Molecular motion in solid ammonia trimethylalane

Abstract Solid state 2 H NMR has been used to study molecular motion in deuterated ammonia trimethylalane (CH 3 ) 3 AlND 3 . From analysis of the 2 H NMR lineshape between 123 and 298 K, reorientation of the –ND 3 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 2 H NMR lineshapes shows that the reorientation can be described as a 3-site 120° jump motion. From the temperature dependence of the 2 H 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 2 H or 27 Al NMR data for any site-exchange between the sites occupied by the –ND 3 and –CH 3 groups. The anisotropy of the dynamics of (CH 3 ) 3 AlND 3 indicates that the orientation of the Al–N bond is highly constrained, presumably by a strong interaction between the electric dipoles of neighboring molecules.