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The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 ± 2.1% donated and accepted hydrogen bonds per molecule, pointing to a continuousdistribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray-matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscalelength correlations within the continuous distribution remains open. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H2O molecular moieties. This is mirrored in a manifold of the H2O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H2O symmetric normal mode and the local asymmetric O–H bond coordinate up to 1 and 1.5 Å, respectively. The comparison of the single H2O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H2O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O–H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen 1s to 4a1 resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1s to 2b2 resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H2O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O–H, H–H, and O–O correlation lengths from neutron scattering. [ABSTRACT FROM AUTHOR]

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 Förster 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. [ABSTRACT FROM AUTHOR]

Photosäuren sind Moleküle, die einen starken Anstieg ihrer Acidität im ersten elektronisch angeregten Zustand aufweisen. Diese Eigenschaft ermöglicht Echtzeitstudien des Protonentransfers in Säure‐Base‐Reaktionen, des Protonentransports in Energiespeichern und biomolekularen Sensorproteinsystemen. Erklärungsansätze für die Ursachen der Photoacidität reichen von Variationen der freien Solvatationsenergie bis hin zu Veränderungen der elektronischen Struktur während der vier Phasen des Förster‐Zyklus. Wir berichten hier Veränderungen der elektronischen Struktur der Protonen‐Donor Gruppe in einer protonierten aromatischen Amino‐Photosäure in Lösung bei Photoanregung und anschließender Protonentransferdynamik mit Hilfe von Pikosekunden‐Stickstoff‐K‐Kanten‐Spektroskopie. Anhand der Orbitalspezifität von Rumpf‐Valenz‐Übergängen an der funktionellen Aminogruppe weisen wir deutlich ausgeprägte Veränderungen der elektronischen Struktur, des Dipolmoments und der Energiezustände auf der Seite der konjugierten Photobase nach. Diese Ergebnisse bilden die Grundlage für eine zukünftige detaillierte Charakterisierung des Phänomens der Photoacidität im Lichte der elektronischen Struktur. [ABSTRACT FROM AUTHOR]

Es wird gezeigt, dass Kovalenz die Ladungstrennung nach Photooxidation des Metallzentrums in einem Metall‐Ligand‐Ladungstransferzustand eines Eisensensibilisators ausgleicht. Die Fähigkeit der Liganden zur σ‐Bindung kompensiert den Verlust einer 3d‐Elementarladung am Eisenzentrum, sodass die ursprüngliche Ladungsdichte und lokale Edelgaskonfiguration am Metall erhalten bleibt. Diese Erkenntnisse werden durch elementspezifische und orbitalselektive zeitaufgelöste Röntgenabsorptionsspektroskopie an der Eisen‐L‐Kante ermöglicht. Die Population von Valenzorbitalen um das Metallzentrum wird so direkt zugänglich. Mit Hilfe von Dichtefunktionaltheorie stellen wir fest, dass das Bild einer lokalisierten Ladungstrennung inadäquat ist. Die ungepaarte Spindichte bietet jedoch eine geeignete Beschreibung des mit dem Elektronentransferprozess assoziierten Elektron‐Loch‐Paares. [ABSTRACT FROM AUTHOR]

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process. [ABSTRACT FROM AUTHOR]

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 N 1s X‐ray absorption spectroscopy and multi‐configurational spectrum simulations. Photoexcitations to the singlet S2 and S4 states both relax promptly through intersystem crossing to the triplet T1 state. The T1 state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S0 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. Intersystem crossing: Efficient intersystem‐crossing (ISC) drives proton‐transfer in 2‐thiopyridone (2‐TP, aq). Transient N 1s X‐ray absorption spectroscopy reveals prompt ISC upon photoexcitation of 2‐TP. The dominant population of the T1 state mediates excited‐state proton‐transfer on sub‐nanosecond timescales. Comparison to existing data indicates invariance of the pathway to excitation wavelength and solvent environment (see scheme). [ABSTRACT FROM AUTHOR]

Die Femtosekundendynamik nach resonanten Photoanregungen mit optischen und Röntgenpulsen ermöglicht eine selektive Verformung von chemischen N ‐ H ‐ und N ‐ C ‐ Bindungen in 2 ‐ Thiopyridon in wässriger Lösung. Die Untersuchung der orbitalspezifischen elektronischen Struktur und ihrer Dynamik auf ultrakurzen Zeitskalen mit resonanter inelastischer Röntgenstreuung an der N1s ‐ Resonanz am Synchrotron und dem Freie ‐ Elektronen ‐ Laser LCLS in Kombination mit quantenchemischen Multikonfigurationsberechnungen erbringen den direkten Nachweis dieser kontrollierten photoinduzierten Molekülverformungen und ihrer ultrakurzen Zeitskala. Die photoinduzierte Dynamik in 2 ‐ Thiopyridon (2 ‐ TP) wurde mit zeitaufgelöster resonanter inelastischer Röntgenstreuung untersucht. Die spektralen Fingerabdrücke für die N ‐ Deprotonierung geben Einblick in die ersten Schritte eines Protonentransferprozesses im valenzangeregten Zustand. Röntgenanregung von 2 ‐ TP verursacht eine Schwächung der C ‐ N ‐ Bindung, wodurch eine selektive Bindungsverformung durch Anpassung der Photonenenergie möglich wird. [ABSTRACT FROM AUTHOR]

