Structure and dynamics of the hydrated proton in liquid acetonitrile and water

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^1,2. 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 studies show that up to at least n =8 all water molecules are complexed with protons with a narrow cluster size distribution which is centered about n, the average water solvate size. Put together, we unequivocally determine the structure of the n=1, 2, 3 protonated water solvates in acetonitrile. Our main conclusion is that above n > 2 the main motif of the protonate water complexes is the protonated water trimer H₇⁺O₃ which in acetonitrile we demonstrate is a very robust unit. Unlike the situation for ultracold isolated H₇⁺O₃ the protonated water trimer solvation motif is qualitatively described in room-temperature polar-solutions as a protonated water dimer which is strongly hydrogen bonded to an additional water molecule. According to recent QM/MM calculations, the proton translocate between the two dimeric units within the protonated trimer³. Further 2^nd solvation shell rearrangements may then be the rate determining step for full proton transport.

References

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