Molecular Motion of Cations and Phase Transitions in Tetramethylammonium Hexachloroplatinate (IV), ‐tellurate (IV), and ‐stannate (IV) as Studied by CW and Pulsed ¹H NMR and DTA Measurements

Abstract

Abstract The temperature dependences of the 1 H spin‐lattice relaxation time T 1 , at 20 MHz and of the second moment of 1 H NMR absorptions were studied in tetramethylammonium hexachloroplatinate (IV), ‐tellurate (IV), and ‐stannate (IV), which are referred below as (tetMA) 2 PtCl 6 , (tetMA) 2 TeCl 6 , and (tetMA) 2 SnCl 6 , respectively. From the measurements of DTA and pulsed 1 H NMR carried out below room temperature, four phases were found in (tetMA) 2 PtCl 6 while two phases in both of the remaining complexes. The 1 H NMR studies revealed that, with decreasing temperature, the most part of the rotational or reorientational freedom of tetMA cations freezes at almost the same temperature, but a small fraction of the motional freedom still remains even at lower temperatures. From the analysis of the shallow T 1 minimum clearly observed for (tetMA) 2 TeCl 6 , this motion activated in this complex and also in (tetMA) 2 PtCl 6 is unambiguously attributed to a small‐angle reorientation of four methyl groups in the cation around their staggered conformation. The mechanism of the phase transitions detected in these complexes is discussed in relation to the motion of tetMA cations.