Brown, M.A.; Vila, F.; Sterrer, M.; Thürmer, S.; Winter, B.; Ammann, M.; Rehr, J.J.; van Bokhoven, J.A.: Electronic Structures of Formic Acid (HCOOH) and Formate (HCOO-) in Aqueous Solutions. Journal of Physical Chemistry Letters 3 (2012), p. 1754-1759
10.1021/jz300510r

Abstract:
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.