Reorientations in crystalline (CH₃)₃NHCl studied by quasielastic neutron scattering

Abstract

Reorientations of the CH₃ groups and of the cation are studied in the two phases (phase transition at 308 K) of (CH₃)₃NH⁺Cl^- by quasielastic neutron scattering using a time of flight and a backscattering instrument. In the high temperature phase the experimental results can be described by rotational diffusion of the cation around its threefold axis with D _r = 0·49 × 10¹² s^-1 (τ_c = 1/D _r = 2·05 ¢ 10^-12 s) at 333 K. The reorientation of the methyl groups is too slow to produce in this phase a quasielastic broadening detectable by the time of flight experiment. In the low temperature phase the observed spectra can be explained by simultaneous 120° jumps of the cation and of the methyl groups. The correlation times for the two reorientations are about the same: τ_c ≈ 4 × 10^-10 s at 273 K.