| 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, 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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| 4. |
- 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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| 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., 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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| 7. |
- Coates, Michael R., 1994-, et al.
(författare)
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Theoretical Investigation of Transient Speciesfollowing Photodissociation of Ironpentacarbonylin Ethanol Solution
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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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| 8. |
- Das, Sambit Kumar, 1994-, et al.
(författare)
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Simulating non-adiabatic dynamics of photoexcited phenyl azide : Investigating electronic and structural relaxation en route to the formation of phenyl nitrene
- 2024
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 30:7
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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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| 9. |
- Das, Sambit, 1994-, et al.
(författare)
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Theoretical simulations of the photodissociation of phenyl azide
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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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| 10. |
- Dunlop, David, et al.
(författare)
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Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha's Rule
- 2023
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 145:39, s. 21569-21575
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Tidskriftsartikel (refereegranskat)abstract
- Fluorescence exclusively occurs from the lowest excited state of a given multiplicity according to Kasha's rule. However, this rule is not obeyed by a handful of anti-Kasha fluorophores whose underlying mechanism is still understood merely on a phenomenological basis. This lack of understanding prevents the rational design and property-tuning of anti-Kasha fluorophores. Here, we propose a model explaining the photophysical properties of an archetypal anti-Kasha fluorophore, azulene, based on its ground- and excited-state (anti)aromaticity. We derived our model from a detailed analysis of the electronic structure of the ground singlet, first excited triplet, and quintet states and of the first and second excited singlet states using the perturbational molecular orbital theory and quantum-chemical aromaticity indices. Our model reveals that the anti-Kasha properties of azulene and its derivatives result from (i) the contrasting (anti)aromaticity of its first and second singlet excited states (S-1 and S-2, respectively) and (ii) an easily accessible antiaromaticity relief pathway of the S-1 state. This explanation of the fundamental cause of anti-Kasha behavior may pave the way for new classes of anti-Kasha fluorophores and materials with long-lived, high-energy excited states.
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