Rotational tunneling of NH4 and NH3D and dipolar interaction in (NH3.6D0.4)2PtCl6

Deuteron NMR spectra and relaxation in fully and partly deuterated (NH4)2ZnCl4

Deuteron NMR spectra and relaxation were studied at the resonance frequency of 46MHz in polycrystalline fully and partly deuterated (NH(4))(2)ZnCl(4) between 300 and 5K. Spectral components confirm existence of ammonium positions with different potential symmetry, resulting in two- and threefold reorientation of ammonium ions. The temperature dependence of the spin-lattice relaxation rate discloses two time constants in the whole range. The fitting procedure allows the separation into contributions from subsystems of ions in respective potentials. Two relaxation rate maxima are attributed to ions performing threefold uniaxial reorientation at low temperatures. The lower-temperature maximum is observed at T36K. With increasing temperature reorientations about remaining axes start to contribute leading to the other maximum near 100K. The other category of ammonium ions gives rise to the maximum at about 50K. Below this temperature the dominant motion seems to be 180( composite function) reorientations about one twofold axis according to observed spectra. Consistent picture of ion mobility is accomplished for 5%, 30%, 70% and 100% deuterated compounds.

Deuteron spin-lattice relaxation in partly and fully deuterated (NH(4))(2)PdCl(6)

Deuteron spin-lattice relaxation and spectra were studied in partially and fully deuterated (NH(4))(2)PdCl(6) in the temperature range 5-300K. The relaxation rate maximum was observed at 45K in (ND(4))(2)PdCl(6). Its value is reduced due to limited jumps by about 33% relative to the theoretical value expected for threefold reorientations. Limited jumps correspond to an N-D vector jumping between six directions on a cone around a Pd-N vector, the angle between the N-D and Pd-N vectors being denoted Delta. This motion makes a part of the quadrupole interaction ineffective in relaxation thus reducing the maximum rate at 45K. The observed reduction leads to the value Delta=21( composite function). Limited jumps are quenched to a large extent at the order-disorder phase transition and consequently a decrease is observed in the rate. Below the transition ND(4)(+) ions reorient between the tetrahedral orientations of the ordered phase, therefore the quadrupole interaction has the full relaxing efficiency. In the 10% deuterated sample the temperature of the rate maximum is shifted to 35K and below 20K the rate itself is one order of magnitude larger than in (ND(4))(2)PdCl(6). The increase is related to (1) the absence of the order-disorder phase transition and (2) to the enhanced mobility of NH(3)D(+) because of its electric dipole moment. Limited jumps are claimed to be the dominant relaxation mechanism below 20K. The relaxation in the disordered 30% deuterated sample is quite similar to that in 10% sample. The 50% and 70% deuterated samples undergo a transition to the ordered phase. The relaxation is biexponential with the characteristic rates somewhat smaller than those in (ND(4))(2)PdCl(6), but approaching them with increasing deuteration. This variation can be explained with different mobilities and varying relative numbers of the various isotopomers NH(4-n)D(n)(+), n=1-4.

Deuteron NMR spectra of ammonium ion isotopomers at low temperatures

Partially deuterated ammonium compounds contain ammonium ion isotopomers with relative abundances given by the binomial distribution of protons and deuterons. All isotopomers with deuterons contribute characteristic deuteron NMR spectra at 5K. Experimental NMR spectra were separated and respective contributions of isotopomers were determined. The derived contributions agree with expected values for a given deuteration in the case of ammonium hexafluorophosphate. In ammonium hexachlorotellurate both NH2D2+ and about 50% of NH3D+ ions are rigid, while the remaining NH3D+ perform limited jumps. NHD3+ and ND4+ ions undergo tunnelling rotation, NH3D+ ions perform either jumps about C2 axis or limited jumps, but some stay rigid in ammonium hexachlorostannate. NH2D2+, NHD 3+ and ND4+ undergo rotational tunnelling. In the case of ammonium perchlorate, the NH3D+ ions perform either jumps about C3 axis or limited jumps whilst some remain rigid. Very low values of activation energies were derived for all spectral components from the temperature dependence of their spectra, up to about 20K, which indicates an incoherent tunnelling nature of the observed dynamic processes. The diverse mobility of NH3D+ ions appears to be the most interesting and new feature.