Broad-line n.m.r. investigation of molecular motion in solid boron trifluoride amine complexes

JOHN A. RIPMEESTER. Can. J. Cnem. 54, 3453 (1976). The solid pyridinium chloride, bromide, and iodide salts were studied using 1H nuclear magnetic resonance and differential scanning calorimetry. Phase transitions were observed at 345 K for the chloride, 269 K for the bromide, and 247 K for the iodide. Well below each transition, the pyridinium ions are held rigidly in the crystal lattice, whereas above each transition the ions reorientate rapidly about an axis at right angles to the ring planes. From the temperature dependence of the spin-lattice relaxation times the high temperature phase reorientational activation energies were determined to be 1.55, 2.30, and 4.20 kcal/mol for the chloride, bromide, and iodide, respcctively. [Traduit par le journal]

“…Methods of temperature measurement and control, as well as methods of data treatment have been described previously (12,13).…”

Section: (B) Equipment and Experimental Techniquesmentioning

confidence: 99%

Molecular motion, phase transitions, and disorder in the pyridinium halides

Ripmeester¹

1976

“…Previous measurements of the linewidths and the second moments of both 'H and ' 9~ down to 68 K did not reach the rigid-lattice values (9,10). We therefore extended the temperature region of the continuous-wave experiments down to 53 K. The results indicate that the second moment of 'H reaches an almost flat plateau below 60 K at (31 * 1) G2.…”

Section: Second Momentmentioning

confidence: 84%

Molecular motion in the crystalline donor–acceptor complex trimethylamine–trifluoroborane as studied by proton and fluorine magnetic resonance

Chihara¹,

Nakamura²,

Chihara³

1988

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...

“…'National kesearch Council of Canada Scholarship holder 1967-1970. been described elsewhere (9). The adiabatic rapid passage experiments were done at 8 MHz using the methods reported by Janzen et al (10).…”

Section: Methodsmentioning

confidence: 99%

Nuclear Magnetic Resonance Studies of Molecular Motion in a Number of Stearates and Oleates

Ripmeester¹,

Dunell²

1971

Self Cite

The broad line p.m.r. spectra of the alkali metal oleates have been observed from 77 "K up to or beyond the crystalline to waxy phase transition. Lower phase transition temperatures are observed in the oleates than in the stearates. This fact is attributed to larger entropies of transition in the oleates than in the stearates. The n.m.r. second moments indicate that in the stearates the packing of chains is probably not closer than in the oleates and consequently that the barriers to chain reorientation are not significantly higher in the stearates than in the oleates. Sodium oleate and stearate both appear to behave irregularly in the series of alkali metal soaps. Spin-lattice relaxation times have been measured by adiabatic rapid passage methods for both alkali metal oleates and stearates. Values of TI and of the activation energy barrier for the reorientation of end methyl groups are compared with values obtained by other workers. N o significant difference is seen between relaxation processes in stearates and oleates, at least in the lower temperature range. Soaps which seem to have some amorphous character have a second relaxation mechanism, in addition to end methyl group rotation, which is evidently important in the region of about 150-250 OK.