Theoretical studies of photoelectron spectra of SO42−(H2O)n clusters and the extrapolation to bulk solution

Abstract

Photoelectron spectra of hydrated doubly charged anion clusters, SO42−(H2O)n, have been studied by performing first-principles electronic structure calculations on SO42−(H2O)n (n=3–6, 12, and 13). The calculated adiabatic electron ionization energies are in good agreement with available experimental data. A detailed analysis of the calculated results suggests that for n⩾12 the observed threshold ionization energy of the low binding energy band in the recently reported photoelectron spectra of SO42−(H2O)n is associated with the electron ionization from the solute, SO42−, whereas the observed threshold ionization energy of the high binding energy band is associated with the electron ionization from the water molecules in the first solvation shell of SO42−. For n⩽6, both threshold ionization energies of the low and high binding energy bands are all associated with the electron ionizations from the solute. This shows that the bulk solution value (n→∞) extrapolated from those threshold ionization energies of the high binding energy band of the clusters should refer to the first ionization energy of the water molecules in the first solvation shell of SO42− in aqueous solution and, therefore, should be significantly smaller than the measured threshold ionization energy of liquid water. This differs from the recent result that the value of 10.05 eV extrapolated from the threshold ionization energies of the high binding energy band based on a simple 1/Rc model was nearly identical to the measured threshold ionization energy (10.06 eV) of liquid water. To address this difference, we have used a new approach for the extrapolation of solvated ion cluster data to bulk solution. We show that the new extrapolation approach consistently produces extrapolated bulk solution results in significantly better agreement with those observed directly in bulk solution for the first ionization energies of the ions in SO42−(H2O)n, Br−(H2O)n, and I−(H2O)n. The same extrapolation approach predicts a bulk solution value of 7.20 eV extrapolated from the threshold ionization energies of the high binding energy band, consistent with our assignment of the high binding energy band.