1H and 13C NMR Studies on New Ionic Plastic Crystals Constructed by Ellipsoidal Cations with [BEt3Me] Anion

Abstract

Abstract Two new ionic plastic crystals of [NEtMe2Pr][BEt3Me] and [NEt2MePr][BEt3Me] were found. In contrast, the highest-temperature solid-phase of [NEtMe2Bu][BEt3Me] and [NEt2MeBu][BEt3Me] were assigned to rotator phases. Solid-state 1H and 13C nuclear magnetic resonance (NMR) measurements revealed that both the cations and anions perform isotropic reorientations in the plastic phase. Conversely, the cations of [NEtMe2Bu] and [NEt2MeBu] undergo rotation about an axis. Based on these results, it is revealed that ellipsoidal cations of [NEtMe2Pr]+ and [NEt2MePr]+ can form plastic crystalline phases with [BEt3Me]−. In the lower temperature solid-phase of the plastic phase, a rotator phase was also found in [NEtMe2Pr][BEt3Me] and [NEt2MePr][BEt3Me] salts. This is rarely reported in alkylammonium compounds with [BEt3Me]. 1H NMR spin-lattice relaxation time (T1) measurements showed that activation energies of isotropic reorientation were slightly large when compared to those reported in other ionic plastic crystals constructed with globular cations. This difference can be explained by assuming the aspect ratio. On differential scanning calorimetry (DSC) charts, small entropy changes were recorded at melting points of four compounds. These results support the observation that cations and anions have large degrees of freedom of motion in the highest-temperature solid-phases (plastic and rotator phases).