The spin-lattice relaxation times of the solid conlplexes of trimethylamine with I?, IC1, Br,, BCl:,? and BBr:, and the relaxation null tinles of trimethylphosphine -boron trichloride and -boron tribromide have been measured over a range of temperature by pulsed proton magnetic resonance. A minimum in T , corresponding to reorientation of the methyl groups ;~baut the C-N bonds is observed in each complex, although i n the case of Me,,NBr, the n~ininium corresponds to a combination of methyl group reorientation and reorientation of the whole Me:,N moiety. The complexes with the boron trihalides show a second minimum in T I . These minima have been analyzed in terms of either of two possible motions, reorientation of NMe,, about its threefold axis, o r that same motion combined with isotropic nlolecular tumbling. We favor the interpretation involving the simpler motion. Activation energies have been measured for all the motions. T h e barriers to reorientation of methyl groups about the C-N bond i n the moiety Me,,N appear to increase as the thermal stability of the complex decreases.Les temps de relaxation spin-rCseau des complexes solides de la trimkthylamine avec I,, ICI, Bra, BCl::, et BBr:! et les temps ztro de relaxation du trimtthylphosphine -trichlorure de bore et tribromure de bore ont CtC mesurts dans un intervalle de temptrature par la "rCsonance magnCtique d u proton par pulsation". Un minimum de TI qui correspond B la rkorientation des groupes methyles autour des liaisons C-N est observC dans chacun des complexes bien que dans le cas de Me:,NBr:: le minimum correspond 5 la combinaison de la rCorientation du groupe mCthyl et de la rtorientation de tout I'ensemble de Me,,N. Les conlplexes avec les trihalogtnures de bore nlontrent un second minimum de Ti. Ces minima ont C t t analysts en fonction de I'un des deux mouvements possibles, rkorientation de NMe,, autour de son axe ternaire, ou encore le mbme mouvement associC B l'agitation molCculaire isotrope. Nous prCftrons I'interprB tation impliquant le mouvement le plus simple. Les Cnergies d'activation ont Ctb rnesurCes pour tous les mouvements. Les barrikres la reorientation des groupes mCthyles autour de la liaison C-N dans la partie Me,,N tentent B augmenter lorsque la stabilitC thermique du coniplexe diminue.[Traduit par le journal]Can. J. Chem., 52, 1840 (1974) Nuclear magnetic resonance studies of me- (2), also by the wide-line method. These two thy1 group reorientation and other molecular groups also estimated activation energies from motions in solid addition complexes involving their line width data, but as they point out, there methyl compounds of elements from main are approximations and uncertainties inherent in groups 111 and V have been made in several this method of obtaining activation energies so laboratories (1-5). As a further contribution to that the derived results must be regarded with these studies, we have measured activation some reservation. energies for the molecular motions in the comExperimental plexes of trimethylamine wi...