The solvation species of the hydrated proton in acetonitrile/water mixtures were determining by combining FTIR and NMR studies of the mixtures. Both the OH and the CN stretch vibration were analyzed as a function of n=[H2O]/[H+] in acetonitrile. We find excellent agreement between the two types of measurements which essentially reports on the same protonated water structures as a function of n. The 1H NMR studies show that above 0.3 M of water all water molecules are complexed with protons with a narrow cluster size distribution which is centered around n, the average water solvate size. Put together, it has become possible to unequivocally determine the structure of the n=1, 2, 3 protonated water solvates in acetonitrile and to demonstrate the step-by-step build-up of the size of the protonated water complex as a function of n. Our main conclusion is that above n &gt; 2 the main motif of the protonate water complexes is the protonated water trimer H7+O3. This dominating proton solvation motif may be qualitatively described as a protonated water dimer which is hydrogen bonded to an additional water molecule. The persistence of this motif up to at least n =10 where our spectroscopic analysis loose its structural resolution.

Structure of protonated water solvates in acetonitrile reveals the importance of the protonated water trimer H7+O3

2019 Fall National Meeting

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