Rotational Tunneling and Neutron Spectroscopy: A Compilation.

The $^{35}\mathrm{Cl}$ nuclear quadrupole resonance (NQR) and the quadrupole perturbed $^{2}\mathrm{D}$ NMR have been investigated in (${\mathrm{ND}}_{4}$)${\mathrm{TeCl}}_{6}$ in the temperature range 4.2\char21{}85 K. The $^{2}\mathrm{D}$ spin-lattice relaxation rate shows a classical maximum at about 80 K and a broad peak at about 25 K, which is attributed to the influence of a phase transition at the same temperature. An extra contribution to the chlorine NQR spin-lattice relaxation rate is observed near the cubic-to-trigonal transition at 85 K. The disappearance of the NQR signal before the second transition at about 42 K is accompanied by a divergence of the spin-spin relaxation rate. The results are discussed in terms of a localization of the deuterium in a triangle-shaped potential well in the plane perpendicular to the N-D bond direction.

2D NMR and 35Cl nuclear-quadrupole-resonance study of ammonium-ion motion and phase transitions in natural and deuterated (NH4)2TeCl6.

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

In situ solid-state 1H NMR studies of hydration of the solid acid catalyst ZSM-5 in its ammonium form

Rotational tunneling effects are analyzed in 2H‐NMR spectra of deuterated ammonium ions in powders. The spectra are generally composed of a strong central line from A symmetry species and weaker T sidebands. At a high magnetic field (∼6.5 T) the central line contains well‐resolved doublets. Their separations supply a direct measure of the rotational tunneling frequency νt in the range 0.1<νt<6 MHz. At low magnetic fields the spectra depend both on νt and the Larmor frequency ν0. Level crossing related effects may be detected in a ±0.75 or ±0.35 MHz range around that Larmor frequency which is equal to νt or 1/2 νt, respectively. Generally, only the fitting of the theoretical spectra to those measured at different ν0 allows an estimation of the tunneling frequency. The measurement of νt up to about 20 MHz becomes thus possible. The theoretical predictions are verified by results on a (ND4)2SnCl6 powder sample. High and low field spectra lead to slightly different values for the tunneling frequency at 4.2 K ...

Rotational tunneling effects in 2H‐NMR spectra of polycrystalline (ND4)2SnCl6

Theory of2H-NMR spectra is developed for a ND4+ ion. Quadrupole and dipole-dipole interactions for four deuterons of the tetrahedral ion as well as the rotational tunneling are taken into account. Features in the single crystal spectra at the high magnetic field of 6.7 T are analyzed. Their central doublets (±5 kHz range around the Larmor frequency) are attributed to theA symmetry species and exhibit sensitivity to theA−T tunneling frequencyvt. This allows the measurement ofvt in a wide rangevt <10 MHz. The shape of sidebands (±140 kHz range) is related to the T levels structure. Theoretical predictions are verified on a (ND4)2SnCl6 single crystal at 44 MHz. The tunneling frequency at temperatures below 10 K equals 7.5 MHz, which is about 100 times smaller than for (NH4)2SnCl6.vt was measured up to 40 K. At still higher temperatures motional narrowing effects were observed for theA spectral components preventing the determination ofvt.

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: I. Tunneling frequency measurements

Deuteron NMR spectrum of ND4VO3 and some other ammonium compounds

In the preceding paper (Z. Phys. B — Condensed Matter66, 363 (1987)) we had shown that2H-NMR spectra of ND4+ in a cubic potential were separable into components originating fromA, E andT symmetry species. Separations of theA andE doublets supply currently an unique way to measure theA−T tunneling splitting up to about 10 MHz. Now we analyse tunneling effects and angular dependence ofT spectral components. Calculated spectra show angular dependence which varies withT levels structure. The spectra depend on theT−T splittings, allowing their evaluation up to about 1 MHz. The threeT levels of ND4+ in (ND4)2SnCl6 are found to be split asymmetrically by 91.5 kHz and 49.5 kHz at 4.2 K. The isotope reduction for theT level splittings amounts to about 670, in contrast to a factor 100 found for theA−T splitting.

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: II.T levels structure

Because of the quadrupole interaction, the2H-NMR spectra of a tunneling ND
 4
 +
 ion depend on the Larmor frequencyv
0 and the tunneling frequencyv

 t
 , when both frequencies are less than 20 MHz. Calculated single crystal spectra visualize features to be expected. Significant broadening and asymmetry are observed at level crossing regions aroundv

 t
 =v
0 orv

 t
 =2v
0. Angular, frequency and temperature dependences of the spectra are studied using an (ND4)2SnCl6 single crystal. Experimental spectra obtained for different frequencies at 4.2K are fitted withv

 t
 as a parameter. The tunneling frequency is also checked by measurements of the deuteron spin-lattice relaxation timeT
1 as a fuction ofv
0. Both methods give consistentlyv

 t
 =7.15±0.10 MHz.

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal

The high field 2H-NMR spectra of the tunneling ND 4 + ion are found to be separable into contributions of A, E and T symmetry species. Their temperature dependence as well as that of the tunneling frequency and spin-lattice relaxation are followed in an (ND4)2SnCl6 single crystal. A common correlation time scale exists.

Z. T. Lalowicz

Papers

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: I. Tunneling frequency measurements

Deuteron NMR spectrum of ND4VO3 and some other ammonium compounds

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal: II.T levels structure

2H-NMR study of ammonium ion rotational tunneling and reorientation in (ND4)2SnCl6 single crystal

Tunneling to Reorientation Transition Effects in High Field 2H-NMR Spectra of ND 4+ Ions