Kβ X-ray Emission Spectroscopy Research Articles

Structural origin of the anomalous temperature dependence of the local magnetic moments in the CaFe2As2 family of materials.

We report a combination of Fe Kβ x-ray emission spectroscopy and density functional reduced Stoner theory calculations to investigate the correlation between structural and magnetic degrees of freedom in CaFe2(As1-xPx)2. The puzzling temperature behavior of the local moment found in rare earth-doped CaFe2As2 [H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013)] is also observed in CaFe2(As1-xPx)2. We explain this phenomenon based on first-principles calculations with scaled magnetic interaction. One scaling parameter is sufficient to describe quantitatively the magnetic moments in both CaFe2(As1-xPx)2 (x=0.055) and Ca0.78La0.22Fe2As2 at all temperatures. The anomalous growth of the local moments with increasing temperature can be understood from the observed large thermal expansion of the c-axis lattice parameter combined with strong magnetoelastic coupling. These effects originate from the strong tendency to form As-As dimers across the Ca layer in the CaFe2As2 family of materials. Our results emphasize the dual local-itinerant character of magnetism in Fe pnictides.

Study of iron dimers reveals angular dependence of valence-to-core X-ray emission spectra.

Transition-metal Kβ X-ray emission spectroscopy (XES) is a developing technique that probes the occupied molecular orbitals of a metal complex. As an element-specific probe of metal centers, Kβ XES is finding increasing applications in catalytic and, in particular, bioinorganic systems. For the continued development of XES as a probe of these complex systems, however, the full range of factors which contribute to XES spectral modulations must be explored. In this report, an investigation of a series of oxo-bridged iron dimers reveals that the intensity of valence-to-core features is sensitive to the Fe–O–Fe bond angle. The intensity of these features has a well-known dependence on metal–ligand bond distance, but a dependence upon bond angle has not previously been documented. Herein, we explore the angular dependence of valence-to-core XES features both experimentally and computationally. Taken together, these results show that, as the Fe–O–Fe angle decreases, the intensity of the Kβ″ feature increases and that this effect is modulated by increasing amounts of Fe np mixing into the O 2s orbital at smaller bond angles. The relevance of these findings to the identification of oxygenated intermediates in bioinorganic systems is highlighted, with special emphasis given to the case of soluble methane monooxygenase.

Catalytic defluorination of perfluorinated aromatics under oxidative conditions using N-bridged diiron phthalocyanine.

Carbon-fluorine bonds are the strongest single bonds in organic chemistry, making activation and cleavage usually associated with organometallic and reductive approaches particularly difficult. We describe here an efficient defluorination of poly- and perfluorinated aromatics under oxidative conditions catalyzed by the μ-nitrido diiron phthalocyanine complex [(Pc)Fe(III)(μ-N)Fe(IV)(Pc)] under mild conditions (hydrogen peroxide as the oxidant, near-ambient temperatures). The reaction proceeds via the formation of a high-valent diiron phthalocyanine radical cation complex with fluoride axial ligands, [(Pc)(F)Fe(IV)(μ-N)Fe(IV)(F)(Pc(+•))], which was isolated and characterized by UV-vis, EPR, (19)F NMR, Fe K-edge EXAFS, XANES, and Kβ X-ray emission spectroscopy, ESI-MS, and electrochemical techniques. A wide range of per- and polyfluorinated aromatics (21 examples), including C6F6, C6F5CF3, C6F5CN, and C6F5NO2, were defluorinated with high conversions and high turnover numbers. [(Pc)Fe(III)(μ-N)Fe(IV)(Pc)] immobilized on a carbon support showed increased catalytic activity in heterogeneous defluorination in water, providing up to 4825 C-F cleavages per catalyst molecule. The μ-nitrido diiron structure is essential for the oxidative defluorination. Intramolecular competitive reactions using C6F3Cl3 and C6F3H3 probes indicated preferential transformation of C-F bonds with respect to C-Cl and C-H bonds. On the basis of the available data, mechanistic issues of this unusual reactivity are discussed and a tentative mechanism of defluorination under oxidative conditions is proposed.

Kβ mainline X-ray emission spectroscopy as an experimental probe of metal-ligand covalency.

