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O K-edge inner-shell spectra of polymers in solutions from the snapshots of molecular dynamics simulations

For obtaining the O K-edge inner-shell spectra of poly(N-isopropylacrylamide) (PNIPAM) in solutions, the 5-mer PNIPAM chains with terminated H atoms, including the second coordination shells of the solvent methanol and water molecules, were extracted from the 40-mer PNIPAM chains in solutions, whose snapshots were obtained by the molecular dynamics simulations. The inner-shell spectra of PNIPAM in aqueous methanol solutions were obtained by averaging those of 9700 extracted polymer structures. This calculation method can be precisely evaluated the energy shifts of the C=O π* peaks of PNIPAM caused by the structural changes of the polymer chains, the substitutions of the hydrogen bonds of the C=O groups in PNIPAM from methanol to water molecules, and the increases of the coordination numbers of solvent molecules with the C=O groups, which were observed in the O K-edge XAS experiments.

  1. M. Nagasaka et al., J. Chem. Phys. 162, 054901 (2025).

Metal-ligand delocalization of metal porphyrin complexes probed by N K-edge XAS

To investigate metal‒ligand delocalization of metal porphyrin complexes in aqueous solutions, this study probes the electronic structures of both the metal and ligand sides using XAS at the metal L2,3-edges and N K-edges, respectively. In the N K-edge XAS spectra of the ligands, the C=N π* peaks of cobalt protoporphyrin IX (CoPPIX) show higher energy shifts than those of iron protoporphyrin IX (FePPIX), owing to the different electronic configurations and spin multiplicities of metal porphyrin complexes. These spectra are useful for discussing the central-metal dependence of metal‒ligand delocalization. Meanwhile, the N K-edge XAS of CoPPIX and the inner-shell calculations of different hydration models reveal that CoPPIX maintains its five-coordination geometry in aqueous solution.

  1. M. Nagasaka et al., Phys. Chem. Chem. Phys. 26, 23636 (2024).

Probing isolated water molecules in aqueous acetonitrile solutions using XAS

O K-edge XAS of an aqueous acetonitrile solution exhibited a sharp peak at approximately 537 eV, which was similar to that of water vapor and was not observed in liquid water. The inner-shell spectra of isolated water molecules and water clusters of different sizes surrounded by acetonitrile molecules were obtained by extracting these water structures from the liquid structures of aqueous acetonitrile solutions, as calculated using molecular dynamics simulations. The sharp peak profiles of the O K-edge XAS spectra were derived not from water clusters but from isolated water molecules surrounded by acetonitrile molecules. The electronic structures of the isolated water molecules can be analyzed using O K-edge XAS spectra, separating the contributions of small water clusters.

  1. M. Nagasaka, J. Phys. Chem. Lett. 15, 5165 (2024).

Mechanism of poly(N-isopropylacrylamide) cononsolvency in aqueous methanol solutions studied by O K-edge XAS

The cononsolvency mechanism of poly(N-isopropylacrylamide) (PNIPAM), dissolving in pure methanol (MeOH) and water but being insoluble in aqueous MeOH solutions, were investigated by O K-edge XAS with theoretical calculations executed in molecular dynamics simulations and inner-shell calculations. It was found that the cononsolvency emerges from the aggregation of PNIPAM with MeOH clusters owing to the hydrophobic interactions between PNIPAM and MeOH clusters leading to the collapse of the hydrophobic hydration of PNIPAM.

  1. M. Nagasaka et al., Phys. Chem. Chem. Phys. 26, 13634 (2024).

Time-resolved XAS for photoexcitation processes of metal complexes in solutions

Time-resolved XAS is an effective method to investigate photoexcitation processes of metal complexes in solutions. The time-resolved XAS measurements were realized from the almost coaxially introduction of the laser pulses to the liquid cell with soft X-ray pulses. The N K-edge XAS spectra of [Fe(phen)3]2+ aqueous solutions in the ground and photoexcited states were obtained by scanning the delay time of the soft X-ray pulses relative to the laser pulses. The relaxation process of the [Fe(phen)3]2+ complexes from the high spin state to the low spin states has been obtained from the ligand side at the N K-edge, which is close to that obtained by the Fe K-edge.

  1. F. Kumaki et al., J. Chem. Phys. 158, 104201 (2023).

C K-edge inner-shell spectra of liquid alcohols obtained from the snapshots of molecular dynamics simulations

For reproducing C K-edge XAS spectra of liquid alcohols such as ethanol and methanol, the inner-shell calculations were performed with the snapshots of the liquid structures obtained by molecular dynamics simulations. The C K-edge inner-shell spectrum of liquid ethanol was obtained by the summation of one thousand calculated spectra including neighbor molecules within the CH2‒CH2 distance of 6 Å. The calculated C K-edge inner-shell spectra of liquid alcohols well reproduced the spectral shapes of the experimentally obtained XAS spectra and the spectral changes from gas to liquid phases.

