Proton magnetic resonance study of molecular motion in trimethylamine-trimethylaluminium and trimethylamine-trimethylgallium

Abstract

Proton magnetic resonance second moments and spin-lattice relaxation times of trimethylamine-trimethylaluminium and trimethylamine-trimethylgallium have been measured between liquid nitrogen temperature and their melting points. The second moments show that the methyl groups attached to Al or Ga are reorientating fast enough to narrow the n.m.r. line at the lowest observed temperatures. Reorientation of the N-methyl groups about their C₃ axes and of the whole molecule about the N—Al or N—Ga bond at n.m.r. line width frequencies can be distinguished from the second moment curves as separate processes occurring at higher temperatures. The variation with temperature of the spin-lattice relaxation time has been interpreted for each compound in terms of reorientation with one correlation time of methyl groups on Al or Ga, reorientation with a longer correlation time (at given temperature) of methyl groups on N, and reorientations with yet longer correlation times about the central bond. Separate motions are distinguished for reorientation of the two moieties in the gallium complex. Phase transitions are found in both solids, and in the highest temperature phase of each solid the spin-lattice relaxation results, augmented by values of the spin-lattice relaxation time in the rotating frame, have been interpreted as showing a combined motion of isotropic rotation and self-diffusion. Activation energies have been found for the several motional processes.