th  - 1987 - 51st ENC Conference

WF46
DEUTERIUM NMR STUDY OF METHYL GROUP DYNAMICS
IN L-ALANINE
Kebede Beshah*, Bdvard T. OleJniczak +, and Robert G. Griffin
Francis Bitter National Nagnet Laboratory
Massachusetts Institute of Technology
Cambridge, NA 02139
Deuterium quadrupole echo spectroscopy is used to study the
dynamics of the CD 3 group in polycrystalline L-alanine-d 3. Temperature-
dependent quadrupole echo lineshapes, their spectral intensities and x-
dependence, and the anisotropy of the 2H spln-lattice relaxatlon were
employed to determine the rate and mechanism of the -CD 3 group motion.
The rigid lattice Pake pattern observed at low temperature (T < -120°C)
transforms to a triplet spectrum characteristic of threefold Jumps in the
intermediate exchange regime (-120°C to -70°C) and this in turn to a
Pake pattern of reduced breadth at higher temperatures (T > -70°C). The
quadrupole echo lineshapes and their x-dependence, which are especially
sensitive to the rate and mechanism of the motion, can be simulated
quantitatively vith the threefold jump model. Ve find ga = 20.0 kJlmole
for this process vhich is higher than is observed for most methyl groups,
probably because of steric crowding in the L-bla molecule. Finally, we
observe a lineshape due to the presence of multiple crystallographic
forms which suggest that this technique can be extended to studies of the
dynamics of more complex systems.
+Present address: Abbott Laboratories
North Chicago, IL 60046