pre-edge peaks using published calibrations and compared to semi-empirical thermodynamic model calculations and Mössbauer measurements on quench products. Whilst showing positive correlation, the comparisons highlight the limitations involved in applying XANES calibrations and models for Fe3+/ΣFe derived from measurements on glasses, to high temperature basic melts. Fe3+/ΣFe varies from approximately zero up to about 65% in the high temperature melts and 75% in the glasses.

Effect of aging on the structure and phosphate retention of Fe(III)-precipitates formed by Fe(II) oxidation in water

Iron(III)-precipitates formed by Fe(II) oxidation in aqueous solutions affect the cycling and impact of Fe and other co-precipitated elements in environmental systems. Fresh Fe(III)-precipitates are metastable and their transformation into more stable phases during aging may result in the release of initially co-precipitated ions. Phosphate, silicate, Mg and Ca play key roles in determining the structure and composition of fresh Fe(III)-precipitates. Here we examine how these ions affect the structure and phosphate retention of Fe(III)-precipitates formed by oxidation of 0.5mM dissolved Fe(II) at pH 7.0 after aging for 30days at 40°C.Iron K-edge X-ray absorption spectroscopy (XAS) shows that aged precipitates consist of the same structural units as fresh precipitates: Amorphous Fe(III)- or Ca-Fe(III)-phosphate, ferrihydrite, and poorly crystalline lepidocrocite. Mg, Ca, and dissolved phosphate stabilize (Ca-)Fe(III)-phosphate against transformation into ferrihydrite. Silicate further attenuates (Ca-)Fe(III)-phosphate transformation. The crystallinity of lepidocrocite formed in phosphate- and silicate-free solutions slightly increases during aging. The transformation of Fe(III)- and Ca-Fe(III)-phosphate into ferrihydrite and ongoing ferrihydrite crystallization during aging result in the release of co-precipitated phosphate. Dissolved Ca on the other hand limits phosphate concentrations to values consistent with solubility control by octacalciumphosphate. Owing to the combined effects of Ca and silicate, phosphate is most effectively retained by Fe(III)-precipitates formed and aged in Ca- and silicate-containing solutions. The results from this study contribute to an improved understanding of the formation and transformation of Fe(III)-precipitates and emphasize that the complexity of Fe(III)-precipitate dynamics in the presence of multiple interfering solutes must be considered when addressing their impact on major and trace elements in environmental systems.

X-ray Absorption and Emission Spectroscopic Studies of [L2Fe2S2](n) Model Complexes: Implications for the Experimental Evaluation of Redox States in Iron-Sulfur Clusters.

Herein, a systematic study of [L2Fe2S2]n model complexes (where L = bis(benzimidazolato) and n = 2-, 3-, 4-) has been carried out using iron and sulfur K-edge X-ray absorption (XAS) and iron Kβ and valence-to-core X-ray emission spectroscopies (XES). These data are used as a test set to evaluate the relative strengths and weaknesses of X-ray core level spectroscopies in assessing redox changes in iron–sulfur clusters. The results are correlated to density functional theory (DFT) calculations of the spectra in order to further support the quantitative information that can be extracted from the experimental data. It is demonstrated that due to canceling effects of covalency and spin state, the information that can be extracted from Fe Kβ XES mainlines is limited. However, a careful analysis of the Fe K-edge XAS data shows that localized valence vs delocalized valence species may be differentiated on the basis of the pre-edge and K-edge energies. These findings are then applied to existing literature Fe K-edge XAS data on the iron protein, P-cluster, and FeMoco sites of nitrogenase. The ability to assess the extent of delocalization in the iron protein vs the P-cluster is highlighted. In addition, possible charge states for FeMoco on the basis of Fe K-edge XAS data are discussed. This study provides an important reference for future X-ray spectroscopic studies of iron–sulfur clusters.

Combined Mössbauer spectroscopic, multi-edge X-ray absorption spectroscopic, and density functional theoretical study of the radical SAM enzyme spore photoproduct lyase.

