Proton spin–lattice relaxation study of molecular motion in tetramethylarsonium halides

Abstract

Tetramethylarsonium chloride, bromide and iodide have been prepared and their spin–lattice relaxation times measured by proton magnetic resonance pulse spectrometry in the temperature range 77–540 K. The two minima in T₁ found for each salt have been assigned to reorientation of methyl groups about the carbon to central atom bonds and to a tumbling motion of the whole cation which approximates isotropic reorientation but is probably reorientation in random sequence about each of the four threefold axes of the tetrahedral cations. The activation energies and other relaxation parameters have been calculated. The activation energies have been compared with those of the corresponding ammonium and phosphonium salts, and where possible, the variations from compound to compound related to the crystal structures. To this end, the X-ray powder diffraction photographs of the arsonium salts were examined and the chloride and bromide indexed in the hexagonal system. The unit cell dimensions are calculated. The iodide is apparently of lower symmetry. At least three phase transitions have been noted in the iodide.