| Popis: |
Rotational motions of pivalic acid are analysed in its solid plastic phase, using time of flight neutron spectroscopy. Four different partially deuterated compounds are studied, to analyse motions of the different parts of the molecule. From a study of the compound (CH 3 ) 3 CCOOD, an elastic incoherent structure factor is extracted which corresponds to a model based upon uniaxial rotational diffusion of the t-butyl groups about their threefold symmetry axis, together with fluctuations of the long axes of the dimer units, about their mean direction of orientation along the [110] lattice axes. The width of the angular distribution is found equal to about 10°. A phenomenological analysis of the low-energy part of the inelastic spectra in terms of a single overdanped oscillator confirms these results, leading to a mean oscillatory amplitude of about 8°. Subtraction of the spectra obtained with the fully deuterated compound (CD 3 ) 3 CCOOD, from those measured with (CD 3 ) 3 CCOOH enables the study of the motion of the carboxylic group. This is found to consist of 180° jumps about the central molecular C−C bond. These results are entirely consistent with the measurements performed on the fully hydrogenated compound (CH 3 ) 3 CCOOH Etude des mouvements de rotation dans la phase plastique par spectrometrie de neutrons par temps de vol, sur des echantillons partiellement deuteries |