Spore photoproduct lyase (SPL), a member of the radical S-adenosyl-L-methionine (SAM) superfamily, catalyzes the direct reversal of the spore photoproduct, a thymine dimer specific to bacterial spores, to two thymines. SPL requires SAM and a redox-active [4Fe-4S] cluster for catalysis. Mössbauer analysis of anaerobically purified SPL indicates the presence of a mixture of cluster states with the majority (40%) as [2Fe-2S](2+) clusters and a smaller amount (15%) as [4Fe-4S](2+) clusters. On reduction, the cluster content changes to primarily (60%) [4Fe-4S](+). The speciation information from Mössbauer data allowed us to deconvolute iron and sulfur K-edge X-ray absorption spectra to uncover electronic (X-ray absorption near-edge structure, XANES) and geometric (extended X-ray absorption fine structure, EXAFS) structural features of the Fe-S clusters, and their interactions with SAM. The iron K-edge EXAFS data provide evidence for elongation of a [2Fe-2S] rhomb of the [4Fe-4S] cluster on binding SAM on the basis of an Fe···Fe scatterer at 3.0Å. The XANES spectra of reduced SPL in the absence and presence of SAM overlay one another, indicating that SAM is not undergoing reductive cleavage. The X-ray absorption spectroscopy data for SPL samples and data for model complexes from the literature allowed the deconvolution of contributions from [2Fe-2S] and [4Fe-4S] clusters to the sulfur K-edge XANES spectra. The analysis of pre-edge features revealed electronic changes in the Fe-S clusters as a function of the presence of SAM. The spectroscopic findings were further corroborated by density functional theory calculations that provided insights into structural and electronic perturbations that can be correlated by considering the role of SAM as a catalyst or substrate.

Coupling of arsenic mobility to sulfur transformations during microbial sulfate reduction in the presence and absence of humic acid

Microbial sulfate reduction is an important terminal electron accepting process in arsenic-contaminated subsurface environments. Humic acids are ubiquitous in such environments, yet their impact on arsenic mobility under sulfate-reducing conditions is poorly understood. In this study, we examined the effects of microbial sulfate reduction and humic acid on arsenic mobilization via a series of advective-flow column experiments. The initial solid-phase in these experiments comprised quartz sand that was coated with As(III)-sorbed goethite (α-FeOOH). The effect of humic acid was assessed by comparing columns that received artificial groundwater in which humic acid was either absent or present at 100mgL−1, whilst the effect of microbial sulfate reduction was investigated by comparing columns that were inoculated with the sulfate-reducer Desulfovibrio vulgaris (ATCC strain 7757) versus abiotic control columns. The presence of high concentrations of humic acid alone did not enhance the overall extent of arsenic release from either the abiotic or the inoculated (sulfate reducing) columns. This is consistent with similar arsenic concentrations in porewaters filtered to both <0.45μm and <3kDa, demonstrating that aqueous arsenic did not form mobile colloidal humic acid complexes. In contrast, microbial sulfate reduction was found to mobilize substantial levels of arsenic relative to those observed in the corresponding abiotic control columns. Iron and sulfur K-edge X-ray absorption spectroscopy (XAS) showed that reaction between goethite and microbially-produced sulfide lead to accumulation of mackinawite (FeS) and elemental S. Microbial sulfate reduction also caused important changes in arsenic speciation, especially the formation of aqueous dithioarsenate and monothioarsenate. However, arsenic K-edge XAS showed that arsenic sulfide mineral phases (orpiment and realgar) did not form during the 60day advective-flow experiment. The formation of poorly-sorbing thioarsenate species appeared to contribute to the observed enhancement of arsenic mobilization from the inoculated columns. Dithioarsenate and monothioarsenate were relatively stable, and were found to make up>40% of aqueous arsenic even at very low porewater sulfide concentrations (i.e. <10μmolL−1). Accordingly, the formation, stability and sorption–desorption of thioarsenate species need to be considered when evaluating and predicting subsurface arsenic mobility.

Structure of RPE65 isomerase in a lipidic matrix reveals roles for phospholipids and iron in catalysis

RPE65 is a key metalloenzyme responsible for maintaining visual function in vertebrates. Despite extensive research on this membrane-bound retinoid isomerase, fundamental questions regarding its enzymology remain unanswered. Here, we report the crystal structure of RPE65 in a membrane-like environment. These crystals, obtained from enzymatically active, nondelipidated protein, displayed an unusual packing arrangement wherein RPE65 is embedded in a lipid-detergent sheet. Structural differences between delipidated and nondelipidated RPE65 uncovered key residues involved in substrate uptake and processing. Complementary iron K-edge X-ray absorption spectroscopy data established that RPE65 as isolated contained a divalent iron center and demonstrated the presence of a tightly bound ligand consistent with a coordinated carboxylate group. These results support the hypothesis that the Lewis acidity of iron could be used to promote ester dissociation and generation of a carbocation intermediate required for retinoid isomerization.

