Solid-solid phase transitions determined by differential scanning calorimetry. III. Organosilicon compounds

High-resolution solid-state 13C and 29Si NMR investigations of the dynamic properties of tetrakis(trimethylsilyl)silane

“…Above the phase transition, TTMSS is a plastic crystal2 and each molecule undergoes rapid isotropic motion about a fixed centre of mass, rendering all Me carbons equivalent on the 13C NMR timescale. The crystal structure of TTMSS below the phase transition temperature is not known, but it is assumed from the observation of two peaks with 3 : 1 intensity ratio in the 13C and 29Si NMR spectra that in the low-temperature phase one SiMe3 group (denoted type [l]) becomes crystallographically inequivalent from the other three SiMe3 groups (denoted type [2]), at least on the timescale of these NMR experiments. This suggests that, in this temperature regime, the TTMSS molecule lies on a crystallographic threefold symmetry axis which is coincident (on averge) with the Si(c)-Si(1) bond [where Si(c) denotes the central Si].…”

“…This suggests that, in this temperature regime, the TTMSS molecule lies on a crystallographic threefold symmetry axis which is coincident (on averge) with the Si(c)-Si(1) bond [where Si(c) denotes the central Si]. In terms of the dynamic properties, it is clear that rapid rotations of the type [2] SiMe3 groups [ie. motion of type ( b ) for these groups] is required to render all nine Me groups equivalent.…”

“…We now consider the spectral changes associated with the type [2] SiMe3 groups occurring from ca. 208 to 152 K. In view of the earlier discussion, it is plausible to assume that, on passing from the high-temperature phase to the low-temperature phase, the symmetry of the TTMSS molecule in the crystal is lowered to C3, with the threefold axis coincident with the Si(c)-Si(1) bond.…”

“…208 to 152 K. In view of the earlier discussion, it is plausible to assume that, on passing from the high-temperature phase to the low-temperature phase, the symmetry of the TTMSS molecule in the crystal is lowered to C3, with the threefold axis coincident with the Si(c)-Si(1) bond. Each of the type [2] SiMe3 groups is therefore no longer located in a local potential field with threefold rotational symmetry but in a local potential field with mirror symmetry [the mirror plane being the plane containing the Si( l)-Si(c)-Si(2) atoms]. For any general rotation angle of the %Me3 group about the Si(c)-Si(2) bond, all three Me groups will be inequivalent, although for the specific rotation angles in which one Si(2)-C bond lies in the mirror plane, two of the Me groups become equivalent to each other, but inequivalent to the other Me group.…”

“…For any general rotation angle of the %Me3 group about the Si(c)-Si(2) bond, all three Me groups will be inequivalent, although for the specific rotation angles in which one Si(2)-C bond lies in the mirror plane, two of the Me groups become equivalent to each other, but inequivalent to the other Me group. At sufficiently high temperature, rapid type (b) rotation of the type [2] SiMe3 group will render all of its Me groups equivalent on the NMR timescale. When this rotational motion of the type [2] SiMe3 groups becomes hindered, two situations can be envisaged.…”