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)

Abstract

Eight salts of [NEt_xMe_(3−x)(i-Pr)][BEt_(4−y)Me_y] (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 (E_a rot) and ion diffusion (E_a diff). The lattice constant a in the cubic structure indexed to a CsCl-type was larger than that of the corresponding plastic crystals of [NEt_xMe_(4−x)][BEt_(4−y)Me_y] (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 E_a rot and E_a diff values were estimated using ¹H NMR spin–lattice relaxation time (T₁) and electrical conductivity measurements, respectively.