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 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.
†Institute for Chemical and Bioengineering, ETH Zürich, CH-8093 Zürich, Switzerland.
Matthew A
MA
Brown
‡Department of Physics, University of Washington, Seattle, Washington, 98195, United States.
Fernando
F
Vila
§Department of Chemical Physics, Fritz-Haber-Institute der Max-Planck-Gesellschaft, D-14195 Berlin, Germany.
Martin
M
Sterrer
∥Helmholtz-Zentrum Berlin für Materialien und Energie and BESSY, D-12489 Berlin, Germany.
Stephan
S
Thürmer
∥Helmholtz-Zentrum Berlin für Materialien und Energie and BESSY, D-12489 Berlin, Germany.
Bernd
B
Winter
⊥Laboratory for Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
Markus
M
Ammann
‡Department of Physics, University of Washington, Seattle, Washington, 98195, United States.
John J
JJ
Rehr
†Institute for Chemical and Bioengineering, ETH Zürich, CH-8093 Zürich, Switzerland.|#Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
Jeroen A
JA
van Bokhoven
DFT; PEY XAS; XAS; XPS; carboxylic acid; complementary experimental probes; in situ; liquid−vapor interface; solute