Rotational potentials of bridging and terminal methyl groups in trimethylaluminum-dimers

Trimethylaluminum is composed of dimers linked via bridging methyl groups. Neutron spectroscopy is used to determine internal and external modes, espe cially rotational tunnelling. Librations of methyl groups are assigned on t he basis of the isotope effect. Rotational potentials are derived. The brid ging CH3 group shows the strongest potential: Steric hindrance by the five- fold coordination of its C atom overcompensates the barrier reduction by th e elongated AI-C bond. (C) 2000 Elsevier Science B.V. All rights reserved.