On the protonation of water

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 683.2(2) +/- 0.2(5) kJ mol(-1) at 0 K, which converts to 688.8(1) +/- 0.2(5) kJ mol(-1) at room temperature. The experimental results are corroborated by a hierarchy of coupled-cluster calculations up to pentuple excitations and septuple-zeta 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.