Tuning the intermolecular proton bond in the H5O2+ 'Zundel ion' scaffold

► Vibrational spectra reveal quantum levels of proton trapped between oxygen atoms. ► Asymmetry in intermolecular proton bond correlated with shifts in proton frequency. ► Solvent molecules attached to the H2O·H+·H2O scaffold similar to methyl substitution. ► Ar and H2O surprisingly similar solvents despite 10× difference in bond strength.