| 1. |
- Banerjee, Ambar, 1985-, et al.
(författare)
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Accessing metal-specific orbital interactions in C–H activation with resonant inelastic X-ray scattering
- 2024
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Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 15:7, s. 2398-2409
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Tidskriftsartikel (refereegranskat)abstract
- Photochemically prepared transition-metal complexes are known to be effective at cleaving the strong C–H bonds of organic molecules in room temperature solutions. There is also ample theoretical evidence that the two-way, metal to ligand (MLCT) and ligand to metal (LMCT), charge-transfer between an incoming alkane C–H group and the transition metal is the decisive interaction in the C–H activation reaction. What is missing, however, are experimental methods to directly probe these interactions in order to reveal what determines reactivity of intermediates and the rate of the reaction. Here, using quantum chemical simulations we predict and propose future time-resolved valence-to-core resonant inelastic X-ray scattering (VtC-RIXS) experiments at the transition metal L-edge as a method to provide a full account of the evolution of metal–alkane interactions during transition-metal mediated C–H activation reactions. For the model system cyclopentadienyl rhodium dicarbonyl (CpRh(CO)2), we demonstrate, by simulating the VtC-RIXS signatures of key intermediates in the C–H activation pathway, how the Rh-centered valence-excited states accessible through VtC-RIXS directly reflect changes in donation and back-donation between the alkane C–H group and the transition metal as the reaction proceeds via those intermediates. We benchmark and validate our quantum chemical simulations against experimental steady-state measurements of CpRh(CO)2 and Rh(acac)(CO)2 (where acac is acetylacetonate). Our study constitutes the first step towards establishing VtC-RIXS as a new experimental observable for probing reactivity of C–H activation reactions. More generally, the study further motivates the use of time-resolved VtC-RIXS to follow the valence electronic structure evolution along photochemical, photoinitiated and photocatalytic reactions with transition metal complexes.
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| 2. |
- Banerjee, Ambar, et al.
(författare)
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Photoinduced bond oscillations in ironpentacarbonyl give delayed synchronous bursts of carbonmonoxide release
- 2022
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- Early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed mechanistic details in the sub-picosecond regime. Hence, in this study the photoexcitation of ironpentacarbonyl Fe(CO)5 is simulated with semi-classical excited state molecular dynamics. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces synchronous Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)5 acts as a CO geyser, as a result of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered (MC) excited states. The predominant release of axial CO ligands and delayed release of equatorial CO ligands are explained in a unified mechanism based on the σ*(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states.
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| 3. |
- Banerjee, Ambar, et al.
(författare)
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Spectroscopic Signature of Dynamical Instability of the Aqueous Complex in the Brown-Ring Nitrate Test
- 2022
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 28:54
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Tidskriftsartikel (refereegranskat)abstract
- The chemistry of the brown-ring test has been investigated for nearly a century. Though recent studies have focused on solid state structure determination and measurement of spectra, mechanistic details and kinetics, the aspects of solution structure and dynamics remain unknown. We have studied structural fluctuations of the brown-ring complex in aqueous solution with ab-initio molecular dynamics simulations, from which we identified that the classically established pseudo-octahedral [Fe(H2O)5(NO)]2+ complex is present along with a square-pyramidal [Fe(H2O)4(NO)]2+ complex. Based on the inability in multi-reference calculations to reproduce the experimental UV-vis spectra in aqueous solution by inclusion of thermal fluctuations of the [Fe(H2O)5(NO)]2+ complex alone, we propose the existence of an equilibrium between pseudo-octahedral and square-pyramidal complexes. Despite challenges in constructing models reproducing the solid-state UV-vis spectrum, the advanced spectrum simulation tool motivates us to challenge the established picture of a sole pseudo-octahedral complex in solution.
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| 4. |
- Coates, Michael R., 1994-
(författare)
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Fundamental interactions in transition metal reactions
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transition metal complexes that participate in homogeneous reactions often perform the role of a catalyst, facilitating novel reaction pathways. When these complexes are pushed away from their equilibrium, the arrangement of the coordinating ligands around the metal center is perturbed and new reaction pathways are opened. By using a light-induced “trigger” to push the metal complex away from its equilibrium, this process can be initiated with precision. This thesis is concerned with the theoretical understanding of light-induced “triggered” reactions that generate transient, short-lived photoproducts in solution, capable of reacting with the surrounding solvent medium. A combination of theoretical and experimental tools are employed to give precise information about the formation and decay of these transient photoproducts.In an effort to understand the innate differences between a broad range of transition metal complexes, electron configurations of the metal and its coordinating ligands are a natural starting point. These distinct electronic structures define the physical structure of the transition metal complex and explain the reactivity or lack of reactivity of the transition metal complex. To describe these electronic structures, robust quantum chemistry methods are required. Coinciding with these methods is a theoretical framework that aims to simulate the evolution of molecules by means of a molecular dynamics simulation.The present work involves the study of ironpentacarbonyl or Fe(CO)5 which we use to explain the reactive landscape of a broad class of carbonyl coordinated transition metal complexes. The part of the thesis devoted to Fe(CO)5 is divided into distinct sections (i) the short-time (femtosecond-to-picosecond) gas-phase excited state molecular dynamics that produces the transient species, (ii) the long-time (picosecond-to-nanosecond) liquid-phase ground state molecular dynamics which describes the intermediates formed by the transient species and (iii) the experimental probes of the former sections. A final part of the thesis connects carbonyl containing metal complexes to another broad and detailed class of nitrosyl containing metal complexes.
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| 5. |
- Coates, Michael R., et al.
