Electronic Structures of Formic Acid (HCOOH) and Formate (HCOO-) in Aqueous Solutions

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 Is 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 -> pi* state excitations occur at 288.0 eV and may lead to the incorrect conclusion that the energy levels of the pi* 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 pi* orbitals of deprotonated HCOO- compared to neutral HCOOH in aqueous solutions.