A. Detken et H. Zimmermann, REORIENTATIONAL TUNNELING OF PARTIALLY DEUTERATED METHYL-GROUPS - A SINGLE-CRYSTAL DEUTERON NMR-STUDY OF ASPIRIN-CH2D, The Journal of chemical physics, 108(14), 1998, pp. 5845-5854
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.