The mainline feature in metal Kβ X-ray emission spectroscopy (XES) has long been recognized as an experimental marker for the spin state of the metal center. However, even within a series of metal compounds with the same nominal oxidation and spin state, significant changes are observed that cannot be explained on the basis of overall spin. In this work, the origin of these effects is explored, both experimentally and theoretically, in order to develop the chemical information content of Kβ mainline XES. Ligand field expressions are derived that describe the behavior of Kβ mainlines for first row transition metals with any dn count, allowing for a detailed analysis of the factors governing mainline shape. Further, due to limitations associated with existing computational approaches, we have developed a new methodology for calculating Kβ mainlines using restricted active space configuration interaction (RAS–CI) calculations. This approach eliminates the need for empirical parameters and provides a powerful tool for investigating the effects that chemical environment exerts on the mainline spectra. On the basis of a detailed analysis of the intermediate and final states involved in these transitions, we confirm the known sensitivity of Kβ mainlines to metal spin state via the 3p–3d exchange coupling. Further, a quantitative relationship between the splitting of the Kβ mainline features and the metal–ligand covalency is established. Thus, this study furthers the quantitative electronic structural information that can be extracted from Kβ mainline spectroscopy.

Study of Kβ X-ray emission spectroscopy applied to Mn(2−x)V(1+x)O4 (x=0 and 1/3) oxyspinel and comparison with XANES

Oxidation state and coordination of transition metal cations seems to be hard to assess when considering multiple cations, each one with different possible oxidation states. In fact, this is the case of the spinel-type double oxides family. High resolution Kβ X-ray fluorescence spectra were measured in Mn(2−x)V(1+x)O4 (x=0 and 13) spinels-type double oxides in order to determine the oxidation state and coordination of V and Mn cations. The relative intensity of radiative Auger effect KM2,3M4,5 to the total intensity and the integral absolute difference value were used as reference parameters for the characterization of Mn oxidation states. The coordination of Mn ions was inferred by the intensity of the Kβ5 line. In the case of V compounds, it was used as the intensity of the line Kβ′ relative to the total area of Kβ region. The obtained results were further compared with X-ray absorption spectra analysis, showing good agreements regarding the oxidation state characterization. However, there were found some discrepancies in coordination, due to customary oversimplifications in the Kβ5 line origin. The obtained results might represent valuable and useful data for chemical scopes of characterizing spinel-type oxides family.

Sensitivity of X-ray Core Spectroscopy to Changes in Metal Ligation: A Systematic Study of Low-Coordinate, High-Spin Ferrous Complexes

In order to assess the sensitivity and complementarity of X-ray absorption and emission spectroscopies for determining changes in the metal ligation sphere, a systematic experimental and theoretical study of iron model complexes has been carried out. A series of high-spin ferrous complexes, in which the ligation sphere has been varied from a three-coordinate complex, [L(tBu)Fe(SPh)] (1) (where L(tBu) = bulky β-diketiminate ligand; SPh = phenyl thiolate) to four-coordinate complexes [L(tBu)Fe(SPh)(X)] (where X = CN(t)Bu (2); 1-methylimidazole (3); or N,N-dimethylformamide (DMF) (4)), has been investigated using a combination of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES) spectroscopies. The Fe K XAS pre-edge and edge of all four complexes are consistent with a high-spin ferrous assignment, with the largest differences in the pre-edge intensities attributed to changes in covalency of the fourth coordination site. The X-ray emission spectra show pronounced changes in the valence to core region (V2C) as the identity of the coordinated ligand is varied. The experimental results have been correlated to density functional theory (DFT) calculations, to understand key molecular orbital contributions to the observed absorption and emission features. The calculations also have been extended to a series of hypothetical high-spin iron complexes to understand the sensitivity of XAS and XES techniques to different ligand protonation states ([L(tBu)Fe(II)(SPh)(NHn)](3-n) (n = 3, 2, 1, 0)), metal oxidation states [L(tBu)Fe(SPh)(N)](n-) (n = 3, 2, 1), and changes in the ligand identity [L(tBu)Fe(IV)(SPh)(X)](n-) (X = C(4-), N(3-), O(2-); n = 2, 1, 0). This study demonstrates that XAS pre-edge data have greater sensitivity to changes in oxidation state, while valence to core (V2C) XES data provide a more sensitive probe of ligand identity and protonation state. The combination of multiple X-ray spectroscopic methods with DFT results thus has the potential to provide for detailed characterization of complex inorganic systems in both chemical and biological catalysis.

