Spin-lattice relaxation of the nuclear dipolar energy and spin diffusion in NH4ClO4

Translation and reorientation of CD4 molecules in nanoscale cages of zeolites as studied by deuteron spin-lattice relaxation

Deuteron spin-lattice relaxation was applied to study translational and rotational mobility of CD(4) molecules trapped in the cages of zeolites. Tetrahedral methane molecules are treated as quantum rotators. Relaxation rates related to the intraquadrupole interaction are derived for the T and A+E symmetry species in the presence of large tunneling splittings, consistently with the assumption that A and E species molecules relax at the same rate. An exchange model is presented, which describes the effect on relaxation of CD(4) jumping between two positions characterized by different potentials. While staying at either position bonded to an atom or ion at the cage wall, the molecule has some freedom to move in the vicinity. This causes a time-dependent external electric field gradient, which contributes to the deuteron relaxation rate via the electric quadrupole interaction. Spin conversion transitions couple the relaxation of magnetizations M(T) and M(AE), which is taken into account by reapplying the presented model under somewhat different conditions. Such a two-step procedure leads to successful fits with the experimental results obtained in the range of temperatures roughly 20-200 K for zeolites HY, NaA, and NaMordenite. At higher temperatures CD(4) molecules fly freely across zeolite cages and relaxation changes accordingly, while incoherent tunneling dominates for immobile molecules below 20 K.

Proton double quantum coherence and cross-relaxation in (NH4)2SnBr6

A proton double quantum coherence signal can be observed exclusively from the T species NH4 groups (with the total spin I = 1) in ammonium compounds at low temperatures by the three-pulse sequence 90x degrees - tpr - 90x degrees - tev - 90x degrees - ta, where tpr and ta are of the magnitude of the inverse line width and tev very short. The usefulness of this pulse sequence, preceded by the additional pulse sequence 90x degrees - t1 -90(-x) degrees - t2 for creating an unbalance between the A and T species magnetizations, was demonstrated by applying it to cross-relaxation studies in (NH4)2SnBr6.

Anisotropy of the proton T1 and the low-field relaxation in NH4 ClO4 below 20K

The spin-lattice relaxation of protons in NH4ClO4 at low temperatures has been studied theoretically and experimentally. The NH4 librational tunneling determines the spin-librational wavefunctions, which are derived first. The dominant transition rates related to the magnetic dipolar interaction between the NH4 protons are then obtained. They reproduce well the angular dependence of the proton relaxation time. When all the transition rates are taken into account, both the temperature and frequency dependence of the relaxation rate agree with our experimental data for a powder sample. Reasons for the non-exponential relaxation are discussed. Apart from the angular dependence of the relaxation rate, they include the tunnel frequency distribution, the coupling to the tunnel reservoir and the effect of deuterons in their natural concentration.