SOLID-STATE PROTON SPIN RELAXATION AND METHYL REORIENTATION IN ISOPROPYLBENZENE

We have measured the temperature and Larmor frequency dependence of th e proton spin-lattice and spin-spin relaxation rates in solid isopropy lbenzene. The sample melts at too low a temperature to observe the hig h-temperature frequency-independent regime from which a rotational bar rier is normally extracted. By measuring the rates at three Larmor fre quencies, however, we demonstrate that all the relevant parameters tha t characterize methyl reorientation can be overdetermined within the c onfines of a given dynamical model even though the model may not be un ique. The relaxation technique is very sensitive to the state of the s olid, and interesting and unusual thermal history effects are presente d and discussed.