  The transfer of a hydrated proton between water molecules in aqueous solution
is accompanied by the large-scale structural reorganization of the environment
as the proton relocates, giving rise to the Grotthus mechanism. The Zundel
(H5O2+) and Eigen (H9O4+) cations are the main intermediate structures in this
process. They exhibit radically different gas-phase infrared (IR) spectra,
indicating fundamentally different environments of the solvated proton in its
first solvation shell. The question arises: is there a least common denominator
structure that explains the IR spectra of the Zundel and Eigen cations, and
hence of the solvated proton? Full dimensional quantum simulations of these
protonated cations demonstrate that two dynamical water molecules embedded in
the static environment of the parent Eigen cation constitute this fundamental
subunit. It is sufficient to explain the spectral signatures and anharmonic
couplings of the solvated proton in its first solvation shell. In particular,
we identify the anharmonic vibrational modes that explain the large broadening
of the proton transfer peak in the experimental IR spectrum of the Eigen
cation, of which the origin remained so far unclear. Our findings about the
quantum mechanical structure of the first solvation shell provide a starting
point for further investigations of the larger protonated water clusters with
second and additional solvation shells.
