  Infrared (IR) absorption in the 1000-3700 cm-1 range and 1H NMR spectroscopy
reveal the existence of an asymmetric protonated water trimer, H7+O3, in
acetonitrile. The core H7+O3 motif persists in larger protonated water clusters
in acetonitrile up to at least 8 water molecules. Quantum mechanics/molecular
mechanics (QM/MM) molecular dynamics (MD) simulations reveal irreversible
proton transport promoted by propagating the asymmetric H7+O3 structure in
solution. The QM/MM calculations allow for the successful simulation of the
measured IR absorption spectra of H7+O3 in the OH stretch region, which
reaffirms the assignment of the H7+O3 spectra to a hybrid-complex structure: a
protonated water dimer strongly hydrogen-bonded to a third water molecule with
the proton exchanging between the two possible shared-proton Zundel-like
centers. The H7+O3 structure lends itself to promoting irreversible proton
transport in presence of even one additional water molecule. We demonstrate how
continuously evolving H7+O3 structures may support proton transport within
larger water solvates.