(författare)
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Simulations of the Aqueous "Brown-Ring" Complex Reveal Fluctuations in Electronic Character
- 2023
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 62:41, s. 16854-16866
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio molecular dynamics (AIMD) simulations of the aqueous [Fe(H2O)(5)(NO)](2+) "brown-ring" complex in different spin states, in combination with multiconfigurational quantum chemical calculations, show a structural dependence on the electronic character of the complex. Sampling in the quartet and sextet ground states show that the multiplicity is correlated with the Fe-N distance. This provides a motivation for a rigid Fe-N scan in the isolated "brown-ring" complex to investigate how the multiconfigurational wave function and the electron density change around the FeNO moiety. Our results show that subtle changes in the Fe-N distance produce a large response in the electronic configurations underlying the quartet wave function. However, while changes in spin density and potential energy are pronounced, variations in charge are negligible. These trends within the FeNO moiety are preserved in structural sampling of the AIMD simulations, despite distortions present in other degrees of freedom in the bulk solution.
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| 6. |
- Coates, Michael R.
(författare)
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Structure, dynamics and reactivity of low-oxidation state iron complexes in solution studied by ab initio molecular dynamics simulations and advanced quantum chemistry calculations.
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Third row (3d) metals, such as iron have become a candidate for a broad class of photocatalysts that have a large abundance on Earth and a low toxicity to humans and the environment. Unlike many commonly used photocatalysts that contain expensive precious metals, iron is cheap. Many important chemical processes such as the Haber-Bosch process or the Fenton’s reagent have employed an iron catalyst, however, in terms of metal complex photochemistry, this has been overshadowed by 4d and 5d metals with large affinities for unsaturated and saturated hydrocarbons. In an effort to understand the innate differences between a broad range of transition metals, electron configurations of the metal and its’ coordinating ligands are a natural starting point. The d-block orbitals can accommodate at most 10 electrons, while the splittings between the occupied and unoccupied orbitals are determined by the metal and the type of coordinating ligands. This often produces complicated electronic structures, with multiple low-lying spin states that can couple. To describe these electronic structures, robust quantum chemistry methods are required which can describe many geometric configurations of a metal complex in a variety of bonding conifgurations. Often these methods are coupled with dynamical simulation tools that can probe molecular processes in both the ground and excited electronic states in an isolated and bulk liquid environment.The present work aims to address many of these points by considering two different iron complexes: the brown-ring complex ([Fe(H2O)5(NO)]2+) and ironpentacarbonyl (Fe(CO)5). In the brown-ring complex, the ground state molecular dynamics (GSMD) have been simulated using Car-Parrinello molecular dynamics (CPMD) and the electronic properties have been presented. It is shown that a dynamical equilibrium between species have a unique spectroscopic signature, while the multireference character of the complex in the electronic ground state reveals a unique bondingconfiguration. In ironpentacarbonyl the excited state molecular dynamics (ESMD) have been performed to understand the mechanistic details that promote dissociation of one or more carbonyl ligands following excitation. In parallel to this study, the reactivity of the molecular fragments with the surrounding solvent molecules have been characterized.
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| 7. |
- Coates, Michael R., 1994-, et al.
(författare)
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Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution
- 2024
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Ingår i: Inorganic Chemistry. - 0020-1669 .- 1520-510X. ; 63:23, s. 10634-10647
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Tidskriftsartikel (refereegranskat)abstract
- Photodissociation of ironpentacarbonyl [Fe-1(CO)(5)] in solution generates transient species in different electronic states, which we studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound Fe-1(CO)(5) is found to interact weakly with the solvent, whereas the irontetracarbonyl [Fe(CO)(4)] species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state [Fe-1(CO)(4)] and weakly in the triplet state [Fe-3(CO)(4)]. From the simulations, we have gained insights into intersystem crossing in solvated irontetracarbonyl based on the distinct structural differences induced by the change in multiplicity. Alternative forms of coordination between Fe-1(CO)(4) and functional groups of the ethanol molecule are simulated, and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl gives information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for under-coordinated metal carbonyls in solution.
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| 8. |
- Coates, Michael R., 1994-, et al.
(författare)
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Theoretical Investigation of Transient Speciesfollowing Photodissociation of Ironpentacarbonylin Ethanol Solution
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Photodissociation of ironpentacarbonyl in solution generates transient species indifferent electronic states, which we have studied theoretically. From ab initio moleculardynamics simulations in ethanol solutions, the close-shell parent compound 1Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl species,formed after photodissociation, has a strongly spin-dependent behavior. It coordinatesa solvent molecule tightly in the singlet state (1Fe(CO)4) and weakly in the tripletstate (3Fe(CO)4). Alternative forms of coordination between 1Fe(CO)4 and functionalgroups of the ethanol molecule are simulated, and quantum chemical calculations ofthe energy landscape for the coordinated irontetracarbonyl give information aboutthe inter-conversion of different transient species in solution. Furthermore, insights1from the simulations challenge the previously proposed mechanism of chain walkingfor undercoordinate metal carbonyls in solution. Thereby, we gain insight into bothintersystem crossing in solvated irontetracarbonyl and its exchange in coordinationbetween Fe−OH and Fe−HC.
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| 9. |
- 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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| 10. |
- 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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| 11. |
- Jay, Raphael M., et al.
(författare)
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Photochemical Formation and Electronic Structure of an Alkane ?-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- C-H bond activation reactions with transition metals typically proceed via the formation of alkane ?-complexes, where an alkane C-H ?-bond binds to the metal. Due to the weak nature of metal-alkane bonds, ?-complexes are challenging to characterize experimentally. Here, we photochemically prepare the model ?-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photo-induced CO dissociation from Cr(CO)6 to occur within the 100 fs time-resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane ?-complex both from the metal and ligand perspectives. We find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane ?-complex, accompanied with rehybridization between metal and ligand orbitals. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the properties of alkane ?-complexes as the decisive intermediates in C-H bond activation reactions.
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| 12. |
- Jay, Raphael M., et al.
(författare)
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Photochemical Formation and Electronic Structure of an Alkane σ-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy
- 2024
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 146:20, s. 14000-14011
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Tidskriftsartikel (refereegranskat)abstract
- C–H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C–H σ-bond binds to the metal. Due to the weak nature of metal–alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal–ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)6 to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C–H bond to the undercoordinated Cr center in the Cr(CO)5-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal–alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C–H bond activation reactions.
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| 13. |
- Jay, Raphael, et al.