  1. M. Nagasaka, J. Chem. Phys. 158, 024501 (2023).

Site selective analysis of water in hydrogen bond network of aqueous dimethyl sulfoxide solutions by O K-edge XAS

Hydrogen bond (HB) network in aqueous dimethyl sulfoxide (DMSO) solutions at different concentrations has been observed by O K-edge XAS with a site selective analysis that separates donor and acceptor sites of H2O, where the S=O π* peak in DMSO reflects the donor site of H2O and the 4a1 peak in H2O reflects the acceptor site of H2O, respectively. The molecular dynamics simulations and inner-shell calculations revealed that the HB network in aqueous DMSO solutions is influenced with not only the HB interaction of the S=O group with the donor site of H2O but also the dipole interaction of the S atom with the acceptor site of H2O, which breaks the HB network between H2O. Four concentration regions were found in the HB network of aqueous DMSO solutions, which would be related to the anomalies of physical properties and solvent effects in chemical and biological reactions.

  1. M. Nagasaka, J. Mol. Liq. 366, 120310 (2022).

Hydrophobic cluster formation in aqueous ethanol solutions probed by XAS

Hydrophobic cluster structures in aqueous ethanol solutions at different concentrations have been investigated by XAS. The lower energy features in the C K-edge XAS spectra arise from a transition from the terminal methyl C 1s electron to an unoccupied orbital of 3s Rydberg character, which is sensitive to the nearest neighbor intermolecular interactions. From the comparison of C K-edge XAS with the inner-shell calculations, it is found that ethanol clusters are easily formed in the middle concentration region due to the hydrophobic interaction of the ethyl group in ethanol, resulting in the enhancement of the hydrogen bond structures among water molecules.

  1. M. Nagasaka et al., J. Phys. Chem. B 126, 4948 (2022).

XAS in the low-energy region explored using an argon gas window

The soft X-ray region below 200 eV is important for investigating chemical and biological phenomena since it covers K-edges of Li and B and L-edges of Si, P, S and Cl. In this study, the argon gas window is proposed as a new soft X-ray transmission window in the low-energy region. High-order X-rays are removed by the absorption of the Ar L-edge (240 eV), and first-order X-rays become the major contribution of transmitted soft X-rays in the low-energy region. The XAS measurements confirmed that the argon gas window is effective for soft X-ray transmission in the low-energy region from 60 to 240 eV.

  1. M. Nagasaka, J. Synchrotron Rad. 27, 959 (2020).

Microheterogeneity in aqueous acetonitrile solution probed by XAS

Chemical processes in solutions are influenced by microheterogeneity, where two liquids seem to be mixed in a macroscopic scale but are microscopically inhomogeneous. In aqueous acetonitrile solutions, which show microheterogeneity, molecular interactions of acetonitrile were revealed by the C and N K-edge XAS at different concentrations, and those of solvent water were separately revealed by the O K-edge XAS. The peak energy shift at the C K-edge shows three characteristic concentration regions and a phase transition-like behavior between them. The inner-shell calculations found that the dipole interaction between acetonitrile and water is the key structure to emerge microheterogeneity in the middle concentration region, which continues until the predominance of the dipole interaction over the hydrogen bond interactions.

  1. M. Nagasaka et al., J. Phys. Chem. B 124, 1259 (2020).

Laminar flow in microfluidics observed by spatially-resolved XAS

The XAS measurements in the soft X-ray regions were applied to the element-selective analyses of liquid mixtures in a microfluidic flow. The microfluidic cell consists of the T-shaped microfluidics with the width and depth of 50 μm made in PDMS resin and covered by the Si3N4 membrane with a thickness of 100 nm. XAS spectra was measured by detecting fluorescent soft X-rays using a silicon drift detector. In the O Kα fluorescent image of the T-shaped microfluidics, water and pyridine were merged at the center position, and a laminar flow of pyridine and water was clearly observed in the mixed part. From the N K-edge XAS spectra with the spatial resolution of 30 × 30 μm2, the distributions of the molar fractions of pyridine and water near the liquid-liquid interface in the laminar flow have been determined from the energy shifts probed at different geometric positions.

