SOLID-STATE DYNAMIC PROPERTIES OF TETRAKIS(TRIMETHYLSILYL)METHANE - HIGH-RESOLUTION SOLID-STATE C-13 AND SI-29 NMR INVESTIGATIONS

The dynamic properties of solid tetrakis(trimethylsilyl)methane (C(Si(CH3)3)4; TTMSM) have been determined using high-resolution solid state C-13 and Si-29 NMR spectroscopy. TTMSM is known to undergo a phase transition from a high-temperature ''rotator'' phase, in which there is isotropic reorientational motion of the molecule within the solid, to a low-temperature phase in which the molecular motion becomes anisotropic. In this low-temperature phase, one SiMe3 group of the TTMSM molecule becomes inequivalent from the other three SiMe3 groups and the molecules undergo two different types of dynamic process. These are classified as (1) exchange between the crystallographically unique SiMe3 group and an SiMe3 group within the set of three equivalent SiMe3 groups, and (2) hindered rotation of each SiMe3 group about the relevant C[central]-Si bond. The energy barriers for these dynamic processes are discussed, and the dynamic behavior of TTMSM is compared with that reported previously for solid tetrakis(trimethylsilyl) silane. The C-13 NMR results strongly suggest that the molecular symmetry of the TTMSM molecule in the low-temperature phase is C3. (C) 1994 Academic Press, Inc.