Solid-state 1H and 13C NMR studies of new ionic plastic-crystals with branched structures: [NEtxMe(3-x)(i-Pr)][BEt(4-y)Mey] (x=1-3, y=0, 1)

Eight salts of [NEtxMe(3-x)(i-Pr)][BEt(4-y)Mey] (x = 1-3, y = 0, 1) were prepared to reduce Coulombic interactions in the crystals. These novel ionic plastic crystals exhibited low activation energies for isotropic rotational motion (Ea rot) and ion diffusion (Ea diff). The lattice constant a in the cubic structure indexed to a CsCl-type was larger than that of the corresponding plastic crystals of [NEtxMe(4-x)][BEt(4-y)Mey] (nonbranched sample). Solid-state 1H and 13C nuclear magnetic resonance (NMR) spectra of the salts revealed that both cations and anions undergo isotropic reorientation motions in the plastic crystalline phase. This result is consistent with the differential scanning calorimetry data, which showed that the compounds have large entropy changes at the transition temperature between the ordinal and plastic crystalline phases, except for [NEt2Me(i-Pr)][BEt3Me] and [NEt3(i-Pr)][BEt3Me]. The Ea rot and Ea diff values were estimated using 1H NMR spin-lattice relaxation time (T1) and electrical conductivity measurements, respectively.