A New Model for the Structure and Dynamics of the Hydrated Proton in Liquid Acetonitrile and Water

A new proton-solvation and -transport model in aqueous solutions is proposed based on our combined NMR and IR experiments and theoretical quantum–classical-molecular-dynamics findings. The H₇⁺O₃ solvate is at the center of the emerging model of the aqueous proton transport which is based on measurements made on protonated water solvates in liquid acetonitrile up to water-cluster size of at least 12 water molecules. The existence of structurally well-defined protonated water clusters in these solutions is unequivocally verified by NMR and FTIR spectroscopies of the hydrated proton. In contrast to the gas-phase where the H₇⁺O₃ unit is symmetric, the core H₇⁺O₃ unit in polar liquids is characterized by a chain of 3 water molecules with asymmetrically distorted oxygen-oxygen distances due the fluctuating solvent environment which does not allow a complete isotropic solvation of the proton. Such solvent-field-induced asymmetric distortion favors a Zundel-type, dimeric active proton solvation structure within the protonated water trimer unit. The dimeric solvation structure determines the (ultrafast) IR vibrational response of the genuine proton transfer mode in the ~ 1200 cm^-1 region putting the model in harmony with recent fs-resolved 2D-IR experiments.