Phase behaviour and dynamics in ethylammonium halide salts

Abstract

1H and 2H NMR spin-lattice relaxation times (T1) have been studied as a function of temperature to obtain activation energies for the threefold reorientations of CH3 groups (Ea = 22.83, 18.95, and 18.39 kJ mol–1) and NH3 groups (Ea = 47.64, 45.31, and 28.74 kJ mol–1) in the β-phases of ethylammonium chloride, -bromide, and -iodide, respectively, for comparison with barriers determined from Raman torsional frequencies. T1, 2H NMR lineshapes, and DSC studies have been used to clarify the existence of a β → α phase transition above room temperature in all three salts, and the absence of a transition at low temperature. 2H NMR lineshape analysis indicates a rapid n-fold (probably fourfold) reorientation of the whole molecule in the α-phase. This motion occurs about an axis in the molecular CCN mirror plane in the bromide and iodide. In the chloride, the axis is in the plane perpendicular to the bisector of the DCD bond angle.Key words: ethylammonium halides, solid state 1H and 2H NMR, molecular dynamics, rotational barriers, phase behaviour.