REORIENTATIONAL TUNNELING OF PARTIALLY DEUTERATED METHYL-GROUPS - A SINGLE-CRYSTAL DEUTERON NMR-STUDY OF ASPIRIN-CH2D

Partially deuterated methyl groups in single crystals of aspirin are i nvestigated by deuteron NMR at temperatures between 8 K and room tempe rature. The CH2D groups perform reorientations which are governed by a rotational potential with three wells, two of which are almost equall y deep whereas the third is significantly deeper. At temperatures belo w 20 K, a so far unobserved type of incoherent tunneling process is id entified. This process consists in reorientations between the two uppe r potential wells which are fast on the time scale of the quadrupolar interaction, whereas transitions into the deeper well are slow on this time scale. At temperatures above 35 K, the methyl groups perform the rmally activated stochastic reorientations between all three potential wells. By determining the relative populations of the three wells as a function of temperature, the energy difference between the lower and the two upper wells is found to be 3.3 meV. This amounts to almost 8% of the average barrier height, which is determined from the temperatu re dependence of the spin-lattice relaxation time to be 43 meV. (C) 19 98 American Institute of Physics.