High-Resolution Solid-State Nuclear Magnetic Resonance and X-ray Structure Study of Choline Chloride, Bromide, and Iodide

Article abstract of DOI:10.1021/j100229a017

13C and 2H high-resolution solid-state NMR studies of choline chloride and bromide in their radiation-sensitive α phases indicate the onset of reorientational motions at temperatures preceding the transition to their radiation-stable (higher-temperature) β phases.These motions are effectively isotropic both below and above the α-to-β phase transitions.The early onset of the rotational motion and the accompanying drop in the radiation sensitivity strongly suggest that the extreme difference in radiation sensitivity between the α and β phases is principally due to processes affected by the rotational motions and not to crystallographic differences.Choline iodide, which is radiation normal at all temperatures so far studied, shows motions of the two methylene carbons even at room temperature.An X-ray study of the latter compound at ambient temperature indicates P2₁/m at the possible space group with appreciable disordering between the symmetric positions related by the reflection plane defined by the O, N, and one of the methyl carbons.The reorientational motion in choline iodide becomes completely isotopic only at temperatures close to its third phase transition (ca. 163 deg C).An analysis of the low-temperature spectra shows that in choline chloride and bromide the chemical shift tensor of the N-13CH2 carbon is directed with ?₃₃ approximately parallel to the N-CH2 bond.The 2H spectra (OH position) show that the electric field gradient tensors of all three compounds have similar principal values, are approximately axial, and show the same effects of motion as the 13C.