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 K – 150 K. The activation energies describing the methyl and hydroxyl motions show broad distributions. The relaxation data were interpreted by improving a recent model [Solid State Nucl. Magn. Reson. 49−50, 33–41 (2013)], 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 s) was assumed for each essentially different methyl and hydroxyl position. The correlation times were calculated from the Arrhenius equation (containing the pre-exponential factor t0), 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 wt. The temperature dependence of wt 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, s and t0 obtained, which describe the methyl and hydroxyl motions in different positions in zeolite.  </p

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 ωt=ω0 and 28% at ωt=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.</div

Structure of the hydroxyl groups and adsorbed D2OD_2O sites in the DX zeolite : DFT and experimental NMR data

We report on density functional theory computations combined with the results of measurements of the quadrupole coupling constants for 17O, hydroxyl deuterons, and adsorbed water deuterons in DX zeolite (n(Si)/n(Al) = 1). Calculations are performed for the crystalline periodic model of the DX zeolite. The local structural parameters are found for various oxygen positions at equilibrium. The quadrupole coupling constants for 17O are in the range CQ(17O) = 7.75 ± 0.5 and 3.75 ± 0.5 MHz for (Si–OD–Al) and (Si–O–Al) substructures, respectively. Related values of the asymmetry parameter η fall in the ranges of 0.9 ± 0.5 and 0.15 ± 0.1, respectively. The quadrupole coupling constant for deuterons depends on the OD distance according to the relation CQ(D)[MHz] = −2.732dOD[MHz/Å] + 2.938 MHz. Deuterons are assigned to labeled oxygen positions according to the decreasinng quadrupole coupling constant CQ(D) as O1 ≥ O4 > O2 > O3, with their relative abundances of 54.4, 7.6, 26.6, and 11.4%, respectively. The binding energy of deuteron in hydroxyl groups and for adsorbed water molecules is analyzed. Formation of the water hexamer in the plane of the 12-ring window was confirmed. Results of the calculations are compared with NMR experimental data for 17O and D