  The solvation structure of protons in aqueous media is highly relevant to
electric properties and to proton transport in liquids and membranes. At
ambient temperature, polar liquids display structural fluctuations on femto- to
picosecond time scales with a direct impact on proton solvation. We apply
two-dimensional infrared (2D-IR) spectroscopy for following proton dynamics in
acetonitrile/water mixtures with the Zundel cation H$_5$O$_2^+$ prepared in
neat acetonitrile as a benchmark. The 2D-IR spectra of the proton transfer mode
of H$_5$O$_2^+$ demonstrate stochastic large-amplitude motions in the
double-minimum proton potential, driven by fluctuating electric fields. In all
cases the excess proton is embedded in a water dimer, forming an H$_5$O$_2^+$
complex as major solvation species. This observation is rationalized by quantum
mechanics/molecular mechanics molecular dynamics simulations including up to 4
water molecules embedded in acetonitrile. The Zundel motif interacts with its
closest water neighbor in an H$_7$O$_3^+$ unit without persistent proton
localization.
