| 1. |
- Eckert, Sebastian, et al.
(författare)
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Electronic Structure Changes of an Aromatic Amine Photoacid along the Forster Cycle
- 2022
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Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 61:27
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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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| 2. |
- Ekimova, Maria, et al.
(författare)
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Aqueous Solvation of Ammonia and Ammonium : Probing Hydrogen Bond Motifs with FT-IR and Soft X-ray Spectroscopy
- 2017
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 139:36, s. 12773-12783
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Tidskriftsartikel (refereegranskat)abstract
- In a multifaceted investigation combining local soft X-ray and vibrational spectroscopic probes with ab initio molecular dynamics simulations, hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH3) and ammonium ion (NH4+), are quantitatively assessed in terms of electronic structure, solvation structure, and dynamics. From the Xray measurements and complementary determination of the IR-active hydrogen stretching and bending modes of NH3 and NH4+ in aqueous solution, the picture emerges of a comparatively strongly hydrogen-bonded NH4+ ion via N-H donating interactions, whereas NH3 has a strongly accepting hydrogen bond with one water molecule at the nitrogen lone pair but only weakly N-H donating hydrogen bonds. In contrast to the case of hydrogen bonding among solvent water moleCules, we find that energy mismatch between occupied orbitals of both the solutes NH3 and NH4+ and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding and, thus, inhibits substantial charge transfer between solute and solvent. A close inspection of the calculated unoccupied molecular orbitals, in conjunction with experimentally measured N K-edge absorption spectra, reveals the different nature of the electronic structural effects of these two key principal amine compounds imposed by hydrogen bonding to water, where a pH-dependent excitation energy appears to be an intrinsic property. These results provide a benchmark for hydrogen bonding of other nitrogen-containing acids and bases.
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| 3. |
- Ekimova, Maria, et al.
(författare)
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From Local Covalent Bonding to Extended Electric Field Interactions in Proton Hydration
- 2022
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Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 61:46
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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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| 4. |
- Ekimova, Maria, et al.
(författare)
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Soft X-ray Spectroscopy of the Amine Group : Hydrogen Bond Motifs in Alkylamine/Alkylammonium Acid-Base Pairs
- 2018
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 122:31, s. 7737-7746
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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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| 5. |
- Kubin, Markus, et al.
(författare)
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Cr L-Edge X-ray Absorption Spectroscopy of CrIII(acac)3 in Solution with Measured and Calculated Absolute Absorption Cross Sections
- 2018
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 122:29, s. 7375-7384
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Tidskriftsartikel (refereegranskat)abstract
- X-ray absorption spectroscopy at the L-edge of 3d transition metals is widely used for probing the valence electronic structure at the metal site via 2p–3d transitions. Assessing the information contained in L-edge absorption spectra requires systematic comparison of experiment and theory. We here investigate the Cr L-edge absorption spectrum of high-spin chromium acetylacetonate CrIII(acac)3 in solution. Using a transmission flatjet enables determining absolute absorption cross sections and spectra free from X-ray-induced sample damage. We address the challenges of measuring Cr L absorption edges spectrally close to the O K absorption edge of the solvent. We critically assess how experimental absorption cross sections can be used to extract information on the electronic structure of the studied system by comparing our results of this CrIII (3d3) complex to our previous work on L-edge absorption cross sections of MnIII(acac)3 (3d4) and MnII(acac)2 (3d5). Considering our experimental uncertainties, the most insightful experimental observable for this d3(CrIII)–d4(MnIII)–d5(MnII) series is the L-edge branching ratio, and we discuss it in comparison to semiempirical multiplet theory and ab initio restricted active space calculations. We further discuss and analyze trends in integrated absorption cross sections and correlate the spectral shapes with the local electronic structure at the metal sites.
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| 6. |
- Kubin, Markus, et al.
(författare)
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Direct Determination of Absolute Absorption Cross Sections at the L-Edge of Dilute Mn Complexes in Solution Using a Transmission Flatjet
- 2018
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Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:9, s. 5449-5462
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Tidskriftsartikel (refereegranskat)abstract
- The 3d transition metals play a pivotal role in many charge transfer processes in catalysis and biology. X-ray absorption spectroscopy at the L-edge of metal sites probes metal 2p–3d excitations, providing key access to their valence electronic structure, which is crucial for understanding these processes. We report L-edge absorption spectra of MnII(acac)2 and MnIII(acac)3 complexes in solution, utilizing a liquid flatjet for X-ray absorption spectroscopy in transmission mode. With this, we derive absolute absorption cross-sections for the L-edge transitions with peak magnitudes as large as 12 and 9 Mb for MnII(acac)2 and MnIII(acac)3, respectively. We provide insight into the electronic structure with ab initio restricted active space calculations of these L-edge transitions, reproducing the experimental spectra with excellent agreement in terms of shapes, relative energies, and relative intensities for the two complexes. Crystal field multiplet theory is used to assign spectral features in terms of the electronic structure. Comparison to charge transfer multiplet calculations reveals the importance of charge transfer in the core-excited final states. On the basis of our experimental observations, we extrapolate the feasibility of 3d transition metal L-edge absorption spectroscopy using the liquid flatjet approach in probing highly dilute biological solution samples and possible extensions to table-top soft X-ray sources.
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