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