The Born Oppenheimer-Potential for the Hydrogen Bond in the H5O2+ Complex: Comparison of Theories and Experiment

The Born-Oppenheimer potential surface for motion of the central H atom in the hydrogen bond in the H$_{5}$O$_{2}^{+}$ ion is of considerable interest. This quantity has been elusive experimentally and computations are of unknown reliability. By measuring the momentum distribution of the protons in the centrosymmetric aquonium perchlorate, H$_{5}$O$_{2}^{+}$ ClO$_{4}^{-}$, and subtracting the ice-like contribution from the non-hydrogen bonded protons, we are able to measure the momentum distribution of the proton in the bond. This distribution is calculated for several ab-initio treatments of the potential surface and compared with the measurements as a means of distinguishing between these methods.