Spin-lattice relaxation in ammonium compounds with a complex molecular dynamics

Deuteron NMR Study of the Diverse Mobility of the Ammonium Ions in the Ordered Phase of (ND4)2PtCl4

“…Proton NMR spectra of ðNH 4 Þ 2 PtCl 4 at liquid helium temperature were interpreted as resulting from rotational tunnelling with a preference for one of the C 2 axes [9].…”

“…All these observations led us to decide to put ammonium tetrachloroplatinate on the list of compounds to be studied at low temperatures with regard to uniaxial reorientations of ammonium ions [9]. Given this type of motion, the plot of the spin-lattice relaxation time T 1 vs. temperature will exhibit two minima both for protons and deuterons.…”

“…The low-temperature minimum is produced by uniaxial reorientation about the preferred axis, while that at a higher temperature results from fast reorientations about the other C 3 and C 2 axes. When the theoretically derived ratio of the relaxation times at the two minima is compared with the experiment, the symmetry of the uniaxial reorientations can, in principle, be determined [9].…”

“…4 of [9]. However, in this case the model of uniaxial ammonium ion reorientations had to be rejected because single-crystal deuteron NMR spectra showed a narrow central component down to the lowest temperature explored, 4:2 K; see Fig.…”

“…However, in this case the model of uniaxial ammonium ion reorientations had to be rejected because single-crystal deuteron NMR spectra showed a narrow central component down to the lowest temperature explored, 4:2 K; see Fig. 9 of [9]. The underlying idea of all motional models considered in the previous relaxation studies of NH þ 4 and ND þ 4 ions (thought to undergo uniaxial reorientations) was that all ammonium ions in the compound are dynamically equivalent.…”