  1. M. Nagasaka et al., J. Chem. Phys. 151, 114201 (2019).

Temperature-dependent structural changes in liquid benzene

Benzene is the simplest aromatic molecule with π−π interactions. This study addresses fundamental questions regarding whether ordered structures of benzene are formed in the liquid state using C K-edge XAS. The π* peak in liquid benzene unexpectedly shows an opposite temperature behavior, approaching the solid peak apart from the gaseous benzene with increasing temperature. This is rationalized by inner-shell calculations providing insights that structural changes from parallel displaced structures to sandwich (parallel nondisplaced) structures cause the unexpected temperature-dependent spectral shift of the π* peak. These consistent results confirmed that there are temperature-dependent changes of ordered structures of benzene in the liquid state that may affect the mechanisms of chemical and biological phenomena.

  1. M. Nagasaka et al., J. Phys. Chem. Lett. 9, 5827 (2018).

Molecular interactions of pyridine in aqueous solutions studied by XAS

Molecular interactions of pyridine in aqueous solutions (C5H5N)x(H2O)1−x at different molar fractions were studied by XAS at the C, N, and O K-edges. In the pyridine-rich region (x > 0.7), the π* peak energies are not different from neat pyridine (x = 1.0), indicating that antiparallel displaced structures of pyridine molecules are still dominant. In the water-rich region (0.7 > x), the N peaks show higher energy shifts, and the C peaks of the meta and para sites show lower energy shifts by increasing the molar fraction of water. The hydrogen bond (HB) network of bulk water is dominant in this region, but quantum chemical inner-shell calculations indicate that small pyridine clusters still exist in the HB network of water even in dilute solutions.

  1. M. Nagasaka et al., Z. Phys. Chem. 232, 705 (2018).

Temperature dependence of hydrogen bond networks in liquid water and aqueous salt solutions

Interaction between water molecules and alkali metal ions in aqueous salt solutions has been studied by the O K-edge XAS. The pre-edge peaks of the hydration water molecules in aqueous salt solutions show higher energy shifts depending on cations but not on anions. The energy shifts of the pre-edge peaks in liquid water at different temperatures represent the structural changes of the hydrogen bond network between water molecules. The pre-edge peaks arising from water molecules in the first hydration shell of lithium cations in aqueous LiCl solutions is not evidently dependent on the temperature, indicating that the hydration water molecules are more strongly bound with lithium cations than other water molecules.

  1. M. Nagasaka et al., J. Phys. Chem. B 121, 10957 (2017).

Local structures of aqueous methanol solutions studied by XAS

Local structures of aqueous methanol (MeOH) solutions were studied by O and C K-edge XAS, where the hydrogen bond (HB) structures between MeOH and water molecules were obtained at the O K-edge, and the local structures around the methyl group of MeOH molecules were obtained at the C K-edge. The C K-edge XAS spectra of (MeOH)x(H2O)1−x show three concentration regions, whose molecular structures were discussed with the help of the molecular dynamics simulations. In the region I (1.0 > x > 0.7), a small amount of water molecules exists separately around HB networks of MeOH clusters. In the region II (0.7 > x > 0.3), the hydrophobic interaction of the methyl group is dominant due to the increase of mixed MeOH‒H2O clusters. In the region III (0.3 > x > 0.05), MeOH molecules are separately embedded in HB networks of water.

  1. M. Nagasaka et al., J. Phys. Chem. B 118, 4388 (2014).

Operando XAS measurements of electrochemical reactions in solutions

For the operando XAS measurements of electrochemical reactions, the electrochemical cell including working, counter, and reference electrodes were developed [1, 2]. The Fe L-edge XAS spectra of aqueous iron sulfate solutions were successfully measured at different potentials with a scan rate of 100 mV/s. The mechanisms of the Fe redox reactions with the change in valence of Fe ions were discussed by correlating the results of cyclic voltammetry with the same scan rates.

  1. M. Nagasaka et al., J. Phys. Chem. C 117, 16343 (2013).
  2. M. Nagasaka et al., Rev. Sci. Instrum. 85, 104105 (2014).

The XAS measurements of liquid samples in transmission mode with a precise thickness control method

Soft X-ray absorption spectroscopy (XAS) of liquid samples in transmission mode has been realized by developing a liquid cell [1, 2], where the liquid layer is sandwiched between two Si3N4 membranes and the liquid thickness is precisely controlled from 20 nm to 40 μm by adjusting the helium pressure around the liquid cell. As shown in the O K-edge XAS of liquid water at different thicknesses, the XAS spectra of various solutions over a wide concentration region can be measured using the precise thickness control method, where the liquid layer becomes thicker in the dilute solution, and vice verse, for obtaining the appropriate absorbance of soft X-rays.

  1. M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 177, 130 (2010).
  2. M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 224, 93 (2018).