X-ray Spectroscopic Observation of an Interstitial Carbide in NifEN-Bound FeMoco Precursor

The iron-molybdenum cofactor (FeMoco) of nitrogenase contains a biologically unprecedented μ(6)-coordinated C(4-) ion. Although the role of this interstitial atom in nitrogenase catalysis is unknown, progress in understanding its biosynthetic origins has been made. Here we report valence-to-core Fe Kβ X-ray emission spectroscopy data to show that this C(4-) ion is present in the Fe(8)S(9) "L-cluster," which is the immediate precursor to FeMoco prior to the insertion of molybdenum and coordination by homocitrate. These results accord with recent evidence supporting a role for the S-adenosylmethionine-dependent enzyme NifB in the incorporation of carbon into the FeMoco center of nitrogenase.

Spin-polarized electronic structure of the core–shell ZnO/ZnO:Mn nanowires probed by X-ray absorption and emission spectroscopy

The combination of X-ray spectroscopy methods complemented with theoretical analysis unravels the coexistence of paramagnetic and antiferromagnetic phases in the Zn0.9Mn0.1O shell deposited onto array of wurtzite ZnO nanowires. The shell is crystalline with orientation toward the ZnO growth axis, as demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than 90% of Mn atoms substituted Zn in the shell while a fraction of secondary phases was below 10%. The value of manganese spin magnetic moment was estimated from the Mn Kβ X-ray emission spectroscopy to be 4.3 μB which is close to the theoretical value for substitutional MnZn. However the analysis of L2,3 X-ray magnetic circular dichroism data showed paramagnetic behaviour with saturated spin magnetic moment value of 1.95 μB as determined directly from the spin sum rule. After quantitative analysis employing atomic multiplet simulations such difference was explained by a coexistence of paramagnetic phase and local antiferromagnetic coupling of Mn magnetic moments. Finally, spin-polarized electron density of states was probed by the spin-resolved Mn K-edge XANES spectroscopy and consequently analyzed by band structure calculations.

Fast Detection Allowing Analysis of Metalloprotein Electronic Structure by X-ray Emission Spectroscopy at Room Temperature

The paradigm of "detection-before-destruction" was tested for a metalloprotein complex exposed at room temperature to the high x-ray flux typical of third generation synchrotron sources. Following the progression of the x-ray induced damage by Mn Kβ x-ray emission spectroscopy, we demonstrated the feasibility of collecting room temperature data on the electronic structure of native Photosystem II, a trans-membrane metalloprotein complex containing a Mn(4)Ca cluster. The determined non-damaging observation timeframe (about 100 milliseconds using continuous monochromatic beam, deposited dose 1*10(7) photons/µm(2) or 1.3*10(4) Gy, and 66 microseconds in pulsed mode using pink beam, deposited dose 4*10(7) photons/µm(2) or 4.2*10(4) Gy) is sufficient for the analysis of this protein's electron dynamics and catalytic mechanism at room temperature. Reported time frames are expected to be representative for other metalloproteins. The described instrumentation, based on the short working distance dispersive spectrometer, and experimental methodology is broadly applicable to time-resolved x-ray emission analysis at synchrotron and x-ray free-electron laser light sources.

Identification of a Single Light Atom within a Multinuclear Metal Cluster Using Valence-to-Core X-ray Emission Spectroscopy

Iron valence-to-core Fe Kβ X-ray emission spectroscopy (V2C XES) is established as a means to identify light atoms (C, N, O) within complex multimetallic frameworks. The ability to distinguish light atoms, particularly in the presence of heavier atoms, is a well-known limitation of both crystallography and EXAFS. Using the sensitivity of V2C XES to the ionization potential of the bound ligand, energetic shifts of ~10 eV in the ligand 2s ionization energies of bound C, N, and O may be observed. As V2C XES is a high-energy X-ray method, it is readily applicable to samples in any physical form. This method thus has great potential for application to multimetallic inorganic frameworks involved in both small molecule storage and activation.

