| 1. |
- Eckert, Sebastian, et al.
(författare)
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Ultrafast Independent N-H and N-C Bond Deformation Investigated with Resonant Inelastic X-Ray Scattering
- 2017
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Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 56:22, s. 6088-6092
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Tidskriftsartikel (refereegranskat)abstract
- The femtosecond excited-state dynamics following resonant photoexcitation enable the selective deformation of N-H and N-C chemical bonds in 2-thiopyridone in aqueous solution with optical or X-ray pulses. In combination with multiconfigurational quantum-chemical calculations, the orbital-specific electronic structure and its ultrafast dynamics accessed with resonant inelastic X-ray scattering at the N 1s level using synchrotron radiation and the soft X-ray free-electron laser LCLS provide direct evidence for this controlled photoinduced molecular deformation and its ultrashort time-scale.
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| 2. |
- Ekimova, Maria, et al.
(författare)
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Aqueous Solvation of Ammonia and Ammonium : Probing Hydrogen Bond Motifs with FT-IR and Soft X-ray Spectroscopy
- 2017
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 139:36, s. 12773-12783
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Tidskriftsartikel (refereegranskat)abstract
- In a multifaceted investigation combining local soft X-ray and vibrational spectroscopic probes with ab initio molecular dynamics simulations, hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH3) and ammonium ion (NH4+), are quantitatively assessed in terms of electronic structure, solvation structure, and dynamics. From the Xray measurements and complementary determination of the IR-active hydrogen stretching and bending modes of NH3 and NH4+ in aqueous solution, the picture emerges of a comparatively strongly hydrogen-bonded NH4+ ion via N-H donating interactions, whereas NH3 has a strongly accepting hydrogen bond with one water molecule at the nitrogen lone pair but only weakly N-H donating hydrogen bonds. In contrast to the case of hydrogen bonding among solvent water moleCules, we find that energy mismatch between occupied orbitals of both the solutes NH3 and NH4+ and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding and, thus, inhibits substantial charge transfer between solute and solvent. A close inspection of the calculated unoccupied molecular orbitals, in conjunction with experimentally measured N K-edge absorption spectra, reveals the different nature of the electronic structural effects of these two key principal amine compounds imposed by hydrogen bonding to water, where a pH-dependent excitation energy appears to be an intrinsic property. These results provide a benchmark for hydrogen bonding of other nitrogen-containing acids and bases.
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| 3. |
- Jay, Raphael, et al.
(författare)
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Following Metal-to-Ligand Charge-Transfer Dynamics with Ligand and Spin Specificity Using Femtosecond Resonant Inelastic X-ray Scattering at the Nitrogen K-Edge
- 2021
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:28, s. 6676-6683
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate for the case of photoexcited [Ru(2,2'-bipyridine)(3)](2+) how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal-ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 +/- 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.
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| 4. |
- Jay, Raphael M., et al.
(författare)
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Disentangling Transient Charge Density and Metal-Ligand Covalency in Photoexcited Ferricyanide with Femtosecond Resonant Inelastic Soft X-ray Scattering
- 2018
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:12, s. 3538-3543
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Tidskriftsartikel (refereegranskat)abstract
- Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal-ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. pi-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences sigma-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes.
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| 5. |
- Jay, Raphael M., et al.
(författare)
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The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(II) complexes
- 2018
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 20:44, s. 27745-27751
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Tidskriftsartikel (refereegranskat)abstract
- Understanding and controlling properties of transition metal complexes is a crucial step towards tailoring materials for sustainable energy applications. In a systematic approach, we use resonant inelastic X-ray scattering to study the influence of ligand substitution on the valence electronic structure around an aqueous iron(II) center. Exchanging cyanide with 2-2'-bipyridine ligands reshapes frontier orbitals in a way that reduces metal 3d charge delocalization onto the ligands. This net decrease of metal-ligand covalency results in lower metal-centered excited state energies in agreement with previously reported excited state dynamics. Furthermore, traces of solvent-effects were found indicating a varying interaction strength of the solvent with ligands of different character. Our results demonstrate how ligand exchange can be exploited to shape frontier orbitals of transition metal complexes in solution-phase chemistry; insights upon which future efforts can built when tailoring the functionality of photoactive systems for light-harvesting applications.
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| 6. |
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| 7. |
- Kubin, Markus, et al.
(författare)
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Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers
- 2017
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Ingår i: Structural Dynamics. - : AMER INST PHYSICS. - 2329-7778. ; 4:5
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Tidskriftsartikel (refereegranskat)abstract
- X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn similar to 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions. (C) 2017 Author(s).
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| 8. |
- Kunnus, Kristjan, et al.
(författare)
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A setup for resonant inelastic soft x-ray scattering on liquids at free electron laser light sources
- 2012
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 83:12
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Tidskriftsartikel (refereegranskat)abstract
- We present a flexible and compact experimental setup that combines an in vacuum liquid jet with an x-ray emission spectrometer to enable static and femtosecond time-resolved resonant inelastic soft x-ray scattering (RIXS) measurements from liquids at free electron laser (FEL) light sources. We demonstrate the feasibility of this type of experiments with the measurements performed at the Linac Coherent Light Source FEL facility. At the FEL we observed changes in the RIXS spectra at high peak fluences which currently sets a limit to maximum attainable count rate at FELs. The setup presented here opens up new possibilities to study the structure and dynamics in liquids. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772685]
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| 9. |
- Kunnus, Kristjan, et al.
(författare)
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Anti-Stokes resonant x-ray Raman scattering for atom specific and excited state selective dynamics
- 2016
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 18
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast electronic and structural dynamics of matter govern rate and selectivity of chemical reactions, as well as phase transitions and efficient switching in functional materials. Since x-rays determine electronic and structural properties with elemental, chemical, orbital and magnetic selectivity, short pulse x-ray sources have become central enablers of ultrafast science. Despite of these strengths, ultrafast x-rays have been poor at picking up excited state moieties from the unexcited ones. With time-resolved anti-Stokes resonant x-ray Raman scattering (AS-RXRS) performed at the LCLS, and ab initio theory we establish background free excited state selectivity in addition to the elemental, chemical, orbital and magnetic selectivity of x-rays. This unparalleled selectivity extracts low concentration excited state species along the pathway of photo induced ligand exchange of Fe(CO)(5) in ethanol. Conceptually a full theoretical treatment of all accessible insights to excited state dynamics with AS-RXRS with transform-limited x-ray pulses is given-which will be covered experimentally by upcoming transform-limited x-ray sources.
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| 10. |
- Kunnus, Kristjan, et al.
(författare)
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From Ligand Fields to Molecular Orbitals : Probing the Local Valence Electronic Structure of Ni2+ in Aqueous Solutions with Resonant Inelastic X-ray Scattering
- 2013
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 117, s. 16512-16521
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Tidskriftsartikel (refereegranskat)abstract
- Bonding of the Ni2+(aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d–3d electron correlation interactions in the Ni2+(aq) complex. The experimentally determined ligand-field strength is 10Dq = 1.1 eV. This and the Racah parameters characterizing 3d–3d Coulomb interactions B = 0.13 eV and C = 0.42 eV as readily derived from the measured energies match very well with the results from UV–vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra.
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