Hydrogen-Bonded Structures of Water Molecules in Hydroxy-Functionalized Nanochannels of Columnar Liquid Crystalline Nanostructured Membranes Studied by Soft X‑ray Emission Spectroscopy

Here, we report a synchrotron-based high-resolution soft X-ray emission spectroscopy study on hydrogen-bonded structures of water molecules in the self-organized, hydroxy-group-functionalized one-dimensional nanochannels of liquid crystalline nanostructured membranes. The water molecules confined in the uncharged hydroxy-functionalized nanochannels (which have a diameter of about 1.5 nm) exhibit hydrogen-bonded structures close to those of bulk liquid water, even directly interacting with diol groups. These hydrogen-bonded structures contrast with the more distorted hydrogen bonding of water molecules confined in self-organized channels with a diameter of 0.6 nm formed by an analogous nanostructured membrane with a cationic moiety, which was explained by the ability of the channel functional groups to donate and accept hydrogen bonds in a confined space and the nanochannel diameter

First-Principles Investigation of Strong Excitonic Effects in Oxygen 1s X‑ray Absorption Spectra

We calculated the oxygen 1s X-ray absorption spectra (XAS) of acetone and acetic acid molecules in vacuum by utilizing the first-principles <i>GW</i>+Bethe–Salpeter method with an all-electron mixed basis. The calculated excitation energies show good agreement with the available experimental data without an artificial shift. The remaining error, which is less than 1% or 2–5 eV, is a significant improvement from those of time-dependent (TD) density functional methods (5% error or 27–29 eV for TD-LDA and 2.4–2.8% error or 13–15 eV for TD-B3LYP). Our method reproduces the first and second isolated peaks and broad peaks at higher photon energies, corresponding to Rydberg excitations. We observed a failure of the one-particle picture (or independent particle approximation) from our assignment of the five lowest exciton peaks and significant excitonic or state-hybridization effects inherent in the core electron excitations

Enhancement of the Hydrogen-Bonding Network of Water Confined in a Polyelectrolyte Brush

Water existing in the vicinity of polyelectrolytes exhibits unique structural properties, which demonstrate key roles in chemistry, biology, and geoscience. In this study, X-ray absorption and emission spectroscopy was employed to observe the local hydrogen-bonding structure of water confined in a charged polyelectrolyte brush. Even at room temperature, a majority of the water molecules confined in the polyelectrolyte brush exhibited one type of hydrogen-bonding configuration: a slightly distorted, albeit ordered, configuration. The findings from this study provide new insight in terms of the correlation between the function and local structure of water at the interface of biological materials under physiological conditions

Direct Observation of Cr³⁺ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr³⁺ impurities in Al₂O₃ with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr³⁺ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L₃ and L₂ edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms

Direct Observation of Cr³⁺ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr³⁺ impurities in Al₂O₃ with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr³⁺ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L₃ and L₂ edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms

Enhancement in Kinetics of the Oxygen Reduction Reaction on a Nitrogen-Doped Carbon Catalyst by Introduction of Iron via Electrochemical Methods

The iron (Fe) electrodeposition–electrochemical dissolution has been employed on nitrogen-doped carbon material (P-PI) prepared via multi-step pyrolysis of a polyimide precursor to achieve the introduction of Fe species, and its influence on the oxygen reduction reaction (ORR) is investigated by cyclic and rotating ring-disk electrode voltammetry in 0.5 M H₂SO₄. After the electrochemical treatment, the overpotential and H₂O₂ production percentage of ORR on the P-PI are decreased and the number of electrons transferred is increased in the meanwhile. In combination with the results of X-ray absorption fine structure spectra, the presence of Fe–N_<i>x</i> sites (Fe ions coordinated by nitrogen) is believed to be responsible for the improved ORR performance. Further kinetic analysis indicates that a two-electron reduction of O₂ is predominant on the untreated P-PI with coexistence of a direct four-electron transformation of O₂ to H₂O, while the introduction of Fe species leads to a larger increase in the rate constant for the four-electron reduction than that for the two-electron process, being in good agreement with the view that Fe–N_<i>x</i> sites are active for four-electron ORR

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X‑ray Emission Spectroscopy

We combine Mn <i>L</i>_2,3-edge X-ray absorption, high resolution Mn 2p–3d–2p resonant X-ray emission, and configuration–interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn²⁺ low-spin state separately from that of the Mn²⁺ high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn²⁺ low spin state having a stronger ligand-to-metal charge-transfer effect between Mn <i>t</i>_2g and CN π orbitals than the Mn²⁺ high spin state. The d–d-excitation peak nearest to the elastic scattering was ascribed to the Mn²⁺ LS state by the CIFM calculation, indicating that the Mn²⁺ LS state with a hole on the <i>t</i>_2g orbital locates near the Fermi level

In Situ Hard X‑ray Photoelectron Study of O₂ and H₂O Adsorption on Pt Nanoparticles

