Deuterium NMR study of methyl group dynamics in L-alanine

Abstract

Deuterium quadrupole echo spectroscopy is used to study the dynamics of the CD₃ group in polycrystalline L‐alanine‐d₃. Temperature‐dependent quadrupole echo line shapes, their spectral intensities and τ dependence, and the anisotropy of the ²H spin‐lattice relaxation were employed to determine the rate and mechanism of the –CD₃ 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 to −70 °C) and this in turn to a Pake pattern of reduced breadth at higher temperatures (T>−70 °C). The quadrupole echo line shapes and their τ dependence, which are especially sensitive to the rate and mechanism of the motion, can be simulated quantitatively with the threefold jump model. We find E_a=20.0 kJ/mol for this process which is higher than is observed for most methyl groups, probably because of steric crowding in the L‐Ala molecule. Finally, we observe a line shape due to the presence of multiple crystallographic forms which suggests that this technique can be extended to studies of the dynamics of more complex systems.