On aT1andT1rgr;Study of Molecular Motion and Phase Transitions in the Tetramethylammonium Halides

摘要

The temperature dependences of methyl group and (CH3)4Nplus;ion reorientations in the tetramethylammonium salts (CH3)4Nplus;Xminus;where X is Cl, Br, I were studied by means of the proton spinhyphen;lattice relaxation times in the static and rotating frame (T1andT1rgr;). TheT1values 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 ofT1in the three salts. Rotating frame spinhyphen;lattice relaxation times were also measured over temperatures from 200 to 380deg;K for (CH3)4NCl, giving results in agreement with those forT1. A discontinuity in theT1of (CH3)4NCl was found near 418deg;K. This led us to make a preliminary DTA investigation with results indicating that a firsthyphen;order phase transition may occur near 434deg;K and another definitely at528 plusmn; 1deg;K.The appearance of the phase above 434deg;K is accompanied by a modulation on the free induction envelope which we ascribe to piezohyphen;electric properties. The phase can be supercooled to room temperature and remains for a considerable length of time.