A. Detken et al., DEUTERON NMR OF METHYL-GROUPS IN THE TUNNELING REGIME - A SINGLE-CRYSTAL STUDY OF ASPIRIN-CD3, Zeitschrift fur Naturforschung. A, A journal of physical sciences, 50(1), 1995, pp. 95-116
We report the first single crystal deuteron NMR spectra of CD3 groups
which display the so-called +/-beta, +/-(\alpha\+/-beta) and +/-(2\alp
ha\+/-beta) lines characteristic of rotational tunneling in a sufficie
ntly clear manner to allow a quantitative comparison with the respecti
ve theory developed in 1988 by the group of W. Muller-Warmuth. The mol
ecular system we study is aspirin-CD3. We recorded spectra for differe
ntly oriented single crystals and measured spin-lattice relaxation tim
es T-1 in a wide temperature range. At 12.5 K we exploit the dependenc
e of the +/-(\alpha\+/-beta) and +/-(2\alpha\+/-beta) lines on the ori
entation of the applied field B-0 for determining the equilibrium orie
ntation of the CD3 group in the crystal lattice. The spectra display f
eatures which allow, by comparison with simulated spectra, a measureme
nt of the tunnel frequency nu(t). Its low temperature limit is (2.7+/-
0.1) MHz. It allows to infer the height V-3 Of the potential V(phi) in
which the CD3 group moves, provided that this potential is purely thr
eefold. We get V-3 = (47.2+/-0.5) meV. The transition from the tunneli
ng to the classical, fast reorienting regime occurs in the 15 K less t
han or similar to T less than or similar to 35 K temperature range. In
this range we observe a broadening, merging and eventually narrowing
of the +/-\alpha\ and +/-2\alpha\ lines in very much the way predicted
by Heuer. His theory, however, must be extended by taking into accoun
t all librational levels. The behaviour of the +/-beta lines in the tr
ansition temperature range signalizes a reduction of the observable tu
nnel frequency with increasing temperature. This reduction allows an i
ndependent measurement of the potential height and represents a test o
f the assumption of a purely threefold potential. From the T-1-data we
derive the temperature dependence of the correlation time tau(c) of t
he reorientational jumps. The plot of log tau(c) vs. 1/T follows a str
aight line for more than five decades. From its slope we get yet anoth
er independent number for the potential height. It agrees well with th
e other ones, which confirms the assumption of the essentially threefo
ld potential V(phi) in aspirin-CD3.