(författare)
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Tracking C–H activation with orbital resolution
- 2023
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 380:6648, s. 955-960
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal reactivity toward carbon-hydrogen (C-H) bonds hinges on the interplay of electron donation and withdrawal at the metal center. Manipulating this reactivity in a controlled way is difficult because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved x-ray spectroscopy, we track the charge-transfer interactions during C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex. Changes in oxidation state as well as valence-orbital energies and character emerge in the data on a femtosecond to nanosecond timescale. The x-ray spectroscopic signatures reflect how alkane-to-metal donation determines metal-alkane complex stability and how metal-to-alkane back-donation facilitates C-H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital level provides opportunities for manipulating C-H reactivity at transition metals.
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| 14. |
- Schröder, Henning, et al.
(författare)
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Varying photo-dissociation mechanisms in Fe(CO)5 and Cr(CO)6 from femtosecond valence photoemission and excited-state moleculardynamics simulations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present measured and calculated time-resolved photoelectron spectra of photoexcited gas-phase Fe(CO)5 and Cr(CO)6. Upon electronic excitation with 266 nm pump pulses and by probing with 23 eV photons from a femtosecond high-order har-monic generation source, we observe differences between Fe(CO)5 and Cr(CO)6 that indicate that the excited-state and dissociation dynamics are slower in Fe(CO)5 than in Cr(CO)6. Changing photoelectron intensities and binding energies combined with excited-state molecular dynamics simulations indicate repopulations of excited states from bound excited to dissociative excited states and to the dissociated species. We find that the more open and flexible structure of Fe(CO)5 with larger metal-carbonyl angles enables the photoexcited states of Fe(CO)5 to dissipate energy by angular distortions as observed in longer populations of bound excited states. The more compactand closed structure of Cr(CO)6 does not enable this relaxation resulting in fasterdissociation.
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| 15. |
- Vincent, Jonathan, et al.
(författare)
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Solvent dependent structural perturbations of chemical reaction intermediates visualized by time-resolved x-ray diffraction
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:15, s. 154502-
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast time-resolved wide angle x-ray scattering from chemical reactions in solution has recently emerged as a powerful technique for determining the structural dynamics of transient photochemical species. Here we examine the structural evolution of photoexcited CH2I2 in the nonpolar solvent cyclohexane and draw comparisons with a similar study in the polar solvent methanol. As with earlier spectroscopic studies, our data confirm a common initial reaction pathway in both solvents. After photoexcitation, CH2I2 dissociates to form CH2I center dot+I center dot. Iodine radicals remaining within the solvent cage recombine with a nascent CH2I center dot radical to form the transient isomer CH2I-I, whereas those which escape the solvent cage ultimately combine to form I-2 in cyclohexane. Moreover, the transient isomer has a lifetime approximately 30 times longer in the nonpolar solvent. Of greater chemical significance is the property of time-resolved wide angle x-ray diffraction to accurately determine the structure of the of CH2I-I reaction intermediate. Thus we observe that the transient iodine-iodine bond is 0.07 A +/- 0.04 A shorter in cyclohexane than in methanol. A longer iodine-iodine bond length for the intermediate arises in methanol due to favorable H-bond interaction with the polar solvent. These findings establish that time-resolved x-ray diffraction has sufficient sensitivity to enable solvent dependent structural perturbations of transient chemical species to be accurately resolved.
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| 16. |
- Wernet, Philippe, et al.
(författare)
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Mapping chemical bonding of reaction intermediates with femtosecond X-ray laser spectroscopy
- 2013
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Ingår i: XVIIIth International Conference on Ultrafast Phenomena. - : EDP Sciences. - 9782759809561 ; , s. 05025-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We determine the pathways in the photo-dissociation reactions of Fe(CO)5 both in the gas phase and in solution by mapping the valence electronic structure of the reaction intermediates with femtosecond X-ray laser spectroscopy.
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| 17. |
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| 18. |
- Aquilante, Francesco, et al.
(författare)
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Modern quantum chemistry with [Open]Molcas
- 2020
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:21
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Tidskriftsartikel (refereegranskat)abstract
- MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.
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| 19. |
- Banerjee, Ambar, 1985-, et al.
(författare)
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Simulating fluorine K -edge resonant inelastic x-ray scattering of sulfur hexafluoride and the effect of dissociative dynamics
- 2023
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Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : American Physical Society (APS). - 2469-9926 .- 2469-9934. ; 108:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on a computational study of resonant inelastic x-ray scattering (RIXS), at different fluorine K-edge resonances of the SF6 molecule, and corresponding nonresonant x-ray emission. Previously measured polarization dependence in RIXS is reproduced and traced back to the local σ and π symmetry of the molecular orbitals and corresponding states involved in the RIXS process. Also electron-hole coupling energies are calculated and related to experimentally observed spectator shifts. The role of dissociative S-F bond dynamics is explored to model detuning of RIXS spectra at the |F1s-16a1g1) resonance, which shows challenges to accurately reproduce the required steepness for core-excited potential energy surface. We show that the RIXS spectra can only be properly described by considering breaking of the global inversion symmetry of the electronic wave function and core-hole localization, induced by vibronic coupling. Due to the core-hole localization we have symmetry forbidden transitions, which lead to additional resonances and changing width of the RIXS profile.
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| 20. |
- Bednarska, Joanna, et al.