To improve the efficiency of Pt-based cathode catalysts in polymer electrolyte fuel cells, understanding of the oxygen reduction process at surfaces and interfaces in the molecular level is essential. In this study, H₂O and O₂ adsorption and dissociation as the first step of the reduction process were investigated by in situ hard X-ray photoelectron spectroscopy (HAXPES). Pt 5d valence band and Pt 3d, Pt 4f core HAXPES spectra of Pt nanoparticles upon H₂O and O₂ adsorption revealed that H₂O adsorption has a negligible effect on the electronic structure of Pt, while O₂ adsorption has a significant effect, reflecting the weak and strong chemisorption of H₂O and O₂ on the Pt nanoparticle, respectively. Combined with ab initio theoretical calculations, it is concluded that Pt 5d states responsible for Pt–O₂ bonding reside within 2 eV from the Fermi level

Measurement of the Ligand Field Spectra of Ferrous and Ferric Iron Chlorides Using 2p3d RIXS

Ligand field spectra provide direct information about the electronic structure of transition metal complexes. However, these spectra are difficult to measure by conventional optical techniques due to small cross sections for d-to-d transitions and instrumental limitations below 4000 cm^–1. 2p3d resonant inelastic X-ray scattering (RIXS) is a second order process that utilizes dipole allowed 2p to 3d transitions to access d–d excited states. The measurement of ligand field excitation spectra by RIXS is demonstrated for a series of tetrahedral and octahedral Fe­(II) and Fe­(III) chlorides, which are denoted Fe­(III)-<i>T</i>_<i>d</i>, Fe­(II)-<i>T</i>_<i>d</i>, Fe­(III)-<i>O</i>_<i>h</i>, and Fe­(II)-<i>O</i>_<i>h</i>. The strong 2p spin–orbit coupling allows the measurement of spin forbidden transitions in RIXS spectroscopy. The Fe­(III) spectra are dominated by transitions from the sextet ground state to quartet excited states, and the Fe­(II) spectra contain transitions to triplet states in addition to the spin allowed ⁵Γ → ⁵Γ transition. Each experimental spectrum is simulated using a ligand field multiplet model to extract the ligand field splitting parameter 10Dq and the Racah parameters <i>B</i> and <i>C</i>. The 10Dq values for Fe­(III)-<i>T</i>_<i>d</i>, Fe­(II)-<i>T</i>_<i>d</i>, and Fe­(III)-<i>O</i>_<i>h</i> are found to be −0.7, −0.32, and 1.47 eV, respectively. In the case of Fe­(II)-<i>O</i>_<i>h</i>, a single 10Dq parameter cannot be assigned because Fe­(II)-<i>O</i>_<i>h</i> is a coordination polymer exhibiting axially compressed Fe­(II)Cl ₆ units. The ⁵T → ⁵E transition is split by the axial compression resulting in features at 0.51 and 0.88 eV. The present study forms the foundation for future applications of 2p3d RIXS to molecular iron sites in more complex systems, including iron-based catalysts and enzymes

Probing the Valence Electronic Structure of Low-Spin Ferrous and Ferric Complexes Using 2p3d Resonant Inelastic X‑ray Scattering (RIXS)

Understanding the detailed electronic structure of transition metal ions is essential in numerous areas of inorganic chemistry. In particular, the ability to map out the many particle d–d spectrum of a transition metal catalyst is key to understanding and predicting reactivity. However, from a practical perspective, there are often experimental limitations on the ability to determine the energetic ordering, and multiplicity of all the excited states. These limitations derive in part from parity and spin-selection rules, as well as from the limited energy range of many standard laboratory instruments. Herein, we demonstrate the ability of 2p3d resonant inelastic X-ray scattering (RIXS) to obtain detailed insights into the many particle spectrum of simple inorganic molecular iron complexes. The present study focuses on low-spin ferrous and ferric iron complexes, including [Fe^III/II­(tacn)₂]^3+/2+ and [Fe^III/II­(CN)₆]^3–/4–. This series thus allows us to assess the contribution of d-count and ligand donor type, by comparing the purely σ-donating tacn ligand to the π-accepting cyanide. In order to highlight the conceptual difference between RIXS and traditional optical spectroscopy, we compare first RIXS results with UV–vis and magnetic circular dichroism spectroscopy. We then highlight the ability of 2p3d RIXS to (1) separate d–d transitions from charge transfer transitions and (2) to determine the many particle d–d spectrum over a much wider energy range than is possible by optical spectroscopy. Our experimental results are correlated with semiempirical multiplet simulations and <i>ab initio</i> complete active space self-consistent field calculations in order to obtain detailed assignments of the excited states. These results show that Δ<i>S</i> = 1, and possibly Δ<i>S</i> = 2, transitions may be observed in 2p3d RIXS spectra. Hence, this methodology has great promise for future applications in all areas of transition metal inorganic chemistry