The proton affinity of water dimer was measured, using the kinetic method with nitrile reference bases, to be 808 ± 6 kJ mol^–1. The difference between the measured value and the proton affinity of a single water molecule (690 ± 4 kJ mol^–1) reflects the difference in solvation energy for two neutral water molecules (the hydrogen bond energy of water dimer), and the energy for solvation of hydronium cation and water. Using the measured proton affinity of the dimer along with ancillary data yields an experimental value of the hydrogen bond energy of neutral water dimer, 18 ± 9 kJ mol^–1, in good agreement with previous experimental and theoretical values. The proton affinity of the dimer measured by using alcohol references is ca 100 kJ mol^–1 too low, likely because the structure of the protonated cluster is not well-suited for kinetic method measurements. These results highlight the importance of choosing reference bases that form cluster ions consisting of a proton bound complex between the water dimer and the reference base.

Keywords: kinetic method, proton affinity, water dimer, hydrogen bond energy