pH dependence of aqueous oxalic acid observed by X-ray absorption and emission spectroscopy

[Display omitted] •XAS and XES were used to observe the O K-edge of aqueous oxalic acid.•Using pH, X-ray absorption/emission spectra were theoretically reproduced.•A resonant effect absent in monocarboxylic acids was observed between COOH groups. In this work, X-ray absorption spectroscopy (XAS) and site-selective X-ray emission spectroscopy (XES) were used to measure the oxygen K-edge (OC=O,1s and OCO−,1s) of oxalic acid, with theoretical calculations performed to reproduce the spectra observed. Structure sampling of a cluster model was performed using an ab initio molecular dynamics trajectory. Relative XES intensities from core-hole excited state dynamics simulations were calculated using density functional theory (DFT). It was found that the theoretical X-ray absorption and emission spectra well reproduced the experimental spectra and showed a resonant effect between COOH functional groups which is absent in monocarboxylic acids.