(författare)
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Elucidating the Mechanism of Zn2+ Sensing by a Bipyridine Probe Based on Two-Photon Absorption
- 2016
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 120:34, s. 9067-9075
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we examine, by means of computational methods, the mechanism of Zn2+ sensing by a bipyridine-centered, D-pi-A-pi-D-type-ratiometric molecular probe. According to recently published experimental data [Divya, K. P.; Sreejith, S.; Ashokkumar, P.; Yuzhan, K.; Peng, Q; Maji, S. K.; Tong, Y.; Yu, H.; Zhao, Y.; Ramamurthy, P.; Ajayaghosh, A. A ratiometric fluorescent molecular -probe with enhanced two-photon response upon Zn2+ binding for in vitro and in vivo: bioimaging.= Chem. Sci. 2014, S, 3469-3474], after coordination to zinc ions the -probe exhibits a large enhancement of the two -photon absorption cross section. The goal of our investigation was to elucidate the mechanism behind this phenomenon. For this purpose, linear and nonlinear optical properties of -the unbound (cation-free) and bound probe were calculated, including the influence of solute Solvent interactions, implicitly using a polarizable continuum model and exp-licitely employing the QM/MM approach. Because the results of the calculations indicate that many conformers of the probe are energetically accessible at room temperature in solution and hence contribute to the Signal, structurepteperty relationships were also taken into account. Results of our simulations-demonstrate that the one-photon absorption bands for both the unbound -and bound forms correspond to the bright pi -> pi* transition to the first excited state; which, on the other hand,. exhibits negligible two-photon activity. On the basis of the results of the quadratic respOnse calculations, we put forward-notion that it is the second excited state that gives the strong signal in the experimental nonlinear spectrum. To explain the differenCes in the two-photon absorption activity for the two lowest-lying excited states and nonlinear response enhancement upon binding, we employed the generalized few -state model including the ground, first, and- second excited states. The analysis of the optical channel suggests that the large two-photon response is due to the coordination -induced increase of the, transition- moment from the first to the second excited state.
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| 21. |
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| 22. |
- Blomdahl, Kajsa-My
(författare)
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Theoretical and Numerical Studies of Efimov States
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In contrast to the classical case, the quantum three-body problem is amenable to qualitative analysis and, in some cases, even to analytic solutions. In 1970, Vitaly Efimov predicted that resonant two-body forces could give rise to a series of bound energy levels in three-particle systems. When the short-ranged two-body forces approached resonance, he found a universal long-range three-body attraction emerging, giving rise to an infinite number of trimer states with binding energies obeying a discrete scaling law at resonance. This oddity in the three-body spectrum close to the zero-energy threshold has since become known as the quantum Efimov effect and the term Efimov physics now covers an array of universal phenomena arising in few-body systems, for particles interacting via short-ranged resonant interactions, whose appearance is due to an emergent three-body attractive force.In this thesis I aim to summarize the theory of Efimov physics and the methodology used for developing a computer code that calculates the effective long-range three-body potentials, which give rise to the discrete Efimov energy spectrum. The calculations of these potentials were performed by formulating the problem in hyperspherical coordinates and introducing the adiabatic representation where the hyperradius is treated as an adiabatic parameter.
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| 23. |
- Blum, M., et al.
(författare)
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Ultrafast Proton Dynamics in Aqueous Amino Acid Solutions Studied by Resonant Inelastic Soft X-ray Scattering
- 2012
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 116:46, s. 13757-13764
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Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic soft X-ray scattering (RIXS) has been used to study the electronic structure of glycine and lysine in aqueous solution. Upon variation of the pH value of the solution from acidic to basic, major changes of the nitrogen K edge RIXS data are observed for both amino acids, which are associated with the protonation and deprotonation of the amino groups. The experimental results are compared with simulations based on density functional theory, yielding a detailed understanding of the spectral changes, as well as insights into the ultrafast proton dynamics in the intermediate core-excited/ionized state of the RIXS process.
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| 24. |
- Brandt, Erik G., 1982-
(författare)
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Interactions and dynamics in biophysical model systems
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Computer simulations of simplified model systems provide understanding of how complex biological systems behave. The simulations give detailed information about the systems, with atomistic resolution, that can be used in combination with experimental knowledge to shed light on underlying physical principles. The thesis presents background information about the studies of two important model systems in biological physics. First, metal ion-binding to proteins is investigated in a computational study on a zinc-binding synthetic peptide, to elucidate the binding details. The major scientific contributions from the study are the identification and mapping of the detailed contributions to the binding. A novel correction scheme is worked out, where classic free energy calculations are combined with density functional theory to adjust for quantum mechanical effects. The sensitivity of the zinc-binding to a specific amino acid segment can be explained in terms of the zinc coordination. Second, equilibrium density fluctuations in biological membranes are studied using computer simulations of the lipid bilayer. The fluctuations are linked to several processes; pore formation, membrane permeability and transport of small molecules across themembrane. Because the lipid bilayer behaves similar to a 2D fluid the density fluctuations can be described in the framework of generalized hydrodynamics. The major scientific contributions from the study are the direct calculation of the density-density autocorrelation function from raw data and the observation that the diffusive contribution to the power spectrum (the Rayleigh line) is not single-exponential. In addition, the accuracy of the approximate hydrodynamic solutions is questionable for the propagation of sound waves in the membrane.
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| 25. |
- C. Couto, Rafael, 1987-, et al.
(författare)
-
Selective gating to vibrational modes through resonant X-ray scattering
- 2017
-
Ingår i: Nature Communications. - : Macmillan Publishers Ltd.. - 2041-1723. ; 8, s. 14165-1-14165-7
-
Tidskriftsartikel (refereegranskat)abstract
- The dynamics of fragmentation and vibration of molecular systems with a large number of coupled degrees of freedom are key aspects for understanding chemical reactivity and properties. Here we present a resonant inelastic X-ray scattering (RIXS) study to show how it is possible to break down such a complex multidimensional problem into elementary components. Local multimode nuclear wave packets created by X-ray excitation to different core-excited potential energy surfaces (PESs) will act as spatial gates to selectively probe the particular ground-state vibrational modes and, hence, the PES along these modes. We demonstrate this principle by combining ultra-high resolution RIXS measurements for gas-phase water with state-of-the-art simulations.
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| 26. |
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| 27. |
- Chaudhuri, A, et al.
(författare)
-
The structure of the Au(111)/methylthiolate interface : new insights from near-edge x-ray absorption spectroscopy and x-ray standing waves.
