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

Acetone mobility in zeolite cages with new features in the deuteron NMR spectra and relaxation

We studied deuteron NMR spectra and spin-lattice relaxation of deuterated acetone-d₆, adsorbed into zeolites NaX (1.3) and NaY(2.4) at 100% coverage of sodium cations. At temperatures roughly below 160 K the deuterons are localized and their NMR characteristics are determined by CD₃ rotation and rotational oscillations of acetone molecules. In NaX the CD₃ rotation and rotational oscillations about the twofold axis of acetone dominate the spectra below 100 K, while above it oscillations also about other axes become important. In NaY dominant features are related to methyl tunnelling and to a smaller extent to rigid acetones, before the rotational oscillations about twofold axis start to prevail above 40 K. The analysis of the strongly non-exponential magnetization recovery was done by applying the recently introduced method (Ylinen et al., 2015 [12]), improved here to take into account the limited fast recovery at the level crossings, 10% at ω_t=ω₀ and 28% at ω_t=2ω₀. At first the experimental recovery is fitted by three exponentials with adjustable weights and decay rates. Then these quantities are calculated from activation energy distributions and known expressions for the deuteron relaxation rate. In NaY two distinctly separate activation energy distributions were needed, the dominant one being very broad. The use of three distributions, two of them covering practically the same energies as the broad one, lead to a somewhat better agreement with experiment. In general the theoretical results agree with experiment within experimental scatter. As the final result the mean activation energies and widths are obtained for activation energy distributions.

The effect of a broad activation energy distribution on deuteron spin-lattice relaxation

Deuteron NMR spectra and spin-lattice relaxation were studied experimentally in zeolite NaY(2.4) samples containing 100% or 200% of CD3OH or CD3OD molecules of the total coverage of Na atoms in the temperature range 20-150K. The activation energies describing the methyl and hydroxyl motions show broad distributions. The relaxation data were interpreted by improving a recent model (Stoch et al., 2013 [16]) in which the nonexponential relaxation curves are at first described by a sum of three exponentials with adjustable relaxation rates and weights. Then a broad distribution of activation energies (the mean activation energy A0 and the width σ) was assumed for each essentially different methyl and hydroxyl position. The correlation times were calculated from the Arrhenius equation (containing the pre-exponential factor τ0), individual relaxation rates computed and classified into three classes, and finally initial relaxation rates and weights for each class formed. These were compared with experimental data, motional parameters changed slightly and new improved rates and weights for each class calculated, etc. This method was improved by deriving for the deuterons of the A and E species methyl groups relaxation rates, which depend explicitly on the tunnel frequency ωt. The temperature dependence of ωt and of the low-temperature correlation time were obtained by using the solutions of the Mathieu equation for a threefold potential. These dependencies were included in the simulations and as the result sets of A0, σ and τ0 obtained, which describe the methyl and hydroxyl motions in different positions in zeolite.

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.

1H NMR study of natural and partially deuterated ammonium hexachlorotellurate

The proton spin-lattice relaxation times in natural and partially deuterated ammonium hexachlorotellurate have been measured. At high temperature (above 50 K) the relaxation curve reveals the classical, hindered reorientation of the ammonium ion with the same activation energy for all studied samples Ea = 5.6 kJ/mol. Below 50 K, the minima which appear in the T1 temperature dependencies are related to the level-crossing. The tunnelling splitting of the ground torsional level in isotopic pure ammonium hexachlorotellurate is equal to 55 MHz and increases with increasing deuteron concentration, due to the phase transitions occurring in partially deuterated crystal. The lowest temperature minimum in isotopic pure (NH4)2TeCl6 can be interpreted as a result of the rigid ammonium ion's reorientation about the threefold crystallographic axis. The limited jumps of NH4+ are characterised by the angle delta = 4 degrees between the NH bond and [111] direction.

Deuteron NMR spectra of ND4ClO4 single crystal at low temperatures

2H NMR spectra of ND4ClO4 single crystal were obtained at v0 = 44 MHz. Orientation and temperature (1.9-75 K) dependences were measured. Fitting the spectra gives the effective quadrupole coupling constants for all deuterons and the ground torsional level structure. The isotope reduction of the (A-T) and (A-E) tunnelling splittings, i.e., the ratios of the respective splittings for NH4+ and ND4+, were found to be different. The splittings at T = 24 K are about 60% of the helium temperature values. The spectrum undergoes intermediate narrowing by reorientations between 26 and 34 K and tunnelling related features in the spectra are eradicated. After reaching the extreme narrowing limit, a doublet with gradually decreasing separation was observed, what was attributed to averaging by torsional oscillations of increasing amplitude. At high temperatures (T > 75 K), the narrow spectrum reflects fast multiaxial reorientation of the ammonium ion.