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

Mark V. Sigalov, Noa Kalish, Benny Carmeli, Dina Pines, Ehud Pines

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11 Scopus citations


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.

Original languageEnglish
Pages (from-to)983-1007
Number of pages25
JournalZeitschrift fur Physikalische Chemie
Issue number6-7
StatePublished - 1 Jan 2013


  • Eigen cation
  • Protonated water clusters
  • Zundel cation

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry


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