Josefina Adebahr

N,N-dimethyl-pyrrolidinium iodide has been investigated using differential scanning calorimetry, nuclear magnetic resonance (NMR) spectroscopy, second moment calculations, and impedance spectroscopy. This pyrrolidinium salt exhibits two solid-solid phase transitions, one at $373\mathrm{K}$ having an entropy change, $\Delta S$ , of $38\mathrm{J}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$ and one at $478\mathrm{K}$ having $\Delta S$ of $5.7\mathrm{J}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$ . The second moment calculations relate the lower temperature transition to a homogenization of the sample in terms of the mobility of the cations, while the high temperature phase transition is within the temperature region of isotropic tumbling of the cations. At higher temperatures a further decrease in the ${}^1\mathrm{H}$ NMR linewidth is observed which is suggested to be due to diffusion of the cations.