Molecular motion in solid ammonia trimethylalane

Solid state H-2 NMR has been used to study molecular motion in deuterated ammonia trimethylalane (CH3)(3)AlND3. From analysis of the H-2 NMR lineshape between 123 and 298 K, reorientation of the -ND3 group about the molecular Al-N axis is shown to occur at a rate higher than 10(8) s(-1), and simulation of partially relaxed H-2 NMR lineshapes shows that the reorientation can be described as a 3-site 120degrees jump motion. From the temperature dependence of the H-2 spin-lattice relaxation time, the activation energy for this motion is estimated to be 9.3 +/- 0.3 kJ mol(-1). There is no evidence from either H-2 or Al-27 NMR data for any site-exchange between the sites occupied by the -ND3 and -CH3 groups. The anisotropy of the dynamics of (CH3)(3)AlND3 indicates that the orientation of the Al-N bond is highly constrained, presumably by a strong interaction between the electric dipoles of neighboring molecules. (C) 2003 Elsevier Inc. All rights reserved.