127I NQR and 1H NMR studies of 4-aminopyridinium tetraiodoantimonate(III); molecular motion and phase transition

Abstract

The crystals of 4-NH 2 PyHSbI 4 (Py = C 5 H 4 N) have been investigated by means of 127 I NQR, 1 H NMR T 1 and DTA. The crystals can exist in two modifications of β and α(I) at room temperatures. The α(I)-phase is a metastable state which is obtained when the stable β form is heated. The α(I)-phase undergoes a first-order type phase transition of α(I) ↔ α(II) at 272 K (on heating), while the β-phase is stable down to 77 K. Four and two 127 I (m = ±1/2 ↔ ±3/2) NQR lines have been found for the β-and α(II)-phases, respectively. One half of them is assignable to the terminal I atom(s) and the other to the bridging I atom(s) in each phase. All the resonance lines of the α(II)-phase underwent a disappearance above ca. 240 K and no resonance line was observed in the α(I)-phase. The second moment M 2 value of 1 H NMR spectra with 8 G 2 at 290 K shows that the 4-NH 2 PyH + cations reside in the rigid lattice in the β-phase. In contrast, in the α(I)-phase the cation rotates about an axis more symmetric than pseudo threefold axis. The activation energy of 21 kJ mol −1 was estimated for the reorientational motion in the α(I)-phase from the 1 H NMR T 1 measurements. The nature of phase transitions in the 4-NH 2 PyHSbI 4 is discussed in comparison with that in 4-NH 2 PyHSbBr 4 .