Manganese Kβ X-ray Emission Spectroscopy As a Probe of Metal–Ligand Interactions

A systematic series of high-spin mononuclear Mn(II), Mn(III), and Mn(IV) complexes has been investigated by manganese Kβ X-ray emission spectroscopy (XES). The factors contributing to the Kβ main line and the valence to core region are discussed. The Kβ main lines are dominated by 3p-3d exchange correlation (spin state) effects, shifting to lower energy upon oxidation of Mn(II) to Mn(III) due to the decrease in spin state from S = 5/2 to S = 2, whereas the valence to core region shows greater sensitivity to the chemical environment surrounding the Mn center. A density functional theory (DFT) approach has been used to calculate the valence to core spectra and assess the contributions to the energies and intensities. The valence spectra are dominated by manganese np to 1s electric dipole-allowed transitions and are particularly sensitive to spin state and ligand identity (reflected primarily in the transition energies) as well as oxidation state and metal-ligand bond lengths (reflected primarily in the transition intensities). The ability to use these methods to distinguish different ligand contributions within a heteroleptic coordination sphere is highlighted. The similarities and differences between the current Mn XES study and previous studies of Fe XES investigations are discussed. These findings serve as an important calibration for future applications to manganese active sites in biological and chemical catalysis.

Kβ X-ray Emission Spectroscopy Offers Unique Chemical Bonding Insights: Revisiting the Electronic Structure of Ferrocene

Kβ X-ray emission spectroscopy (XES) is emerging as a powerful tool for the study of chemical bonding. Analyses of the Kβ XES of ferrocene (Fc) and ferrocenium (Fc(+)) are presented as further demonstrations of the capabilities of the technique. Assignments of the valence to core (V2C) region of these spectra as electric dipole-allowed cyclopentadienyl (Cp) → Fe 1s transitions demonstrate that XES affords electronic structural insight into the energetics of ligand-based molecular orbitals (MOs). Combined with K-edge X-ray absorption spectroscopy (XAS), we show that XES can provide analogous information to photoemission spectroscopy (PES). Density functional theory (DFT) analyses reveal that the V2C transitions in Fc/Fc(+) derive their intensity from Fe 4p admixture (on the order of 5-10%) into the Cp-based MOs from which they originate. These 4p admixtures confer bonding character to the Cp-based a(2u) and e(1u) MOs to at least the extent of backbonding contributions to frontier MOs from higher-lying Cp π* MOs.

Valence-to-Core X-ray Emission Spectroscopy: A Sensitive Probe of the Nature of a Bound Ligand

The sensitivity of iron Kβ X-ray emission spectroscopy (XES) to the nature of the bound ligands (σ-donating, π-donating, and π-accepting) has been explored. A combination of experiment and theory has been employed, with a DFT approach being utilized to elucidate ligand effects on the spectra and to assign the spectral intensity mechanisms. Knowledge of the various contributions to the spectra allows for a deeper understanding of spectral features and demonstrates the sensitivity of this method to the identity of the interacting ligands. The potential of XES for identifying intermediate species in nonheme iron enzymes is highlighted.

Probing Valence Orbital Composition with Iron Kβ X-ray Emission Spectroscopy

A systematic study of 12 ferric and ferrous Kbeta X-ray emission spectra (XES) is presented. The factors contributing to the Kbeta main line and the valence to core region of the spectra are experimentally assessed and quantitatively evaluated. While the Kbeta main line spectra are dominated by spin state contributions, the valence to core region is shown to have greater sensitivity to changes in the chemical environment. A density functional theory (DFT) based approach is used to calculate the experimental valence spectra and to evaluate the contributions to experimental intensities and energies. The spectra are found to be dominated by iron np to 1s electric dipole allowed transitions, with pronounced sensitivity to spin state, ligand identity, ligand ionization state, hybridization state, and metal-ligand bond lengths. These findings serve as an important calibration for future applications to iron active sites in biological and chemical catalysis. Potential applications to Compound II heme derivatives are highlighted.

Pressure induced magnetic transition in sideriteFeCO3 studied by x-ray emission spectroscopy

We have investigated the magnetic state of iron in sideriteFeCO3 under highpressure using Kβ x-ray emission spectroscopy. Pressure induced changes in the shape of the ironKβ emission lines indicate that the iron ground state changes from a low pressure magneticstate to a high pressure non-magnetic state. This transition takes place roughly at 50 GPa.This conclusion is supported by charge transfer multiplet calculations of the ironKβ emission line.