We report on the spectral features of protonated water clusters in acetonitrile/water solutions determined by combining FTIR and NMR measurements (ChemPhysChem accepted with a companing front-cover highlighting the study). Three very strong mineral acids, triflic acid CF₃SO₃H, perchloric acid HClO₄ and hydrogen iodide HI were used to protonate the aqueous fraction of acetonitrile-water binary-solutions. Both the water-clusters OH and the solvent CN stretch vibrations were analyzed as a function of x=[H₂O]/[H⁺] in acetonitrile. We find excellent agreement between the two types of measurements one coming from the complex perspective and the other emerging from the surrounding solvent view-point. Complementary ¹H NMR studies reaffirm that all water molecules are complexed with protons up to at least x=6 with the cluster size n narrowly distributed around x. We also analyze the water bend and the shared proton transitions and the analysis together with spectral QM/MM calculations reaffirm the assignment of the asymmetric structure of the H₇⁺O₃ solvation core. In parallel to our studies, inner-shell electron absorption x-ray spectroscopy of the H₂O oxygen atoms by Ekimova et al. provides similar structural information. Based on all the above evidence we unequivocally determine the step-by-step build-up of the protonated-water cluster-size n as a function of x. Our main conclusion is that from n > 2 up to at least n =8 the main core-motif of the protonated water complexes is the asymmetric protonated water trimer H₇⁺O₃. This dominating proton solvation motif may be qualitatively described as a shared proton configuration with one of its H₂O molecules strongly hydrogen bonded to an additional water molecule. This motif is frequently found in solid protonated water hydrates but it is the first time it is unambiguously detected in room-temperature liquid water binary-solutions.

Eve Kozari

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