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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).

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).

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).