Water confinement in faujasite cages: a deuteron NMR investigation in a wide temperature range. 1. Low temperature spectra.

Deuteron NMR spectra were measured for D2O confined in NaX, NaY, and DY faujasites with various D2O loadings at temperatures ranging from T = 70 K to T = 200 K with the aim to study the molecular mobility of confined water as a function of Si/Al ratio and loading. The recorded spectra were fitted with linear combinations of representative spectral components. At low loading, with the number of water molecules per unit cell close to the abundance of sodium cations, a component related to π-jumps of water deuterons about the 2-fold symmetry axis dominated. For loadings at levels 3 times and 5 times higher than the initial loading level, Pake dublets due to rigid water deuterons dominated the recorded spectra. A set of the quadrupole coupling constant values of localized water deuterons was derived from the analysis of the Pake dublets. Their values were attributed to deuteron positions corresponding to the locations at oxygen atoms in the faujasite framework and locations within hydrogen-bonded water clusters inside faujasite cages. The contributions of the different spectral components were observed to change with increasing temperature according to the Arrhenius law with a characteristic dynamic crossover point at T = 165 K. Below T = 165 K a spectral component was observed whose contribution changed with temperature, yielding the activation energy of about 2 kJ/mol, characteristic for jumps between inversion-related water positions in clusters.

Water confinement in faujasite cages: a deuteron NMR investigation in a wide temperature range. 2. Spectra and relaxation at high temperature.

Deuteron NMR spectra and spin-lattice relaxation were measured for D2O confined in NaX, NaY, and DY faujasites with various loadings at temperatures ranging from 200 to 310 K with the aim to study molecular mobility of confined water. Hysteresis of spin-lattice relaxation was observed for both DY and NaY(2.4) samples at 500% loading (280 water molecules per unit cell) in a heating-cooling cycle between 264.5 and 277.7 K. The hysteresis is most likely reflecting formation and decomposition of water clusters at different temperature. Spin-lattice relaxation rates obtained from the experiment are consistent with a picture of the fast magnetization exchange between two dynamically different deuteron populations. The observed relaxation behavior as a function of temperature and loading is most likely an effect of interplay between translational and rotational diffusion. Translational diffusion of water molecules is found to be related to the strength of the electrostatic interaction of water oxygen atoms to faujasite sodium cations, whereas water molecule reorientations seem to depend on the strength of hydrogen bonding to faujasite oxygen atoms and the strength of hydrogen bonds between water molecules, at outer and inner positions in water clusters, respectively.

Deuteron spin-lattice relaxation in the presence of an activation energy distribution: application to methanols in zeolite NaX.

A new method is introduced for analyzing deuteron spin-lattice relaxation in molecular systems with a broad distribution of activation energies and correlation times. In such samples the magnetization recovery is strongly non-exponential but can be fitted quite accurately by three exponentials. The considered system may consist of molecular groups with different mobility. For each group a Gaussian distribution of the activation energy is introduced. By assuming for every subsystem three parameters: the mean activation energy E(0), the distribution width σ and the pre-exponential factor τ(0) for the Arrhenius equation defining the correlation time, the relaxation rate is calculated for every part of the distribution. Experiment-based limiting values allow the grouping of the rates into three classes. For each class the relaxation rate and weight is calculated and compared with experiment. The parameters E(0), σ and τ(0) are determined iteratively by repeating the whole cycle many times. The temperature dependence of the deuteron relaxation was observed in three samples containing CD(3)OH (200% and 100% loading) and CD(3)OD (200%) in NaX zeolite and analyzed by the described method between 20K and 170K. The obtained parameters, equal for all the three samples, characterize the methyl and hydroxyl mobilities of the methanol molecules at two different locations.

Translational and rotational mobility of methanol-d4 molecules in NaX and NaY zeolite cages: a deuteron NMR investigation.

Nuclear magnetic resonance (NMR) provides means to investigate molecular dynamics at every state of matter. Features characteristic for the gas phase, liquid-like layers and immobilized methanol-d(4) molecules in NaX and NaY zeolites were observed in the temperature range from 300 K down to 20K. The NMR spectra at low temperature are consistent with the model in which molecules are bonded at two positions: horizontal (methanol oxygen bonded to sodium cation) and vertical (hydrogen bonding of hydroxyl deuteron to zeolite framework oxygen). Narrow lines were observed at high temperature indicating an isotropic reorientation of a fraction of molecules. Deuteron spin-lattice relaxation gives evidence for the formation of trimers, based on observation of different relaxation rates for methyl and hydroxyl deuterons undergoing isotropic reorientation. Internal rotation of methyl groups and fixed positions of hydrogen bonded hydroxyl deuterons in methyl trimers provide relaxation rates observed experimentally. A change in the slope of the temperature dependence of both relaxation rates indicates a transition from the relaxation dominated by translational motion to prevailing contribution of reorientation. Trimers undergoing isotropic reorientation disintegrate and separate molecules become localized on adsorption centers at 166.7 K and 153.8K for NaX and NaY, respectively, as indicated by extreme broadening of deuteron NMR spectra. Molecules at vertical position remain localized up to high temperatures. That indicates the dominating role of the hydrogen bonding. Mobility of single molecules was observed for lower loading (86 molecules/uc) in NaX. A direct transition from translation to localization was observed at 190 K.

Dynamics of hydroxyl deuterons and bonded water molecules in NaDY(0.8) zeolite as studied by means of deuteron NMR spectroscopy and relaxation.

Deuteron spin-lattice relaxation and spectra were measured for NaDY (0.8) zeolite containing some heavy water. Two subsystems of deuterons with different mobility were disclosed at low temperatures with their respective relaxation rates differing by two orders of magnitude. Spectra exhibit different shapes related directly to a specific motional model. Hydroxyl deuterons perform incoherent tunneling along the hydrogen bond, then on increasing temperature jumps to excited states and over the barrier appear. Hydrogen bonded water molecules perform 180 degrees rotational jumps about the twofold symmetry axis. Spectral amplitudes are consistent with the water content of 13 D(2)O molecules per unit cell. Above about 240K translational mobility becomes significant and finally water molecules diffuse across the free space of cages. Diversity in temperature dependence of hydroxyl deuteron dynamics may indicate location of adsorbed molecules.

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 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.