A study of molecular motion in tetramethylphosphonium halides by proton magnetic resonance

Abstract

T. T. ANG and B. A. DUNELL. Can. J. Chem. 54, 1985 (1976), Spin-lattice relaxation times of tetramethylphosphonium chloride, bromide, and iodide were measured between 100 and 500 K and the two minima in TI found for each compound have been assigned to methyl group reorientation and whole cation tumbling. The second moments also indicate that the cations are tumbling isotropically at nmr frequencies in the upper half of this temperature range, and suggest that librational oscillation of the whole cation occurs at frequencies at least of the order of lo5 s-' near 150 K. The energy barriers for both methyl group reorientation and isotropic tumbling decrease from chloride to bromide but increase when one goes from bromide to iodide. Powder photograph X-ray diffraction analysis indicates that the chloride and bromide have hexagonal crystal structures (a and c measured), but that the iodide has lower, undetermined symmetry.T. T. ANG et B. A. DUNELL. Can. J. Chem. 54, 1985 (1976. On a mesure, entre 100 et 500 K, les temps de relaxation spin-rkseau des chlorures, bromures et iodures de tetramethylphosphonium et on a attribue les deux minima dans TI, trouves pour chacun des composes, a la reorientation du groupe methyle et a un renversement complet du cation. Les moments seconds indiquent aussi que les cations se renversent d'une faqon isotrope aux frequences rrnn dans la partie superieure de l'intewalle de temperature et ces risultats suggerent que I'oscillation librationnelle du cation complet se produit a des frequences au moins de l'ordre de lo5 s-I pres de 150 K. Les barrieres energetiques, pour la reorientation du groupe methyle ainsi que celle pour le renversement isotrope, tous les deux, diminuent lorsque l'on passe du chlorure au bromure mais augmentent quand on passe du bromure a I'iodure. Les analyses de diffraction de rayon-X en poudre indiquent que le chlorure et le bromure ont des structures cristallines et hexagonales (on a mesure a et c); toutefois l'iodure a une symetrie plus basse et non determinee.[Traduit par le journal]Motional properties of compounds of the type (CH3),Z, where Z = C, Si, Ge, Sn, and Pb, have been studied extensively by both broadline nmr and nmr spin-lattice relaxation times (1-5). There has also been considerable interest recently in the motional properties of the tetramethylammonium ion (5-10). In this paper we report a spin-lattice relaxation study of polycrystalline samples of the compounds (CH3),PCX-, where X = C1, Br, and I. Infrared and Raman spectroscopy have shown that the tetramethylphosphonium ion in (CH3),PC1 has the same tetrahedral configuration as the tetramethylammonium ion (1 1). ExperimentalThe tetramethylphosphonium halides were prepared (1 I) by mixing (CH,),P (Pfaltz and Bauer, New York) in 'Visiting Scientist. Permanent address: School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia. dry ether with the appropriate methyl halide (Matheson) and refluxing for 2 days under nitrogen. The precipitated product was filtered off, washed with dry ether,...

Trimethylarsonium iodide

“…[(CH 3 ) 4 As] + cations have been reported with a variety of different counteranions. Tetramethylarsonium halides, for example, have been characterized by X-ray structural analysis for the heavier halides bromide (Collins et al, 1963) and iodide (Assenmacher & Jansen, 1995), whereas the chloride compound has not yet been characterized by single-crystal structure analysis, although Debye-Scherrer investigations have revealed that it is isostructural with (CH 3 ) 4 AsBr (Ang & Dunell, 1976). In contrast, the trimethylarsonium cation has been characterized only twice.…”