  Two-dimensional infrared spectroscopy experiments have presented new results
regarding the dynamics of the hydrated excess proton (aka <q>hydronium</q>
cation solvated in water). It has been suggested by these experiments that the
hydrated excess proton has an anisotropy reorientation timescale of 2.5 ps,
which can be viewed as being somewhat long lived. Through the use of both the
reactive molecular dynamics Multistate-Empirical Valence Bond method and
Experiment Directed Simulation Ab Initio Molecular Dynamics we show that
timescales of the same magnitude are obtained that correspond to proton
transport, while also involving structural reorientations of the hydrated
proton structure that correspond to the so-called <q>special pair dance</q>.
The latter is a process predicted by prior computational studies in which the
central hydrated hydronium in a distorted Eigen cation
(H<sub>9</sub>O<sub>4</sub><sup>+</sup>) structure continually switches special
pair partners with its strongly hydrogen-bonded neighboring water molecules.
These dynamics are further characterized through the time-evolution of
instantaneous normal modes. It is concluded that the hydrated excess proton has
a spectral signature unique from the other protons in the hydrated proton
complex. However, the results call into question the use of a static picture
based on a simple effective one dimensional potential well to describe the
hydrated excess proton in water. Instead, they more conclusively point to a
distorted and dynamic Eigen cation as the most prevalent hydrated proton
species in acid solutions of dilute to moderate concentrations.
