Probing small protonated water clusters in acetonitrile solutions by ¹H NMR

In a previous publication by Kalish et al. (J. Phys. Chem. A 115 (2011) 4063) the existence of well defined small protonated water clusters in acetonitrile has been established by IR spectroscopy. Here we report on a ¹H NMR study of triflic acid, CF₃SO₃H, in acetonitrile-water solutions. Using NMR we are able to corroborate the general solvation scheme we have proposed for the hydrated proton in acetonitrile as a function of the molar ratio between the strong mineral acid and water, n = [H₂O]/[acid]. According to this scheme, backed now by both IR absorption spectroscopy and NMR measurements, the very strong triflic acid completely dissociates in acetonitrile/water solutions to yield the aqueous proton and the triflate anion when n > 1. Furthermore, increasing n results in the proton solvated in increasingly larger water clusters formed within the acetonitrile solution. Clearly distinguishable by NMR are the smallest protonated water clusters, the protonated water monomer, H⁺ ₃ O, and the protonated water dimer, H⁺₅O ₂, which dominate the solution for n = 1,2,3. For larger n the NMR study indicates the gradual increase of the average protonated water cluster size as a function of n while the proton inner solvation core more closely retaining the characteristics of a deformed protonated water dimer, (H ₂O-H⁺⋯OH₂)s than that of the protonated water monomer (H⁺₃ O)_s.