Dynamics, Phase Transitions, and Hydrogen Bonding Motifs in Protic Ionic Liquids: Cations Make the Difference

Compared with their ammonium analogues, phosphonium ionic liquids (PILs) have better thermal stabilities, higher conductivities, and lower viscosities. PILs are therefore more suitable electrolytes for gaining high discharge capacities and rechargeabilities as well as lower Coulombic efficiencies. For understanding the favorable properties at the molecular level in terms of interaction strength and dynamics, we compare the IL tributylammonium methanesulfonate [N₄₄₄-H][OMs] and tributylphosphonium methanesulfonate [P₄₄₄-H][OMs] by means of ²H NMR spectroscopy in both solid and liquid states. The shape of the ²H NMR spectral lines and the spin relaxation times provide information about the interaction strength and hydrogen bonding arrangements, as well as the phase transition phenomena for these ILs. The ²H NMR spin relaxation in the liquid state characterizes the cation dynamics and allows for the investigation of the microscopic viscosity. We support our experimental observations by quantum chemical calculations of IL clusters consisting of different numbers of ion pairs.