Kβ Mainline X-ray Emission Spectroscopy as an Experimental Probe of Metal–Ligand Covalency

Pollock, Christopher J.; Delgado-Jaime, Mario Ulises; Atanasov, Mihail; Neese, Frank; DeBeer, Serena

doi:10.1021/ja504182n

Cited by 134 publications

(252 citation statements)

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“…The steep correlation locates the overall iron bonding in the MB/HB hemes and the porphyrins closer to the noncovalent than covalent limit (32,34,59,60). Agreement of our and earlier Kβ data of MB/HB (45) supports the spin state changes.…”

Section: Discussionsupporting

confidence: 85%

“…Kβ spectra of MB/HB and porphyrin compounds revealed a direct correlation of the formal unpaired Fe(d) spin count to the relative Kβ´intensity, in agreement with results for other iron species (32,34), establishing further that the iron-centered radiative 3p→1s decay is a reliable probe of the valence level configuration. The steep correlation locates the overall iron bonding in the MB/HB hemes and the porphyrins closer to the noncovalent than covalent limit (32,34,59,60).…”

Section: Discussionsupporting

confidence: 82%

“…The core/valence-to-valence/core electronic transitions (ctv/vtc) spectra can be calculated by DFT methods for theory benchmarking (30,31). 3p-3d spin coupling splits the Kβ main-line emission into the Kβ' and Kβ 1,3 features, and the Kβ' intensity is a measure of the metal spin state and bonding covalency (32)(33)(34). XAS has been used to address structural metrics, solution vs. crystalline materials, and redox states in MB/HB (35)(36)(37)(38)(39).…”

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confidence: 99%

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Effective intermediate-spin iron in O ₂ -transporting heme proteins

Schuth

Mebs

Huwald

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Proteins carrying an iron-porphyrin (heme) cofactor are essential for biological O2 management. The nature of Fe-O2 bonding in hemoproteins is debated for decades. We used energy-sampling and rapid-scan X-ray Kβ emission and K-edge absorption spectroscopy as well as quantum chemistry to determine molecular and electronic structures of unligated (deoxy), CO-inhibited (carboxy), and O2-bound (oxy) hemes in myoglobin (MB) and hemoglobin (HB) solutions and in porphyrin compounds at 20–260 K. Similar metrical and spectral features revealed analogous heme sites in MB and HB and the absence of low-spin (LS) to high-spin (HS) conversion. Amplitudes of Kβ main-line emission spectra were directly related to the formal unpaired Fe(d) spin count, indicating HS Fe(II) in deoxy and LS Fe(II) in carboxy. For oxy, two unpaired Fe(d) spins and, thus by definition, an intermediate-spin iron center, were revealed by our static and kinetic X-ray data, as supported by (time-dependent) density functional theory and complete-active-space self-consistent-field calculations. The emerging Fe-O2 bonding situation includes in essence a ferrous iron center, minor superoxide character of the noninnocent ligand, significant double-bond properties of the interaction, and three-center electron delocalization as in ozone. It resolves the apparently contradictory classical models of Pauling, Weiss, and McClure/Goddard into a unifying view of O2 bonding, tuned toward reversible oxygen transport.

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Section: Discussionsupporting

confidence: 85%

Section: Discussionsupporting

confidence: 82%

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confidence: 99%

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Effective intermediate-spin iron in O ₂ -transporting heme proteins

Schuth

Mebs

Huwald

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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“…Stronger metal-ligand covalency may be interpreted in terms of a larger radial distribution of the effective (Wannier) metal d-orbitals used in the description of the electron density, implying a decrease of J which translates into a reduction of the IAD value. Indeed the influence of the metal -ligand covalency on the Kβ CTC spectra has been reported earlier in different works [5,32,33]. Therefore, while J can be assumed to vary very little with Cr-doping, it decreases along the Ba 1-x K x Fe 2 As 2 series with increasing Kcontent as a result of the spread of the Wannier orbitals of Fe 3d character that also manifests…”

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confidence: 61%

Evidence of Mott physics in iron pnictides from x-ray spectroscopy

et al. 2017

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X-ray emission and absorption spectroscopies are jointly used as fast probes to determine the evolution as a function of doping of the fluctuating local magnetic moments in K-and Cr-holedoped BaFe 2 As 2 . An increase in the local moment with hole-doping is found, supporting the theoretical scenario in which a Mott insulating state that would be realized for half-filled conduction bands has an influence throughout the phase diagram of these iron-pnictides.A scenario has recently been proposed in the field of Fe-based superconductors, where the low energy physics is described in terms of a coexistence of strongly and weakly correlated electrons [1]. Such a picture provides a framework to understand a wealth of experiments on quasiparticle mass enhancements (specific heat, low-frequency optical conductivity, angleresolved photoemission spectroscopy, quantum oscillations etc.) in electron-and hole-doped A direct method to probe the local magnetic moment in transition metal compounds is Kβ coreto-core (CTC) x-ray emission spectroscopy (XES) that is sensitive to the local 3d spin because of the intra-atomic 3p-3d exchange interaction [5,6] The unoccupied density of states around Fe can be probed by means of K edge x-ray absorption spectroscopy (XAS) providing information that is complementary to Kβ spectroscopy. We have recorded high-energy resolution fluorescence detected XAS on all K-and Cr-doped samples.All samples were plate-like single crystals grown by the self-flux technique as detailed elsewhere (111) monochromator crystals. The inelastically scattered photons were analyzed using a set of five spherically bent Ge(620) crystals that were arranged with the sample and detector (avalanche photodiode) in a vertical Rowland geometry (R = 1 m). The surface of the samples was aligned to 45º with respect to the incident beam direction, i.e. the angle between the electric field of the linearly polarized x-rays and the sample [001] direction. Non-resonant Fe Kβ XES measurements were recorded at incident energy 7200 eV. High-energy resolution fluorescence-detected x-ray absorption near edge structure (HERFD-XANES) spectra were obtained by setting the emission energy to the maximum of the Kβ 1,3 line while scanning the incoming energy through the absorption edge. The resulting spectrum is an approximation to the 1s photoabsorption cross section with sharper spectral features than in standard XAS [5,6]. The total experimental broadening, determined as full width at half maximum of the elastic profiles, was 1.3 eV. All the measurements were performed at room temperature. Both XES and HERFD-XANES spectra have been normalized in area using the ranges 7025-7080 eV and 7105-7190 eV respectively.Non-resonant Fe Kβ CTC XES spectra have been measured as a function of doping in Ba 1-x K x Fe 2 As 2 (0 ≤ x ≤ 1) and Ba(Fe 1-x Cr x ) 2 As 2 (0.026 ≤ x ≤ 0.47). Figure 1( [20] where J is the exchange integral between the electrons in the 3p and 3d shells. It becomes visible in the difference spectra of Figure 1 and is about 10-...

