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 ± 0.2 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-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. © the Partner Organisations 2014.