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 timescale. [ABSTRACT FROM AUTHOR]

Sebastian Eckert et al. show in their Research Article (e202200709) how time-resolved nitrogen K-edge spectroscopy along the Förster cycle of a photoacid provides key insight into the electronic structural factors responsible for the photoacidity phenomenon. Keywords: Aromaticity; Electronic Structure; Orbital Interactions; Photoacids; Time-Resolved Soft x-Ray Spectroscopy EN Aromaticity Electronic Structure Orbital Interactions Photoacids Time-Resolved Soft x-Ray Spectroscopy 1 1 1 06/29/22 20220704 NES 220704 B Photoacids b have been understood, since the pioneering work of Theodor Förster more than seven decades ago, to show a pronounced increase in acidity when electronically excited, but the underlying microscopic mechanisms have remained elusive. Aromaticity, Electronic Structure, Orbital Interactions, Photoacids, Time-Resolved Soft x-Ray Spectroscopy. [Extracted from the article]

Sebastian Eckert et al. demonstrieren in ihrem Forschungsartikel (e202200709), wie Photoacidität durch Änderungen der elektronischen Struktur einer Photosäure entlang des Förster-Zyklus verursacht wird, die mit zeitaufgelöster Stickstoff-K-Kanten-Spektroskopie verfolgt werden können. Innentitelbild: Änderungen der elektronischen Struktur einer Amino-Photosäure entlang des Förster Zyklus (Angew. Keywords: Aromatizität; Elektronische Struktur; Orbitalwechselwirkung; Photosäuren; Zeitaufgelöste Weichröntgenspektroskopie DE Aromatizität Elektronische Struktur Orbitalwechselwirkung Photosäuren Zeitaufgelöste Weichröntgenspektroskopie 1 1 1 06/29/22 20220704 NES 220704 B Photosäuren b weisen einen starken Anstieg ihrer Säurestärke im elektronisch angeregten Zustand auf, wie von Theodor Förster vor bereits 7 Jahrzehnten gezeigt wurde. [Extracted from the article]

• First quantitative soft X-ray absorption cross sections of aqueous iron cyanides. • Establishing transient soft X-ray signatures of the photo-oxidation. • Quantification of photo-oxidized species at two different absorption edges. It is demonstrated for the case of photo-excited ferrocyanide how time-resolved soft X-ray absorption spectroscopy in transmission geometry at the ligand K-edge and metal L 3 -edge provides quantitatively equivalent valence electronic structure information, where signatures of photo-oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo-oxidized molecules at two soft X-ray absorption edges highlighting the sensitivity of X-ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X-ray range. [ABSTRACT FROM AUTHOR]

Highlights from the article: In ihrem Forschungsartikel auf S. 10588 untersuchen S. K. Kwak, J. Cho et al. dieses Phänomenon in Lithiumionenbatterie-Kathoden. Die Strukturen von Amipurimycin und den Miharamycinen werden von B. Yu et al. in ihrer Zuschrift auf S. 10668 beschrieben. Eine praktikable Methode für die Synthese -chiraler Amine, die auf der Alkylierung von Aminen mit Alkoholen basiert, wird von M. Lei, J. L. Xiao, C. Wang et al. in ihrem Forschungsartikel auf S. 10638 beschrieben.

Elektronische und strukturelle Änderungen bei der Photoanregung von 2 ‐ Thiopyridon (2 ‐ TP) wurden durch inelastische Röntgenstreuung untersucht. S. Eckert, J. Norell et al. beschreiben in der Zuschrift auf S. 6184 ff., dass eine spektrale Signatur für die N ‐ Deprotonierung von 2 ‐ TP mit dem Freie ‐ Elektronen ‐ Laser LCLS auf der Femtosekunden ‐ Zeitskala identifiziert wurde. Quantenchemische Simulationen weisen die Signatur der Bildung des freien Elektronenpaars an N zu und bestätigen, dass durch Röntgenstrahlung induzierte Dynamik von der optisch induzierten Deprotonierung entkoppelt ist. [ABSTRACT FROM AUTHOR]