This paper is dedicated to Professor Charles A. McDowell on the occasion of his 70th birthday YUKIKO N. CHIHARA, NOBUO NAKAMURA, and HIDEAK~ CHIHARA. Can. J. Chem. 66, 1848 (1988). The proton and fluorine second moments and the spin-lattice relaxation times TI (at 25.75 MHz) were measured in the solid complex trimethylamine-trifluoroborane from 53 to 300 K. There is a cross-relaxation effect between 'H and 1 9~ below 120 K. Their TI minima near 140 K led to activation energies of 10.6 and 11.1 kJ mol-I for the reorientation of the methyl and the trifluoride groups, respectively. An additional minimum observed in the TI of protons at about 240 K was attributed to the reorientation of the trimethylamine group about the N-B bond with an activation energy of 25.0 kJ mol-I. A comparison of these potential parameters with those in other trimethylamine complexes led to a notion that the activation energies for the reorientation of the (CH3)3N group in the trimethylamine-trifluoroborane complex are mainly governed by the intramolecular steric effect. Trimethylamine (TMA) forms donor-acceptor complex compounds with a variety of electron accepting materials (1). Some of these complexes have threefold symmetry and different types of intramolecular degrees of freedom. Moreover, some undergo a phase transition or transitions in their crystalline state (2-4) but the nature of the phase transitions has not yet been clarified. To understand the mechanism of phase transition(s) as well as the intricate inter-or intramolecular interactions in the crystals of these molecular complexes, studies are necessary both of the dynamical properties of molecules and of the crystal lattices in relation to detailed structural studies.The 1:l complex compound between TMA and trifluoroborane (TFB) is a typical example of such a complex. It has a molecular threefold symmetry axis and three intramolecular rotational degrees of freedom, and can undergo two different rotations as a whole, i.e., the rotations about the molecular symmetry axis and about the two short axes. The molecule has a large dipole moment (5.76 D) (5) and the crystal belongs to a rhombohedra1 space group with three TMB .TFB complexes per unit cell (6). This compound does not undergo any phase transition between 77 K and its melting point and therefore is a good specimen for examining molecular motion in detail without interruption by the discontinuities of a phase transition. This paper presents the results of proton and fluorine nuclear magnetic resonance and relaxation measurements below room temperature. The molecular modes are assigned and the cross-relaxation effect between the proton and fluorine is discussed. The origin of the activation processes and the possible mechanism of phase transitions in trimethylamine complexes will be considered. ExperimentalTMA-TFB was synthesized according to a method reported in the literature (7); a saturated aqueous solution of KOH was added to trimethylamine hydrochloride from a commercial source and gaseous TMA thus generated was intro...
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