Low temperature vibrational spectroscopy. II. Evidence for order–disorder phase transitions due to weak C–H⋅⋅⋅Cl hydrogen bonding in tetramethylammonium hexachloroplatinate (IV), -tellurate (IV), and -stannate (IV) and the related perdeuterated compounds

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Abstract
The low frequency infrared and Raman spectra of normal and per-deuterated ((CH3)4N)2[MCl6] (M=Pt, Te, or Sn) have been measured at temperatures down to ∼100 K and evidence for phase transitions was found. The spectra have been carefully assigned and it was shown that bands due to forbidden methyl torsions and other noncubic features play a role, especially in spectra at low temperatures. Possible site symmetries of the [PtCl6]2− ion, which cannot have strictly Oh symmetry in either phase, have been deduced. The spectra of a mixed Pt : Te compound showed that the hexachlorometallate anions vibrate approximately independent of each other. The results have been compared with von der Ohe’s recent extensive low temperature Raman study on protonated compounds with M=U, Sn, and Zr, and his conclusions are discussed. It is shown that crystals of this kind can be characterized by methyl–chlorine interaction and it is suggested that the phase transitions are caused by an ordering of rotationally disordered methyl groups via the formation of weak C–H⋅⋅⋅Cl hydrogen bonds at low temperatures. The transition temperatures and hence the interactions are shown to depend on both the kind of hydrogen isotope and metal present in the crystals.

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