On a T1 and T1ρ Study of Molecular Motion and Phase Transitions in the Tetramethylammonium Halides

The temperature dependences of methyl group and (CH3)4N+ ion reorientations in the tetramethylammonium salts [(CH3)4N+X− where X is Cl, Br, I] were studied by means of the proton spin-lattice relaxation times in the static and rotating frame (T1 and T1ρ). The T1 values calculated for the motions are in good agreement with the double minima observed for each compound. Activation energies and frequency factors were obtained for both motions from the temperature dependence of T1 in the three salts. Rotating frame spin-lattice relaxation times were also measured over temperatures from 200 to 380°K for (CH3)4NCl, giving results in agreement with those for T1. A discontinuity in the T1 of (CH3)4NCl was found near 418°K. This led us to make a preliminary DTA investigation with results indicating that a first-order phase transition may occur near 434°K and another definitely at 528 ± 1°K. The appearance of the phase above 434°K is accompanied by a modulation on the free induction envelope which we ascribe to piezo-electric properties. The phase can be supercooled to room temperature and remains for a considerable length of time.