| 1. |
- Das, Sambit Kumar, 1994-, et al.
(author)
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Electronic Fingerprint of the Protonated Imidazole Dimer Probed by X-ray Absorption Spectroscopy
- 2024
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In: The Journal of Physical Chemistry Letters. - 1948-7185. ; 15:5, s. 1264-1272
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Journal article (peer-reviewed)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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| 2. |
- Das, Sambit, 1994-, et al.
(author)
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Probing the electronic structure of imidazole complexes in solution with quantum chemistry and X-ray absorption spectroscopy
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Other publication (other academic/artistic)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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| 3. |
- Eckert, Sebastian, et al.
(author)
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Electronic Structure Changes of an Aromatic Amine Photoacid along the Forster Cycle
- 2022
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In: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 61:27
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Journal article (peer-reviewed)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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| 4. |
- Jay, Raphael M., et al.
(author)
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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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Other publication (other academic/artistic)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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| 5. |
- Jay, Raphael M., et al.
(author)
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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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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 146:20, s. 14000-14011
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Journal article (peer-reviewed)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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