- 2009
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:12, s. 124708-
-
Tidskriftsartikel (refereegranskat)abstract
- The local structure of the Au(111)(√3x√3)R30 degrees-methylthiolate surface phase has been investigated by S K-edge near-edge s-ray absorption fine structure (NEXAFS) both experimentally and theoretically and by experimental normal-incidence x-ray standing waves (NIXSW) at both the C and S atomic sites. NEXAFS shows not only excitation into the intramolecular σ*S-C resonance but also into a σ* S-Au orbital perpendicular to the surface, clearly identifying the local S headgroup site as atop a Au atom. Simulations show that it is not possible, however, to distinguish between the two possible adatom reconstruction models; a single thiolate species atop a hollow-site Au adatom or a dithiolate moiety comprising two thiolate species bonded to a bridge-bonded Au adatom. Within this dithiolate moiety a second σ* S-Au orbital that lies near parallel to the surface has a higher energy that overlaps that of the σ* S-C resonance. The new NIXSW data show the S-C bond to be tilted by 61 degrees relative to the surface normal, with a preferred azimuthal orientation in <211>, corresponding to the intermolecular nearest-neighbor directions. This azimuthal orientation is consistent with the thiolate being atop a hollow-site Au adatom, but not consistent with the originally proposed Au-adatom-dithiolate moiety. However, internal conformational changes within this species could, perhaps, render this model also consistent with the experimental data.
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| 28. |
- da Cruz, Vinicius Vaz, et al.
(författare)
-
Anomalous polarization dependence in vibrationally resolved resonant inelastic x-ray scattering of H2O
- 2018
-
Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : American Physical Society. - 2469-9926 .- 2469-9934. ; 98:1
-
Tidskriftsartikel (refereegranskat)abstract
- It is well established that different electronic channels, in resonant inelastic x-ray scattering (RIXS), display different polarization dependences due to different orientations of their corresponding transition dipole moments in the molecular frame. However, this effect does not influence the vibrational progression in the Franck-Condon approximation. We have found that the transition dipole moments of core excitation and deexcitation experience ultrafast rotation during dissociation in the intermediate core-excited state. This rotation makes the vibrational progression in RIXS sensitive to the polarization of the x-ray photons. We study the water molecule, in which the effect is expressed in RIXS through the dissociative core-excited state where the vibrational scattering anisotropy is accompanied also by violation of parity selection rules for the vibrations.
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| 29. |
- da Cruz, Vinicius Vaz, et al.
(författare)
-
Nuclear dynamics in resonant inelastic X-ray scattering and X-ray absorption of methanol
- 2019
-
Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 150:23
-
Tidskriftsartikel (refereegranskat)abstract
- We report on a combined theoretical and experimental study of core-excitation spectra of gas and liquid phase methanol as obtained with the use of X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). The electronic transitions are studied with computational methods that include strict and extended second-order algebraic diagrammatic construction [ADC(2) and ADC(2)-x], restricted active space second-order perturbation theory, and time-dependent density functional theory-providing a complete assignment of the near oxygen K-edge XAS. We show that multimode nuclear dynamics is of crucial importance for explaining the available experimental XAS and RIXS spectra. The multimode nuclear motion was considered in a recently developed "mixed representation" where dissociative states and highly excited vibrational modes are accurately treated with a time-dependent wave packet technique, while the remaining active vibrational modes are described using Franck-Condon amplitudes. Particular attention is paid to the polarization dependence of RIXS and the effects of the isotopic substitution on the RIXS profile in the case of dissociative core-excited states. Our approach predicts the splitting of the 2a RIXS peak to be due to an interplay between molecular and pseudo-atomic features arising in the course of transitions between dissociative core- and valence-excited states. The dynamical nature of the splitting of the 2a peak in RIXS of liquid methanol near pre-edge core excitation is shown. The theoretical results are in good agreement with our liquid phase measurements and gas phase experimental data available from the literature. (C) 2019 Author(s).
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| 30. |
- da Cruz, Vinicius Vaz, et al.
(författare)
-
Probing hydrogen bond strength in liquid water by resonant inelastic X-ray scattering
- 2019
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10
-
Tidskriftsartikel (refereegranskat)abstract
- Local probes of the electronic ground state are essential for understanding hydrogen bonding in aqueous environments. When tuned to the dissociative core-excited state at the O1s pre-edge of water, resonant inelastic X-ray scattering back to the electronic ground state exhibits a long vibrational progression due to ultrafast nuclear dynamics. We show how the coherent evolution of the OH bonds around the core-excited oxygen provides access to high vibrational levels in liquid water. The OH bonds stretch into the long-range part of the potential energy curve, which makes the X-ray probe more sensitive than infra-red spectroscopy to the local environment. We exploit this property to effectively probe hydrogen bond strength via the distribution of intramolecular OH potentials derived from measurements. In contrast, the dynamical splitting in the spectral feature of the lowest valence-excited state arises from the short-range part of the OH potential curve and is rather insensitive to hydrogen bonding.
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| 31. |
- Das, Sambit, 1994-
(författare)
-
A theoretical perspective on photoinduced reactions - based on quantum chemical models and non-adiabatic molecular dynamics.
- 2023
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The broad range of applications for photochemical reactions is the result of light-matter interaction at the electronic level. The diverse application of photochemistry in various fields, including photovoltaic materials, molecular switches, and biological systems are due to electronic and structural transformations induced by photoexcitation as well as molecular alteration due to electron and charge transfer. An improved understanding of these photochemical events is dependent on the fundamental theoretical evaluation, to model and analyze the ultrafast processes. The studies discussed in this thesis explore such theoretical implementation in two different frontiers.In the first study, dynamic simulations are performed to model the light-induced bond dissociation of phenyl azide. The surface hopping formalism, implemented under the semiclassical molecular dynamics approach helped in tracing the time evolution of the electronic and structural levels, involved in the photodissociation. In the second study, the time-dependent density functional theory has been applied to generate XA spectra of imidazole solutions. The theoretical assessments support experimental measurements and provide more insight into the core excitations and structural influence on the absorption spectra.
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| 32. |
- Das, Sambit Kumar, 1994-, et al.
