Neutron scattering studies of the solid tetramethyl compounds of silicon, germanium and tin

Orientational disorder in plastic neopentane

“…At this point, two different lines of thinking may be followed. i) from a dynamic point of view one could propose that the tetrahedral molecules reorient with a complex motion, which could not be identified in reference [13], and was not observed in the present analysis because the wrong trajectories were chosen. This, however, seems difficult to prove.…”

“…- an inelastic incoherent neutron scattering experiment performed on a powder sample [13] concluded that the most probable reorientations corresponded to jumps from one D2 d orientation to another through 90° rotations around the fourfold axis which is common to the molecule and the crystal.…”

“…The data collection is consequently long. 32 independent reflexions could be measured at 150 K, 31 at 173 K, 13 (3) at 187 K, 27 at 225 K and 21 at 253 K, which is not unusual in a plastic crystal [5]. As many equivalent reflexions as compatible with the angle limits imposed on the Euler mount by the size of the cryostat were collected which amounted to a total of 150 reflexions.…”

Orientational disorder in plastic neopentane (paralipomena)

“…roughly the same value as 0 x 2) (12° and 11° respectively) with only a small improvement in the quality of the fit. Such a result does not favor a tilted orientation of the molecule, but rather suggests that the probability distribution is no longer Gaussian at these tempera- [5,7,10] had concluded in a rapid variation of the orientational disorder with temperature. As p remains equal to zero at all temperatures, one would expect this disorder to show on the thermal variations of (J;) and (J;) ( Fig.…”

“…I is equal to 2.58 x 10-45 kg m2 (1.95 x 10-45 kg m2 in the case of an hydrogenated molecule C (CH3)4), so that Treor varies from approximately 1.8 ps at 150 K to 1.4 ps at 253 K (respectively 1.5 ps and 1.2 ps in C (CH3)4) -Another possible evaluation of Treor is to assume that the kinetic energy of a reorienting molecule is not the ensemble average, but the hindering barrier i.e. approximately 500 K [10]. This supplies Treor = 1 ps for the deuterated molecule, and 0.8 ps for the hydrogenated molecule.…”