Methyl group rotation in trimethylaluminium

The stable molecular unit of trimethylaluminium is the dimer linked via bridging methyl groups. Neutron spectroscopy is used to determine internal and external modes from µeV (rotational tunnelling transitions of methyl groups, stochastic rotational jumps) to meV (deformations, librations, phonons etc). The dimer appears to be non-rigid with a very soft bend mode about an axis connecting the two bridging methyl groups. Rotational potentials are derived from tunnelling modes and barrier heights deduced from quasielastic spectra. Internal modes obtained from ab initio calculations for the isolated dimer are consistent with measured vibrations with respect to eigenenergies and band intensities calculated by the CLIMAX program. The bridging CH3 group shows the strongest and almost purely intramolecular potential due to a loss of a single bond axis of its pentacoordinated carbon atom.