(författare)
-
Electronic Fingerprint of the Protonated Imidazole Dimer Probed by X-ray Absorption Spectroscopy
- 2024
-
Ingår i: The Journal of Physical Chemistry Letters. - 1948-7185. ; 15:5, s. 1264-1272
-
Tidskriftsartikel (refereegranskat)abstract
- Protons in low-barrier superstrong hydrogen bonds are typically delocalized between two electronegative atoms. Conventional methods to characterize such superstrong hydrogen bonds are vibrational spectroscopy and diffraction techniques. We introduce soft X-ray spectroscopy to uncover the electronic fingerprints for proton sharing in the protonated imidazole dimer, a prototypical building block enabling effective proton transport in biology and high-temperature fuel cells. Using nitrogen core excitations as a sensitive probe for the protonation status, we identify the X-ray signature of a shared proton in the solvated imidazole dimer in a combined experimental and theoretical approach. The degree of proton sharing is examined as a function of structural variations that modify the shape of the low-barrier potential in the superstrong hydrogen bond. We conclude by showing how the sensitivity to the quantum distribution of proton motion in the double-well potential is reflected in the spectral signature of the shared proton.
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| 33. |
- Das, Sambit Kumar, 1994-, et al.
(författare)
-
Simulating non-adiabatic dynamics of photoexcited phenyl azide : Investigating electronic and structural relaxation en route to the formation of phenyl nitrene
- 2024
-
Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 30:7
-
Tidskriftsartikel (refereegranskat)abstract
- Excited state molecular dynamics simulations of the photoexcited phenyl azide have been performed. The semi-classical surface hopping approximation has enabled an unconstrained analysis of the electronic and nuclear degrees of freedom which contribute to the molecular dissociation of phenyl azide into phenyl nitrene and molecular nitrogen. The significance of the second singlet excited state in leading the photodissociation has been established through electronic structure calculations, based on multi-configurational schemes, and state population dynamics. The investigations on the structural dynamics have revealed the N−N bond separation to be accompanied by synchronous changes in the azide N−N−N bond angle. The 100 fs simulation results in a nitrene fragment that is electronically excited in the singlet manifold.
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| 34. |
- Das, Sambit, 1994-, et al.
(författare)
-
Probing the electronic structure of imidazole complexes in solution with quantum chemistry and X-ray absorption spectroscopy
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- By combing soft X-ray spectroscopy and theoretical calculations, characterization of the shared proton in the imidazole molecular complex has been done. With nitrogen core-level excitations as a sensitive reporter about the protonation status, a new absorption resonance is observed at a pH where exactly half of the imidazoles are protonated, right between the known absorptions of pure imidazole and pure imidazolium. Supported by TDDFT calculations, the spectral signature has been assigned to the sharing of the excess proton between two imidazole molecules in an asymmetric double minimum potential. Analysis of the discrete core excitations reveals shared electronic attributes between the molecular complex and individual monomers. The theoretical investigation also uncovers the influence of the shared proton on the intrinsic features and the overall spectral outcome.
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| 35. |
- Das, Sambit, 1994-, et al.
(författare)
-
Theoretical simulations of the photodissociation of phenyl azide
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Excited state molecular dynamics simulations of the photoexcitation of phenyl azide have been performed. The semi-classical surface hopping approximation has enabled an unconstrained analysis of the electronic and nuclear degrees of freedom which contribute to the molecular dissociation of phenyl azide into phenyl nitrene and molecular nitrogen. The significance of the S2 state in leading the photodissociation has been established through electronic structure calculations, based on multiconfigurational schemes, and state population dynamics. The investigations on the structural dynamics have revealed the N-N bond separation to be accompanied by synchronous changes in the azide N-N-N bond angle. The 100 fs simulation results in a nitrene fragment that is electronically and vibrationally excited, thus forming a hot nitrene species in the singlet manifold.
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| 36. |
- de Groot, Frank M. F., et al.
(författare)
-
2p x-ray absorption spectroscopy of 3d transition metal systems
- 2021
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 249
-
Tidskriftsartikel (refereegranskat)abstract
- This review provides an overview of the different methods and computer codes that are used to interpret 2p x-ray absorption spectra of 3d transition metal ions. We first introduce the basic parameters and give an overview of the methods used. We start with the semi-empirical multiplet codes and compare the different codes that are available. A special chapter is devoted to the user friendly interfaces that have been written on the basis of these codes. Next we discuss the first principle codes based on band structure, including a chapter on Density Functional theory based approaches. We also give an overview of the first-principle multiplet codes that start from a cluster calculation and we discuss the wavefunction based methods, including multi-reference methods. We end the review with a discussion of the link between theory and experiment and discuss the open issues in the spectral analysis.
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| 37. |
- Eckert, Sebastian, et al.
(författare)
-
Electronic Structure Changes of an Aromatic Amine Photoacid along the Forster Cycle
- 2022
-
Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 61:27
-
Tidskriftsartikel (refereegranskat)abstract
- Photoacids show a strong increase in acidity in the first electronic excited state, enabling real-time studies of proton transfer in acid-base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Forster cycle. Here we use picosecond nitrogen K-edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core-to-valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon.
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| 38. |
- Eckert, Sebastian, et al.
(författare)
-
One-dimensional cuts through multidimensional potential-energy surfaces by tunable x rays
- 2018
-
Ingår i: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : AMER PHYSICAL SOC. - 2469-9926 .- 2469-9934. ; 97:5
-
Tidskriftsartikel (refereegranskat)abstract
- The concept of the potential-energy surface (PES) and directional reaction coordinates is the backbone of our description of chemical reaction mechanisms. Although the eigenenergies of the nuclear Hamiltonian uniquely link a PES to its spectrum, this information is in general experimentally inaccessible in large polyatomic systems. This is due to (near) degenerate rovibrational levels across the parameter space of all degrees of freedom, which effectively forms a pseudospectrum given by the centers of gravity of groups of close-lying vibrational levels. We show here that resonant inelastic x-ray scattering (RIXS) constitutes an ideal probe for revealing one-dimensional cuts through the ground-state PES of molecular systems, even far away from the equilibrium geometry, where the independent-mode picture is broken. We strictly link the center of gravity of close-lying vibrational peaks in RIXS to a pseudospectrum which is shown to coincide with the eigenvalues of an effective one-dimensional Hamiltonian along the propagation coordinate of the core-excited wave packet. This concept, combined with directional and site selectivity of the core-excited states, allows us to experimentally extract cuts through the ground-state PES along three complementary directions for the showcase H2O molecule.
