Proton Transfer and Ionicity: An 15 N NMR Study of Pyridine Base Protonation

Mohammad Hasani, Samrat A. Amin, Jeffery Yarger, Stephen K. Davidowski, Charles Angell

Research output: Contribution to journalArticle

1 Citation (Scopus)

Abstract

Protic ionic liquids (PILs) are made by proton transfer from a Brønsted acid to a base and are of interest for their solvent and electrolyte properties such as high ionic conductivity. Unfortunately, many PILs have been misnamed, because their ionic content is minimal due to an insufficient driving force for the proton transfer. Here we review this problem and introduce a new method, using 15 N NMR spectroscopy, of characterizing the relation between the extent of proton transfer to a given base and the strength of the proton-donating acid. The experimental data reveal a sigmoid "titration type" curve that indicates clearly the acid strength, at which molecule bases, of substituted pyridine type, are fully protonated. We compare results for two bases of similar shape but different basicity, protonated by equimolar amounts of the different acids. The extent of protonation is also reflected in the ionic conductivity, and we show that the important part of the protonation sigmoid is quantitatively reproduced by data for conductivity and viscosity displayed in the form of a Walden plot (log equivalent conductivity vs log fluidity). The acid strength, for this study, is based on gas phase proton affinities, but we note that a similar sigmoid is obtained if we use the condensed phase Hammett acidity functions instead. Our findings allow us to rank the AlCl 4 - anion as the weakest proton acceptor in use in IL studies, consistent with its role in the most conductive ILs.

Original languageEnglish (US)
Pages (from-to)1815-1821
Number of pages7
JournalJournal of Physical Chemistry B
Volume123
Issue number8
DOIs
StatePublished - Feb 28 2019

Fingerprint

Proton transfer
Protonation
Pyridine
Protons
pyridines
Nuclear magnetic resonance
nuclear magnetic resonance
Acids
protons
Sigmoid Colon
Ionic Liquids
acids
Ionic conductivity
Ionic liquids
ion currents
Fluidity
conductivity
Alkalinity
Titration
Acidity

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

Cite this

Proton Transfer and Ionicity : An 15 N NMR Study of Pyridine Base Protonation. / Hasani, Mohammad; Amin, Samrat A.; Yarger, Jeffery; Davidowski, Stephen K.; Angell, Charles.

In: Journal of Physical Chemistry B, Vol. 123, No. 8, 28.02.2019, p. 1815-1821.

Research output: Contribution to journalArticle

Hasani, Mohammad ; Amin, Samrat A. ; Yarger, Jeffery ; Davidowski, Stephen K. ; Angell, Charles. / Proton Transfer and Ionicity : An 15 N NMR Study of Pyridine Base Protonation. In: Journal of Physical Chemistry B. 2019 ; Vol. 123, No. 8. pp. 1815-1821.
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