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

In this work, X-ray absorption spectroscopy (XAS) and site-selective X-ray emission spectroscopy (XES) were used to measure the oxygen K-edge (O-C = O,O-1s and O-CO,O-(1) over bars) 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.