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| 39. |
- Eckert, Sebastian, et al.
(författare)
-
T-1 Population as the Driver of Excited-State Proton-Transfer in 2-Thiopyridone
- 2019
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 25:7, s. 1733-1739
-
Tidskriftsartikel (refereegranskat)abstract
- Excited-state proton transfer (ESPT) is a fundamental process in biomolecular photochemistry, but its underlying mediators often evade direct observation. We identify a distinct pathway for ESPT in aqueous 2-thiopyridone, by employing transient N1s X-ray absorption spectroscopy and multi-configurational spectrum simulations. Photoexcitations to the singlet S-2 and S-4 states both relax promptly through intersystem crossing to the triplet T-1 state. The T-1 state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S-0 potential energy surface. This conclusively establishes a dominant ESPT pathway for the system in aqueous solution, which is also compatible with previous measurements in acetonitrile. Thereby, the hitherto open questions of the pathway for ESPT in the compound, including its possible dependence on excitation wavelength and choice of solvent, are resolved.
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| 40. |
- Ekholm, V., et al.
(författare)
-
Core-hole localization and ultra-fast dissociation in SF6
- 2020
-
Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 53:18
-
Tidskriftsartikel (refereegranskat)abstract
- Resonant inelastic x-ray scattering spectra excited at the fluorine K resonances of SF(6)have been recorded. While a small but significant propensity for electronically parity-allowed transitions is found, the observation of parity-forbidden electronic transitions is attributed to vibronic coupling that breaks the global inversion symmetry of the electronic wavefunction and localizes the core hole. The dependence of the scattering cross section on the polarization of the incident radiation and the scattering angle is interpreted in terms of local pi/sigma symmetry around the S-F bond. This symmetry selectivity prevails during the dissociation that occurs during the scattering process.
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| 41. |
- Ekimova, Maria, et al.
(författare)
-
Aqueous Solvation of Ammonia and Ammonium : Probing Hydrogen Bond Motifs with FT-IR and Soft X-ray Spectroscopy
- 2017
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 139:36, s. 12773-12783
-
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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| 42. |
- Ekimova, Maria, et al.
(författare)
-
From Local Covalent Bonding to Extended Electric Field Interactions in Proton Hydration
- 2022
-
Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 61:46
-
Tidskriftsartikel (refereegranskat)abstract
- Seemingly simple yet surprisingly difficult to probe, excess protons in water constitute complex quantum objects with strong interactions with the extended and dynamically changing hydrogen-bonding network of the liquid. Proton hydration plays pivotal roles in energy transport in hydrogen fuel cells and signal transduction in transmembrane proteins. While geometries and stoichiometry have been widely addressed in both experiment and theory, the electronic structure of these specific hydrated proton complexes has remained elusive. Here we show, layer by layer, how utilizing novel flatjet technology for accurate x-ray spectroscopic measurements and combining infrared spectral analysis and calculations, we find orbital-specific markers that distinguish two main electronic-structure effects: Local orbital interactions determine covalent bonding between the proton and neigbouring water molecules, while orbital-energy shifts measure the strength of the extended electric field of the proton. © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.
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| 43. |
- Ekimova, Maria, et al.
(författare)
-
Soft X-ray Spectroscopy of the Amine Group : Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid-Base Pairs
- 2018
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 122:31, s. 7737-7746
-
Tidskriftsartikel (refereegranskat)abstract
- We use N K-edge absorption spectroscopy to explore the electronic structure of the amine group, one of the most prototypical chemical functionalities playing a key role in acid base chemistry, electron donor-acceptor interactions, and nucleophilic substitution reactions. In this study, we focus on aliphatic amines and make use of the nitrogen is core electron excitations to elucidate the roles of N-H sigma* and N-C sigma* contributions in the unoccupied orbitals. We have measured N K-edge absorption spectra of the ethylamine bases EtxNH3-x (x = 0...3; Et- = C2H5-) and the conjugate positively charged ethylammonium cation acids EtyNH4-y+ (y = 0...4; Et- = C2H5-) dissolved in the protic solvents ethanol and water. Upon consecutive exchange of N-H for ethyl-groups, we observe a spectral shift, a systematic decrease of the N K-edge pre-edge peak, and a major contribution in the post edge region for the ethylamine series. Instead, for the ethylammonium ions, the consecutive exchange of N-H for ethyl groups leads to an apparent reduction of pre-edge and post-edge intensities relative to the main-edge band, without significant frequency shifts. Building on findings from our previously reported study on aqueous ammonia and ammonium ions, we can rationalize these observations by comparing calculated N K-edge absorption spectra of free and hydrogen-bonded clusters. Hydrogen bonding interactions lead only to minor spectral effects in the ethylamine series, but have a large impact in the ethylammonium ion series. Visualization of the unoccupied molecular orbitals shows the consecutive change in molecular orbital character from N-H sigma* to N-C sigma* in these alkylamine/alkylammonium ion series. This can act as a benchmark for future studies on chemically more involved amine compounds.
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| 44. |
- Erbing, Axel, 1991-
(författare)
-
In pursuit of next generation photovoltaics : An electronic structure study of emerging solar cell materials
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of a new generation of photovoltaic technologies is an important task in order to increase the production of clean energy. Perovskite solar cells, with an exceptionally rapid development over the last decade, have transformed into perhaps the most promising candidate to provide a low-cost alternative to conventional cells. While having excellent efficiency, the most successful category of photovoltaic perovskites, the class of hybrid lead-halide perovskites, suffers from poor stability in ambient conditions and gives rise to potential health concerns due to lead toxicity. Because of these issues, studies yielding a better understanding of lead-based perovskites and investigations of new, lead-free materials are likely meaningful steps towards better and more competitive solar cells. This thesis contains studies about established lead-based perovskites, CH3NH3PbI3 and CH(NH2)2PbI3, as well as the lead-free alternatives AgBi2I7 and Cs2AgBiI6. The main method employed is electronic structure calculations through density functional theory under periodic boundary conditions including band structure calculations and projected density of states. A particular focus is given to systems with mixed anion and related effects on the electronic structure.
