Full conformational and energy explorations are conducted on an organic ionic plastic crystal, 1-ethyl-1-methylpyrrolidium tetrafluoroborate [C2mpyr][BF4]. The onsets of various stages of dynamic behaviour, which appear to account for low-temperature solid-solid phase transitions, are investigated by using quantum-chemical simulations. It is suggested that pseudorotation of the pyrrolidine ring occurs in the first instance; the partial rotation of the entire cation subsequently occurs and may be accompanied by reorientation of the ethyl chain as the temperature increases further. A cation-anion configuration, whereby BF4(-) interacts with the C2 mpy cation from the side of the ring, is the most likely structure in the low-temperature phase IV region. These interpretations are supported by (13)C nuclear magnetic resonance chemical-shift analysis.
Keywords: density functional calculations; nmr spectroscopy; organic ionic plastic crystal; solid-state electrolyte; structure and dynamics.
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