  The infrared (IR) spectra of protonated water clusters encode precise
information on the dynamics and structure of the hydrated proton. However, the
strong anharmonic coupling and quantum effects of these elusive species remain
puzzling up to the present day. Here, we report unequivocal evidence that the
interplay between the proton transfer and the water wagging motions in the
protonated water dimer (Zundel ion) giving rise to the characteristic doublet
peak is both more complex and more sensitive to subtle energetic changes than
previously thought. In particular, hitherto overlooked low-intensity satellite
peaks in the experimental spectrum are now unveiled and mechanistically
assigned. Our findings rely on the comparison of IR spectra obtained using two
highly accurate potential energy surfaces in conjunction with highly accurate
state-resolved quantum simulations. We demonstrate that these high-accuracy
simulations are important for providing definite assignments of the complex IR
signals of fluxional molecules.