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| 45. |
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| 46. |
- Erbing, Axel, 1991-, et al.
(författare)
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Spatial microheterogeneity in the valence band of mixed halide hybrid perovskite materials
- 2022
-
Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 13:32, s. 9285-9294
-
Tidskriftsartikel (refereegranskat)abstract
- The valence band of lead halide hybrid perovskites with a mixed I/Br composition is investigated using electronic structure calculations and complementarily probed with hard X-ray photoelectron spectroscopy. In the latter, we used high photon energies giving element sensitivity to the heavy lead and halide ions and we observe distinct trends in the valence band as a function of the I : Br ratio. Through electronic structure calculations, we show that the spectral trends with overall composition can be understood in terms of variations in the local environment of neighboring halide ions. From the computational model supported by the experimental evidence, a picture of the microheterogeneity in the valence band maximum emerges. The microheterogeneity in the valence band suggests that additional charge transport mechanisms might be active in lead mixed halide hybrid perovskites, which could be described in terms of percolation pathways.
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| 47. |
- Eriksson K., Susanna, et al.
(författare)
-
Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed using photoelectron spectroscopy and DFT
- 2016
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 18:1, s. 252-260
-
Tidskriftsartikel (refereegranskat)abstract
- The effects of alkoxy chain length in triarylamine based donor acceptor organic dyes are investigated with respect to the electronic and molecular surface structures on the performance of solar cells and the electron lifetime. The dyes were investigated when adsorbed on TiO2 in a configuration that can be used for dye sensitized solar cells (DSCs). Specifically, the two dyes D35 and D45 were compared using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The differences in solar cell characteristics when longer alkoxy chains are introduced in the dye donor unit are attributed to geometrical changes in dye packing while only minor differences were observed in the electronic structure. A higher dye load was observed for D45 on TiO2. However, D35 based solar cells result in higher photocurrent although the dye load is lower. This is explained by different geometrical structures of the dyes on the surface.
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| 48. |
- Eriksson, Susanna K., et al.
(författare)
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In-Situ Probing of H2O Effects on a Ru-Complex Adsorbed on TiO2 Using Ambient Pressure Photoelectron Spectroscopy
- 2016
-
Ingår i: Topics in catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 59:5-7, s. 583-590
-
Tidskriftsartikel (refereegranskat)abstract
- Dye-sensitized interfaces in photocatalytic and solar cells systems are significantly affected by the choice of electrolyte solvent. In the present work, the interface between the hydrophobic Ru-complex Z907, a commonly used dye in molecular solar cells, and TiO2 was investigated with ambient pressure photoelectron spectroscopy (AP-PES) to study the effect of water atmosphere on the chemical and electronic structure of the dye/TiO2 interface. Both laboratory-based Al K alpha as well as synchrotron-based ambient pressure measurements using hard X-ray (AP-HAXPES) were used. AP-HAXPES data were collected at pressures of up to 25 mbar (i.e., the vapor pressure of water at room temperature) showing the presence of an adsorbed water overlayer on the sample surface. Adopting a quantitative AP-HAXPES analysis methodology indicates a stable stoichiometry in the presence of the water atmosphere. However, solvation effects due to the presence of water were observed both in the valence band region and for the S 1s core level and the results were compared with DFT calculations of the dye-water complex.
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| 49. |
- Eriksson, Susanna K., et al.
(författare)
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Solvent Dependence of the Electronic Structure of I- and I-3(-)
- 2014
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 118:11, s. 3164-3174
-
Tidskriftsartikel (refereegranskat)abstract
- We present synchrotron-based I4d photoelectron spectroscopy experiments of solutions from LiI and LiI3 in water, ethanol, and acetonitrile. The experimentally determined solvent-induced binding energy shifts (SIBES) for the monatomic I- anion are compared to predictions from simple Born theory, PCM calculations, as well as multiconfigurational quantum chemical spectral calculations from geometries obtained through molecular dynamics of solvated clusters. We show that the SIBES for I- explicitly depend on the details of the hydrogen bonding configurations of the solvent to the I- and that static continuum models such as the Born model cannot capture the trends in the SIBES observed both in experiments and in higher-level calculations. To extend the discussion to more complex polyatomic anions, we also performed experiments on I-3(-) and I-/I-3(-) mixtures in different solvents and the results are analyzed in the perspective of SIBES. The experimental SIBES values indicate that the solvation effects even for such similar anions as I- and I-3(-) can be rather different in nature.
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| 50. |
- Ertan, Emelie, 1988-
(författare)
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Ab initio simulations of vibrational and electronic structure evaluated against K-edge resonant inelastic X-ray scattering
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work of this thesis is focused on investigation of electronic and molecular structure and dynamics a molecular species in solutions and minerals in relation to resonant inelastic X-ray scattering (RIXS). The studies have been carried out by means of quantum chemistry calculations using density functional theory (DFT) and multi-configurational methods and quantum dynamics simulations using wave packet dynamics and ab initio molecular dynamics. By applications of these computational tools, I have studied the ultra-fast core-excited state dynamics and the RIXS process. With theoretical spectrum simulations, we can analyse the vibrational information of RIXS.The aim of these investigations is to probe the local electronic structure of ammonia and the hydroxyl group and their chemical interactions with the environment, e.g. hydrogen bonding, in liquid and solid state molecular systems. In Paper I, DFT calculations and wave packet dynamics has been used to simulate the O K-edge X-ray absorption (XAS) spectra and RIXS spectra to investigate the local vibrational structure of the hydroxyl groups of the kaolinite crystal. In Paper II, the N K-edge XAS, XES and RIXS spectra are simulated by application of DFT and multi-configurational methods combined with ab initio molecular dynamics with the aim to study the hydrogen bonding environment of aqueous NH3.
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