Molecular motion in solid ammonia trimethylalane

Solid state ²H NMR has been used to study molecular motion in deuterated ammonia trimethylalane (CH₃)₃AlND₃. From analysis of the ²H 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 10⁸ s^-1, and simulation of partially relaxed ²H NMR lineshapes shows that the reorientation can be described as a 3-site 120° jump motion. From the temperature dependence of the ²H 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 ²H or ²⁷Al NMR data for any site-exchange between the sites occupied by the -ND₃ and -CH₃ groups. The anisotropy of the dynamics of (CH₃)₃AlND₃ indicates that the orientation of the Al-N bond is highly constrained, presumably by a strong interaction between the electric dipoles of neighboring molecules.