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“…In a stimulated Raman process a core hole created by the X-ray pulse on the acceptor is instantaneously filled by a valence electron on the bridge, resulting in a B→A ET. Such an ET process is analogous to the valence-to-core X-ray spontaneous emission observed in transition metal complexes with ligand-to-metal charge transfer (20). 5+ (low spin in the ground state) was investigated recently by transient optical absorption, X-ray absorption, X-ray diffuse scattering, and X-ray emission (21)(22)(23).…”

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confidence: 93%

Coherent control of long-range photoinduced electron transfer by stimulated X-ray Raman processes

Dorfman

Zhang

Mukamel

2016

Proc. Natl. Acad. Sci. U.S.A.

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We show that X-ray pulses resonant with selected core transitions can manipulate electron transfer (ET) in molecules with ultrafast and atomic selectivity. We present possible protocols for coherently controlling ET dynamics in donor-bridge-acceptor (DBA) systems by stimulated X-ray resonant Raman processes involving various transitions between the D, B, and A sites. Simulations presented for a Ru(II)-Co(III) model complex demonstrate how the shapes, phases and amplitudes of the X-ray pulses can be optimized to create charge on demand at selected atoms, by opening up otherwise blocked ET pathways.L ong-range electron transfer (ET) over tens of angstroms in molecular assemblies plays an essential role in many biological processes, artificial light-harvesting schemes, and sensor applications (1-7). Using lasers to precisely control ET pathways and rates has been a long-term goal of chemists (8). The manner in which infrared light can excite molecular vibrations to affect ET in donor-bridge-acceptor (DBA) systems has been studied theoretically (9, 10) and experimentally (11)(12)(13)(14).The rapid development of bright X-ray lasers and high harmonic sources has opened up new opportunities for X-ray spectroscopy (15). We have recently demonstrated that stimulated X-ray Raman spectroscopy (16, 17) with broadband X-ray pulses can reveal the time-evolving oxidation states of various species in the long-range ET process of the protein azurin (18). Here we show that by coupling to core-excited states, resonant X-ray pulses can precisely target either the donor, bridge, or the acceptor site in an ET process by altering the valence electronic states in its vicinity by triggering the bridge-to-acceptor (BA), the donor-to-bridge (DB), or the bridgeto-bridge (BB) ET transfer. We show how the ET pathways in a model DBA system ([(CN) 4 Ru II (tpphz)Co III (CN) 4 ] 3− ) can be coherently manipulated by X-ray pulses resonant with the acceptor.Application is made to a Ru-Co light-harvesting complex (shown in Fig. 1 A and B) where an electron is transferred from the donor Ru II to the acceptor Co III to create Ru III /Co II . X-ray pulses can create valence excitations via a Raman process (19), thus altering the occupied molecular orbitals (MOs). We shall focus on the BA ET coherent control scheme illustrated in Fig. 2A. In a stimulated Raman process a core hole created by the X-ray pulse on the acceptor is instantaneously filled by a valence electron on the bridge, resulting in a B→A ET. Such an ET process is analogous to the valence-to-core X-ray spontaneous emission observed in transition metal complexes with ligand-to-metal charge transfer (20). (Fig. 1 A and B) where the bpy ligands are replaced with (CN) − . This eliminates the complicated spin crossover transition at the Co center (23), because the strong ligands (CN) − favor the low-spin state.The relevant ET parameters were obtained from electronic structure calculations. Because the Ru and Co centers are far apart, electronic structure calculations can be carried out for ...

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Kβ Mainline X-ray Emission Spectroscopy as an Experimental Probe of Metal–Ligand Covalency

Cited by 134 publications

References 75 publications

Effective intermediate-spin iron in O ₂ -transporting heme proteins

Effective intermediate-spin iron in O ₂ -transporting heme proteins

Evidence of Mott physics in iron pnictides from x-ray spectroscopy

Coherent control of long-range photoinduced electron transfer by stimulated X-ray Raman processes

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Kβ Mainline X-ray Emission Spectroscopy as an Experimental Probe of Metal–Ligand Covalency

Cited by 134 publications

References 75 publications

Effective intermediate-spin iron in O 2 -transporting heme proteins

Effective intermediate-spin iron in O 2 -transporting heme proteins

Evidence of Mott physics in iron pnictides from x-ray spectroscopy

Coherent control of long-range photoinduced electron transfer by stimulated X-ray Raman processes

Contact Info

Product

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Effective intermediate-spin iron in O ₂ -transporting heme proteins

Effective intermediate-spin iron in O ₂ -transporting heme proteins