complementary experimental probes; solute; liquid−vapor interface; in situ; XPS; XAS; PEY XAS; DFT; carboxylic acid
The electronic structures of formic acid (HCOOH) and formate (HCOO⁻) have been determined in aqueous solutions over a pH range of 1.88–8.87 using a combination of X-ray photoelectron spectroscopy (XPS), partial electron-yield X-ray absorption spectroscopy (PEY XAS), and density functional theory (DFT). The carbon 1s XPS measurements reveal a binding energy shift of −1.3 eV for deprotonated HCOO⁻ compared with neutral HCOOH. Such distinction between neutral HCOOH and deprotonated HCOO⁻ cannot be made based solely on the respective carbon K-edge PEY XA spectra. Independent of pH, the C1s → π* state excitations occur at 288.0 eV and may lead to the incorrect conclusion that the energy levels of the π* state are the same for both species. The DFT calculations are consistent with the experimental observations and show a shift to higher energy for both the occupied C1s (lower binding energy) and unoccupied π* orbitals of deprotonated HCOO⁻ compared to neutral HCOOH in aqueous solutions.