Controlling the pH of aqueous succinic and maleic acids analyzed by X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) measurements were performed at the oxygen K-edge using aqueous solutions of succinic and maleic acids. Molecular dynamics simulations were employed to perform structural sampling and calculate the theoretical spectra using the framework of the density functional theory. When the pH was varied, the first peak in the XAS profiles exhibited different behaviors for succinic and maleic acids. This peak behavior was reproduced using theoretical calculations. Maleic acid, which has a double bond between its central carbon atoms, exhibits resonance effects between its two carboxyl groups. Because this resonance effect was not observed for succinic acid, it was considered to account for the different peak shifts. Hydrogen bonding analyses of the water molecules surrounding succinic and maleic acids were performed to elucidate the structure of the water network.