On the protonation of water

Abstract

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on isolated water molecules and water dimers establishes a new route to determining the water proton affinity (PA) with unprecedented accuracy. A floating thermochemical cycle constructed from the OH⁺ and H₃O⁺ appearance energies and three other spectroscopic values establishes the water PA as 683.2₂ ± 0.2₅ kJ mol⁻¹ at 0 K, which converts to 688.8₁ ± 0.2₅ kJ mol⁻¹ at room temperature. The experimental results are corroborated by a hierarchy of coupled-cluster calculations up to pentuple excitations and septuple-ζ basis set. Combined with diagonal Born–Oppenheimer and Dirac–Coulomb–Gaunt relativistic corrections, they provide the best theoretical estimate for both the hydronium ion's geometry and a water PA of 683.5 ± 0.4 kJ mol⁻¹ and 689.1 ± 0.4 kJ mol⁻¹ at 0 K and 298.15 K, respectively.