Investigation of sulfur poisoning of CNx oxygen reduction catalysts for PEM fuel cells

The role of the transition metal used during the growth of non-noble metal electrochemical oxygen reduction CNx catalysts was investigated through sulfur treatment, a well-known poison for transition metal-based catalysts. The intent of sulfur poisoning was to show the existence of an electrocatalytic active site in CNx that did not depend on iron. The sulfur treatment was shown to be effective on a platinum catalyst, as seen by the decreasing onset potential. The same treatment, however, not only showed no negative effect on the CNx catalyst, but enhanced its performance, as seen by the increase in the onset potential. This suggests that, if there are iron-based active sites in these catalysts, they are either sulfur tolerant or they do not participate in the electrocatalytic oxygen reduction. The deposition of sulfur onto CNx catalyst was verified by temperature-programmed oxidation and X-ray photoelectron spectroscopy. Iron K-edge X-ray absorption near edge structural analysis of the CNx catalyst suggested that the iron phase, which was primarily composed of nanometer-sized metallic particles, was unchanged by sulfur poisoning, suggesting that the residual iron left in these materials is not catalytically accessible.

Simulating Picosecond Iron K-Edge X-ray Absorption Spectra by ab Initio Methods To Study Photoinduced Changes in the Electronic Structure of Fe(II) Spin Crossover Complexes

Recent time-resolved X-ray absorption experiments probing the low-spin to high-spin photoconversion in Fe(II) complexes have monitored the complex interplay between electronic and structural degrees of freedom on an ultrafast time scale. In this study, we use transition potential (TP) and time-dependent (TD) DFT to simulate the picosecond time-resolved iron K-edge X-ray absorption spectrum of the spin crossover (SCO) complex, [Fe(tren(py)(3))](2+). This is achieved by simulating the X-ray absorption spectrum of [Fe(tren(py)(3))](2+) in its low-spin (LS), (1)A(1), ground state and its high-spin (HS), (5)T(2), excited state. These results are compared with the X-ray absorption spectrum of the high-spin analogue (HSA), [Fe(tren(6-Me-py)(3))](2+), which has a (5)T(2) ground state. We show that the TP-DFT methodology can simulate a 40 eV range of the iron K-edge XANES spectrum reproducing all of the major features observed in the static and transient spectra of the LS, HS, and HSA complexes. The pre-edge region of the K-edge spectrum, simulated by TD-DFT, is shown to be highly sensitive to metal-ligand bonding. Changes in the intensity of the pre-edge region are shown to be sensitive to both symmetry and π-backbonding by analysis of relative electric dipole and quadrupole contributions to the transition moments. We generate a spectroscopic map of the iron 3d orbitals from our TD-DFT results and determine ligand field splitting energies of 1.55 and 1.35 eV for the HS and HSA complexes, respectively. We investigate the use of different functionals finding that hybrid functionals (such as PBE0) produce the best results. Finally, we provide a detailed comparison of our results with theoretical methods that have been previously used to interpret Fe K-edge spectroscopy of equilibrium and time-resolved SCO complexes.

Phenol Nitration Induced by an {Fe(NO)2}10 Dinitrosyl Iron Complex

Cellular dinitrosyl iron complexes (DNICs) have long been considered NO carriers. Although other physiological roles of DNICs have been postulated, their chemical functionality outside of NO transfer has not been demonstrated thus far. Here we report the unprecedented dioxygen reactivity of a N-bound {Fe(NO)(2)}(10) DNIC, [Fe(TMEDA)(NO)(2)] (1). In the presence of O(2), 1 becomes a nitrating agent that converts 2,4,-di-tert-butylphenol to 2,4-di-tert-butyl-6-nitrophenol via formation of a putative iron-peroxynitrite [Fe(TMEDA)(NO)(ONOO)] (2) that is stable below -80 °C. Iron K-edge X-ray absorption spectroscopy on 2 supports a five-coordinated metal center with a bound peroxynitrite in a cyclic bidentate fashion. The peroxynitrite ligand of 2 readily decays at increased temperature or under illumination. These results suggest that DNICs could have multiple physiological or deleterious roles, including that of cellular nitrating agents.

