Motion of methylammonium ions in methylammonium hexachloroplatinate(IV) and hexachlorostannate(IV) as studied by proton magnetic resonance

Add time:08/08/2019    Source:sciencedirect.com

The proton spin—lattice relaxation time T1 at the two frequencies of 20 and 60 MHz and the second moment of proton magnetic resonance were determined for methyl-ammonium hexachloroplatinate(IV), its partially deuterated derivatives, and methylammonium hexachlorostannate(IV) at various temperatures between 4.2 and 400 K. The fully protonated complexes showed narrow PMR signals having the second moment of about 7 G2 even at 77 K, indicating that the CH3 and NH3 groups in methylammonium ions undergo rapid reorientation about the triad axis of the cation. The partially deuterated analogs, (CH3ND3)2PtCl6 and (CD3NH3)2PtCl6 gave rise to a single T1 minimum at nearly the same temperature attributable to the reorientation of the cations about their respective trial axes. On the other hand, the fully protonated complexes yielded two T1 minima, one of which, appearing at a lower temperature, was deep while the other, appearing below the transition point, was shallow. The deep minimum could be assigned to the reorientation of the cation as a whole about its triad axis, and the shallow minimum to the individual reorientations of the CH3 and NH3 groups in the cation. The potential barrier for the internal reorientation of the methylammonium ion could be evaluated to be about 8 kJ mol−1 by use of Woessner's method for the analysis of T1 data.

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