NMR Study on Ion Dynamics and Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid: 1-Butyl-1-methylpiperidinium Bis(fluorosulfonyl)amide

By use of pulse NMR methods, the temperature dependences of the longitudinal and transverse relaxation times for H-1 and F-19 were measured for the three phases of 1-butyl-1-methylpiperidinium bis(fluorosulfonyl)amide ([Pip(1,4)]-LA[FSA]), i.e., liquid or supercooled liquid, Cryst-alpha, and Cryst-beta, to investigate the ion dynamics and phase behavior related to the dynamics. Since the cations and anions in the room -temperature ionic liquid have H-1 and F-19 nuclei, respectively, the dynamics of the [Pip(1,4)] cation and [FSA] anion can be independently observed and the relation between them can be evaluated. The relevant local motions of the ions are fluctuational motion around the chair form of the piperidinium ring, libration or rotation of the alkyl groups, and libration or rotation of the SO2F groups around the N-S axes. Each phase preferentially exhibits these motions. In the Cryst-beta phase, it is thought that O atoms in the SO2F groups form strong hydrogen bonds with the H atoms in the piperidinium ring. As a result, the motions of the SO2F groups and the piperidinium ring are restricted. In the liquid or supercooled liquid states and Cryst-alpha phase, the motion of the anion is more significant than that of the cation and the former works as a trigger for phase changes. Particularly, the motion of the SO2F groups in the Cryst-alpha phase becomes very significant with rising temperature and is directly related to the melting phase transition.