Dynamic aspects of configurational changes in the structure of protonated water clusters in acetonitrile

The ongoing debate on the structure of the core solvation motif of the proton in water is based on the H₅⁺O₂ Zundel-like and the H₉⁺O₄/H₃⁺O Eigen-like proton solvation perspective. Our recent theoretical and experimental observations strongly support a hybrid Eigen/Zundel H₇⁺O₃ protonated water trimer motif being the fundamental solvation core of the proton in water. A key observation in our recent advances is the 3-fold hierarchy in hydrogen bonds which keeps coming out in our spectroscopic and theoretical studies of protonated water. Importantly, our simulations show the 3-fold hierarcy persisting in both the chain-like arrangement and the star-like arrangement of the protonated water tetramer H₉⁺O₄ in water. The protonated water trimer structure encompasses both the so-called deformed Zundel and deformed Eigen configurations features embedded within the 1^st and 2^nd solvation shells of the aqueous proton. We have reached this important conclusion using NMR, IR and X-ray absorption spectroscopies of small, medium-size and large protonated water clusters in acetonitrile. The 3 spectroscopic methods also provide access to the multi-scale configurational space of the hydrated proton and its various reactive stages by snap-shotting specific transient configurations of the hydrated proton and to integrate over all its reactive configurations.