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 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).

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).

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 (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).

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 (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).

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).