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<p>Protic ionic liquids (PILs) can undergo a decomposition process involving proton transfer from the cation to the anion, which reduces ionicity and affects their physicochemical properties. Direct observation of this process remains challenging. Here, we use <sup>2</sup>H NMR spectroscopy to study proton transfer and molecular dynamics in trimethylphosphonium-based ionic liquids with anions of varying basicity: methanesulfonate ([OMs]⁻), triflate ([OTf]⁻), and bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻). By exchanging the acidic proton with deuterium, we selectively probe cation dynamics and detect deuterium transfer to the anion. At low temperatures, two distinct <sup>2</sup>H NMR signals are observed, corresponding to immobile deuterium on the cation and mobile deuterium on the anion for [OMs-D] and [OTf-D]. In [NTf₂-D] no high-amplitude motion is detected due to the larger anion size. These results provide direct NMR evidence for the coexistence of charged and neutral species in phosphonium PILs. The high proportion of neutral species classifies them as “poor” ionic liquids with low ionicity.</p>

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Evidences of Proton Transfer from Cation to Anion in Trialkylphosphonium Ionic Liquids

  • 期刊论文

  • 发表日期：

    2026年7月1日

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  • Alexander E. Khudozhitkov,
  • Iana A. Denisova,
  • Daniil I. Kolokolov,
  • Daniel Rauber,
  • Frederik Philippi,
  • Daniel Schroeder,
  • Ralf Ludwig

摘要

Protic ionic liquids (PILs) can undergo a decomposition process involving proton transfer from the cation to the anion, which reduces ionicity and affects their physicochemical properties. Direct observation of this process remains challenging. Here, we use 2H NMR spectroscopy to study proton transfer and molecular dynamics in trimethylphosphonium-based ionic liquids with anions of varying basicity: methanesulfonate ([OMs]⁻), triflate ([OTf]⁻), and bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻). By exchanging the acidic proton with deuterium, we selectively probe cation dynamics and detect deuterium transfer to the anion. At low temperatures, two distinct 2H NMR signals are observed, corresponding to immobile deuterium on the cation and mobile deuterium on the anion for [OMs-D] and [OTf-D]. In [NTf₂-D] no high-amplitude motion is detected due to the larger anion size. These results provide direct NMR evidence for the coexistence of charged and neutral species in phosphonium PILs. The high proportion of neutral species classifies them as “poor” ionic liquids with low ionicity.