Mario Ulises Delgado-Jaime

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.

Valence‐to‐Core X‐Ray Emission Spectroscopy of Iron–Carbonyl Complexes: Implications for the Examination of Catalytic Intermediates

Valence-to-core X-ray emission spectroscopy (V2C XES) has been applied to a series of compounds relevant to both homogeneous catalysts and intermediates in heterogeneous reactions, namely [Fe(CO)5], [Fe2(CO)9], [Fe3(CO)12], [Fe(CO)3(cod)] (cod=cyclo-octadienyl), [Fe2Cp2(CO)4] (Cp=cyclo-pentadienyl), [Fe2Cp*2(CO)4] (Cp*=tetramethylcyclopentadienyl), and [FeCp(CO)2(thf)][B(ArF)4] (ArF=pentafluorophenyl). DFT calculations of the V2C XES spectra show very good agreement with experiment, which allows for an in depth analysis of the origins of the observed spectral signatures. It is demonstrated that the observed spectral features can be broken down into specific ligand and metal fragment contributions. The relative intensities of the observed features are further explained through a quantitative investigation of the metal 3p and 4p contributions to the spectra. The ability to use V2C XES to separate carbonyl, hydrocarbon, and solvent contributions is highlighted.

Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO3 films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO3 as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering.

Molybdenum L-Edge XAS Spectra of MoFe Nitrogenase†

A molybdenum L-edge X-ray absorption spectroscopy (XAS) study is presented for native and oxidized MoFe protein of nitrogenase as well as Mo-Fe model compounds. Recently collected data on MoFe protein (in oxidized and reduced forms) is compared to previously published Mo XAS data on the isolated FeMo cofactor in NMF solution and put in context of the recent Mo K-edge XAS study, which showed a MoIII assignment for the molybdenum atom in FeMoco. The L3-edge data are interpreted within a simple ligand-field model, from which a time-dependent density functional theory (TDDFT) approach is proposed as a way to provide further insights into the analysis of the molybdenum L3-edges. The calculated results reproduce well the relative spectral trends that are observed experimentally. Ultimately, these results give further support for the MoIII assignment in protein-bound FeMoco, as well as isolated FeMoco.

CTM4DOC: electronic structure analysis from X-ray spectroscopy

CTM4DOC, a new graphical user interface for electronic structure calculations in X-ray spectroscopy, is introduced and examples in transition metal complexes are described.

Kβ Valence to Core X-ray Emission Studies of Cu(I) Binding Proteins with Mixed Methionine – Histidine Coordination. Relevance to the Reactivity of the M- and H-sites of Peptidylglycine Monooxygenase

Biological systems use copper as a redox center in many metalloproteins, where the role of the metal is to cycle between its +1 and +2 oxidation states. This chemistry requires the redox potential to be in a range that can stabilize both Cu(I) and Cu(II) states and often involves protein-derived ligand sets involving mixed histidine-methionine coordination that balance the preferences of both oxidation states. Transport proteins, on the other hand, utilize copper in the Cu(I) state and often contain sites comprised predominately of the cuprophilic residue methionine. The electronic factors that allow enzymes and transporters to balance their redox requirements are complex and are often elusive due to the dearth of spectroscopic probes of the Cu(I) state. Here we present the novel application of X-ray emission spectroscopy to copper proteins via a study of a series of mixed His-Met copper sites where the ligand set varies in a systematic way between the His3 and Met3 limits. The sites are derived from the wild-type peptidylglycine monooxygenase (PHM), two single-site variants which replicate each of its two copper sites (CuM-site and CuH-site), and the transporters CusF and CusB. Clear differences are observed in the Kβ2,5 region at the Met3 and His3 limits. CusB (Met3) has a distinct peak at 8978.4 eV with a broad shoulder at 8975.6 eV, whereas CuH (His3) has two well-resolved features: a more intense feature at 8974.8 eV and a second at 8977.2 eV. The mixed coordination sphere CusF (Met2His) and the PHM CuM variant (Met1His2) have very similar spectra consisting of two features at 8975.2 and 8977.8 eV. An analysis of DFT calculated spectra indicate that the intensity of the higher energy peak near 8978 eV is mediated by mixing of ligand-based orbitals into the Cu d(10) manifold, with S from Met providing more intensity by facilitating increased Cu p-d mixing. Furthermore, reaction of WT PHM with CO (an oxygen analogue) produced the M site CO complex, which showed a unique XES spectrum that could be computationally reproduced by including interactions between Cu(I) and the CO ligand. The study suggests that the valence-to-core (VtC) region can not only serve as a probe of ligand speciation but also offer insight into the coordination geometry, in a fashion similar to XAS pre-edges, and may be sufficiently sensitive to the coordination of exogenous ligands to be useful in the study of reaction mechanisms.

Local vs Nonlocal States in FeTiO3 Probed with 1s2pRIXS: Implications for Photochemistry

Metal–metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron–titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core–hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

In-Situ 2p3d Resonant Inelastic X-ray Scattering Tracking Cobalt Nanoparticle Reduction

In-situ carbon-thermal reduction of cobalt oxide nanoparticles supported on carbon nanotubes was studied by cobalt 2p3d resonant inelastic X-ray scattering (RIXS). The in-situ 2p X-ray absorption spectroscopy (XAS) and RIXS measurements were performed at 500, 600, and 700 °C, where four consistent excitation energies were used for RIXS acquisitions. After 700 °C reduction, the XAS spectrum shows a cobalt metal-like shape, while the RIXS spectra reveal the minority cobalt monoxide phase. The holistic fit on both XAS and RIXS data reveals the respective contributions from metal and monoxide. We show that the relative precision to determine the monoxide content changes from ∼5.6% in XAS results to better than 0.8% in the RIXS analysis, suggesting that RIXS is a useful tool to track the oxidation state of nanoparticles under in situ conditions. We determined a relative radiative ratio (P) factor of approximately 5, where this factor gives the ratio between the relative strengths of the radiative decay channels compared to the nonradiative channels in CoO and Co metal.

Charge-Transfer Analysis of 2p3d Resonant Inelastic X-ray Scattering of Cobalt Sulfide and Halides

We show that with 2p3d resonant inelastic X-ray scattering (RIXS) we can accurately determine the charge-transfer parameters of CoF2, CoCl2, CoBr2, and CoS. The 160 meV resolution RIXS results are compared with charge-transfer multiplet calculations. The improved resolution and the direct observation of the crystal field and charge-transfer excitations allow the determination of more accurate parameters than could be derived from X-ray absorption and X-ray photoemission, both limited in resolution by their lifetime broadening. We derive the crystal field and charge-transfer parameters of the Co2+ ions, which provides the nature of the ground state of the Co2+ ions with respect to symmetry and hybridization. In addition, the increased spectral resolution allows the more accurate determination of the atomic Slater integrals. The results show that the crystal field energy decreases with increasing ligand covalency. The L2 edge RIXS spectra show that the intensity of the (Coster–Kronig induced) nonresonant X-ray emission is a measure of ligand covalency.