Toward Understanding Mobile Proton Behavior from First Principles Calculation: The Short Hydrogen Bond in Crystalline Urea−Phosphoric Acid

The dynamics of the intermolecular short hydrogen bond in the molecular complex of urea and
phosphoric acid are investigated using plane-wave density functional theory. Results indicate migration of
the proton toward the center of the hydrogen bond as temperature is increased, in line with recent
experimental measurements. Computed vibrational frequencies show favorable agreement with experimental
measurement. An analysis of existing neutron diffraction data leads us to conclude that the effective potential
well experienced by the proton is temperature-dependent. Inspired by our computations and theoretical
analysis, we offer a possible explanation for the proton migration phenomenon.