Fading of modern Prussian blue pigments in linseed oil medium

The fading of modern laboratory-synthesized and commercial Prussian blue, iron(III) hexacyanoferrate(II), based pigments in a linseed oil medium during exposure to light has been investigated. The Prussian blue pigments were painted with linseed oil, as a pure pigment and mixed with white lead, (PbCO3)2Pb(OH)2, zinc white, ZnO, or titanium white, TiO2, pigment. The samples were subjected to accelerated ageing for 800 h and the light fastness of the Prussian blue pigment was evaluated by reference to blue wool standards. Pure Prussian blue is extremely light fast whilst it strongly fades when mixed with a white pigment, especially with lead white or zinc oxide. The painted samples were studied by UV-visible, iron K-edge X-ray absorption, iron-57 transmission Mossbauer, and attenuated total reflectance infrared spectroscopy. X-ray absorption results reveal a decrease in the iron coordination number in aged samples in the presence of white pigment. The Mossbauer spectra of the pure Prussian blue and the unaged and aged mixtures of Prussian blue and lead white or zinc oxide at 1:100 and 1:10 dilution ratios, respectively, indicate the presence of iron(II) and iron(III) in a ratio close to one as expected for the bulk stoichiometric KFeIII[FeII(CN)6]; no change in the spectral parameters was observed upon ageing. Combined with the X-ray near edge absorption and infrared studies, these results suggest reduction of the surface iron ions in the Prussian blue with ageing upon exposure to light.

A XANES calibration for determining the oxidation state of iron in mantle garnet

Iron K-edge X-ray absorption near edge structure (XANES) spectra were recorded for synthetic garnets of the almandine–skiagite (Fe 3Al 2Si 3O 12–Fe 3Fe 2Si 3O 12), andradite–skiagite (Ca 3Fe 2Si 3O 12–Fe 3Fe 2Si 3O 12), Fe–knorringite–skiagite (Fe 3Cr 2Si 3O 12–Fe 3Fe 2Si 3O 12), and andradite–grossular (Ca 3Fe 2Si 3O 12–Ca 3Al 2Si 3O 12) solid solutions. Fe 3+/ΣFe varied systematically in these samples from 0 to 1. The variation in the energy and intensity of spectral features as a function of Fe 3+/ΣFe was investigated to identify correlations that could be used to determine Fe 3+/ΣFe of unknowns. The pre-edge energy, which is commonly used for quantification, was found to be relatively insensitive for garnet, particularly at low values of Fe 3+/ΣFe. The best correlation was for the absorption edge energy, which may provide an accurate and precise method for determining Fe 3+/ΣFe of garnets in metapelitic rocks. The resulting calibration curve, however, significantly and non-systematically overestimates the Fe 3+/ΣFe value of mantle garnets (from xenoliths and megacrysts) for which Fe 3+/ΣFe had been determined previously by Mössbauer spectroscopy. This is probably a result of differences in composition between the synthetic and natural garnets. For the mantle garnets, Fe 3+/ΣFe is strongly correlated with the intensity ratio of post-edge features at 7138.4 and 7161.7 eV. The deviation of the Mössbauer results from a linear fit is less than 0.01 (1 σ). This suggests a new method for determining Fe 3+/ΣFe from XANES spectra that does not require precise energy calibration or spectral fitting. The accuracy and precision for mantle garnets (within the compositional range studied: 0.75 ≤ Mg/(Mg + Fe) ≤ 0.86, 3.7–6.3 wt.% CaO, and 0.3–7.4 wt.% Cr 2O 3) is comparable to those of Mössbauer spectrosopy, however, XANES spectra can be acquired in ~ 15 min, allowing large numbers of samples to be analysed or the distribution of Fe 3+/ΣFe to be mapped with micron spatial resolution.

XAFS studies of the synthetic substitutes of hemozoin

The local atomic structure around iron atoms of two synthetic analogues of malarial pigment product is characterized by iron K-edge X-ray absorption fine structure spectroscopy (XAFS). Malarial pigment, also termed hemozoin, is a condensed phase of ferriprotoporphyrin-IX dimers, and is structurally identical to the synthetic material hematin anhydride, which is also termed β-hematin. Synthetic hemozoin analogues might serve as surrogates or models of malaria pigment for a range of fundamental laboratory methods, for example the design of new antimalarials. Two new iron(III)(protoporphyrin-IX) analogues, based on meso- and deuteroporphyrin are considered. Both new condensed phases are slightly soluble in many organic solvents. In the present work, the detailed examination of X-ray absorption spectra of monomer- and dimer-ferriprotoporphyrin-IX-based compounds is discussed. Parameters obtained by quantitative spectra analysis allow us to classify the new synthetic equivalents of the parasite product as dimer materials. X-ray absorption technique was able to distinguish dimer from monomer structures based on the structural disorder parameters of oxygen ligand bonding.