Reorientational Motions in Crystalline (CH3)3NHCl, Studied by Raman Spectroscopy

The IR and Raman vibrational spectra of crystalline (CH3)3NH+X− (X = Cl, Br, I) are recorded. A complete assignment of the measured frequencies to the internal vibrational modes of the (CH3)3NH+ cation in the three compounds is obtained. Above the phase transition of (CH3)3NH+ Cl− (at 308 K) a reorientational motion of the cation around its threefold axis is deduced from the Raman spectrum. This motion is studied by a line shape analysis of the degenerate νCN(E) vibration. A rotational diffusion constant D∥ = (0.38 ± 0.05) · 1012 sec−1 is deduced in agreement with quasielastic neutron scattering results.