ROTATIONAL CORRELATION-FUNCTIONS OF PLASTIC-CRYSTALLINE NEOPENTANE FROM NEUTRON-SCATTERING EXPERIMENTS

The rotational motions of neopentane molecules in the orientationally disordered phase have been studied by incoherent neutron scattering. The measurements were performed with a rotating-crystal spectrometer (incoming wavelength 4.1 Å), covering a wave-vector transfer range of 0.23-2.18 A ̊-1. Its resolution leads to an observation time up to about 5 ps. From the time-of-flight spectra intermediate scattering functions and frequency distributions were obtained. The frequency distributions were fitted with a damped Debye-model for the translational motions. From the intermediate scattering functions the rotational part was deduced and, using the partial-wave expansion, rotational correlation functions could be determined up to third order. These first-and second-order correlation functions are compared with infrared and Raman data and they are found to be in good agreement. From the behaviour of the elastic intensity of the spectra it can be concluded that on the time-scale covered the distribution of the protons of the molecule does not randomize. A comparison of elastic intensities and rotational correlation functions with the predictions of several, symmetry adapted, jump reorientation models is made. The best fit is obtained for a model with D2d symmetry where 90° reorientations about the fourfold axis occur more frequently than 180° jumps about the two perpendicular twofold axes. © 1979.