Low temperature vibrational spectroscopy. III. Structural aspects and detection of phase transitions in crystalline alkali metal and tetramethylammonium hexabromotellurates and platinates

Vibrational study of (CH3)4NSbCl6 and [(CH3)4N]2SiF6

(CH3)4NSbCl6 and [(CH3)4N]2SiF6 are face-centred cubic compounds at ambient temperature with a = 11.548 and 11.172 A, respectively. The vibrational spectra of these two compounds are discussed in relation to their crystal structure and other compounds containing (CH3)4N+ ions. The assignment of the observed bands is discussed. The Raman and infrared spectra of [(CH3)4N]2SiF6 show that the cations and anions are not distorted inside the crystals and are weakly hydrogen-bonded to each other. The infrared spectrum of (CH3)4NSbCl6 confirms a cubic structure for this compound at ambient temperature, in which cations are in octahedral environments and can be interpreted as being disordered groups. No evidence could be found for the existence of hydrogen bonding between cations and anions in this compound. The phase transition of (CH3)4NSbCI6 is studied by means of Raman spectroscopy. It is believed to be governed by the reorientational motions of tetramethylammonium cations and may be classified as an 'order-disorder' type.

35Cl NOR and 19F NMR relaxation studies of CClF2 group dynamics in N(CH3)4H(ClF2CCOO)2

The reorientation of CClF2 groups in N(CH3)4H(ClF2CCOO)2 has been studied using pulsed NQR and NMR techniques. The temperature dependence of both chlorine (35Cl) NQR and fluorine (19F) NMR spin-lattice relaxation has been measured T1Q of chlorine is attributed to the sum of two contributions: the reorientation of CClF2 groups and the modulation of the electric field gradient (EFG) produced by the motion of the N(CH3)4+ cations. The activation energies were determined for both kinds of motion. The fluorine relaxation is dominated by an intramolecular 19F-1H dipolar interaction. In analysis of cross-relaxation effects the spectral density functions have been evaluated using the motional parameters obtained from NQR data.