High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid

Alexander E. Khudozhitkov, Masaki Donoshita, Alexander G. Stepanov, Frederik Philippi, Daniel Rauber, Rolf Hempelmann, Hiroshi Kitagawa, Daniil I. Kolokolov, Ralf Ludwig

Research output: Contribution to journalArticlepeer-review

Abstract

We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C8H17PD3][NTf2] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD3 group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD3 deuterons. A transition enthalpy of about 12 kJ mol−1 from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge-enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded P−D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD3 group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces.

Original languageEnglish
Article numbere202200257
JournalChemistry - A European Journal
Volume28
Issue number23
Early online date20 Feb 2022
DOIs
Publication statusPublished - 22 Apr 2022

Keywords

  • ion pairing
  • ionic liquids
  • molecular mobility
  • quantum tunnelling
  • solid state NMR

OECD FOS+WOS

  • 2.04 CHEMICAL ENGINEERING
  • 1.04 CHEMICAL SCIENCES

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