¹H and ¹⁹F NMR studies on molecular motions and phase transitions in solid triethylammonium tetrafluoroborate

Measurements by differential thermal analysis and differential scanning calorimetry and of the spin-lattice relaxation time (T₁), the spin-spin relaxation time (T₂), and the second moment (M₂) of ¹H and ¹⁹F NMR were carried out in the three solid phases of (CH₃CH₂)₃NHBF₄. X-ray powder patterns were taken in the highest-temperature phase (Phase I) existing above 367 K and the room-temperature phase (Phase II) stable between 220 and 367 K. Phase I formed a NaCl-type cubic structure with a = 11.65(3) Å, Z = 4, V = 1581(13) ų, and D_x = 0.794 g cm⁻³, and was expected to be an ionic plastic phase. In this phase, the self-diffusion of anions and the isotropic reorientation of cations were observed. Phase II formed a tetragonal structure with a = 12.47(1) and c = 9.47(3) Å, Z = 4, V = 1473(6) ų, and D_x = 0.852 g cm⁻³. From the present DSC and NMR results in this phase, the cations and/or anions were considered to be dynamically disordered states. The C₃ reorientation of the cation about the NH bond axis was detected and, in addition, the onset of nutation of the cations and local diffusion of the anions was suggested. In the low-temperature phase (Phase III) stable below 219 K, the C₃ reorientations of the three methyl groups of cations and the isotropic reorientation of anions were observed. The motional parameters for these modes were evaluated.