Application of129Xe 2D-EXSY NMR to intra- and interparticle exchange in zeolites

2D-EXSY NMR methods were used to explore diffusion between different domains in zeolites on two very different length scales. Measurements were made on xenon-containing samples consisting of a mixture of NaX and NaY zeolite particles which give distinct resonance lines. The cross peaks observed were interpreted in terms of interparticle exchange, and exchange rates were measured as a function of particle size. This kind of experiment with a length scale of microns to mm should be useful in model studies of diffusion in packed catalyst beds. Measurements also were made to study exchange on a much smaller length scale between the main-channel and side-pocket sites in samples of microcrystalline Na-mordenite. Exchange rates and site occupancies were measured as a function of temperature, yielding activation energies for xenon desorption and intersite diffusion. Also, information was obtained on the distribution of inequivalent side-pocket sites which leads to the large xenon NMR linewidths for this type of site.     

The Inclusion Mode of tert-Butylbenzene in the 1-D Channel of Microporous β-Bis(1,1,1-trifluoro-5, 5-dimethyl-5-methoxy-acetylacetonato) copper(II)

Inclusion of tert-butylbenzeneinto the l-D channel of the titlehost matrix results in a-[CuL₂]1/3(tert-butylbenzene)inclusion compound(trigonal, space groupR =3;a = 24.495(3),c = 10.510(2) Å,V = 5461(1) ų,Z = 9;R = 0.049). The observed guest–host mole ratio of 1/3 was confirmed by X-ray diffraction and arises from the stoichiometric filling of the larger wide segments of the channel. This contrasts with the benzene inclusion compound studied previously, where both the larger and smaller wide segments were occupied to give a 2/3 guest–host stoichiometry. A comparison of these two structures explains the experimental fact that the guest–host mole ratios for inclusions of the title host lie between values of 1/3 and 2/3for 20 different benzene derivatives.     

On the pressure-induced phase transformation in the structure II clathrate hydrate of tetrahydrofuran

A high-pressure phase of the clathrate hydrate of tetrahydrofuran was prepared by freezing a liquid phase of overall composition THF · 7 H₂O under a pressure of 3.0 kbar, or by pressurizing the solid structure II THF hydrate of 255K to 3.4 kbar. Unfortunately, the products recovered at 77K were always mixed phase materials as shown by X-ray powder diffraction. A number of diffraction lines could be indexed in terms of the cubic structure I hydrate with a slightly expanded lattice parameter, 12.08 Å, giving some support to Dyadin's idea that the high pressure phase transition involves a conversion of Structure II to Structure I. Other phases observed in the recovered product include Ice IX and amorphous materials. The reversion of the high pressure sample to the structure II hydrate was followed by differential scanning calorimetry. At ambient pressure, the high pressure sample converts slowly back to Structure II hydrate event at 77K.NRCC No. 35786.     

Magic angle spinning29Si and27Al NMR study of mordenite dealumination

Magic angle spinning²⁹Si NMR presents a rapid qualitative method of assessing the degree of dealumination of sodium mordenite modified by acid leaching and heat treatment. A quantitative determination of the²⁹Si coordination sphere is hampered by the overlap of chemical shift ranges for Si?O?Al and Si?O?H species. MAS²⁷Al NMR indicates the presence of octahedrally coordinated Al in interstitial sites in all treated samples. On treating mordenite samples at high temperatures (～700°C) much of the Al becomes invisible to the NMR experiment because of its location in sites of low symmetry and large quadrupole coupling constant.     

NMR shielding constants for hydrogen guest molecules in structure II clathrates

Proton NMR shielding constants and chemical shifts for hydrogen guests in small and large cages of structure II clathrates are calculated using density-functional theory and the gauge-invariant atomic-orbital method. Shielding constants are calculated at the B3LYP level with the 6-311++G(d,p) basis set. The calculated chemical shifts are corrected with a linear regression to reproduce the experimental chemical shifts of a set of standard molecules. The calculated chemical shifts of single hydrogen molecules in the small and large structure II cages are 4.94 and 4.84 ppm, respectively, which show that within the error range of the method the H2 guest molecules in the small and large cages cannot be distinguished. Chemical shifts are also calculated for double occupancy of the hydrogen guests in small cages, and double, triple, and quadruple occupancy in large cages. Multiple occupancy changes the chemical shift of the hydrogen guests by approximately 0.2 ppm. The relative effects of other guest molecules and the cage on the chemical shift are studied for the cages with multiple occupancies.     

NMR studies on solid cyclohexaamylose inclusion compounds

Orthorhombic inclusion compounds of cyclohexaamylose with methanol,n-propanol, acetic acid and water as guest molecules were studied using¹³C and²H NMR techniques.¹³C chemical shifts were correlated with structural data, whereas²H NMR lineshapes were used to derive information on guest molecule motions.Issued as NRCC No. 25568. Presented in part at the 2nd International Symposium on Clathrate Compounds and Molecular Inclusion Phenomena, Parma, Italy, 30 Aug.–3 Sept. 1982     

Dielectric and13C NMR studies of the carbon monoxide clathrate hydrate

The structure I clathrate hydrate of carbon monoxide has been studied using dielectric measurements and¹³C NMR spectroscopy. Broad, weak dielectric absorption curves with maxima at 2.2–3.8 K yieldE _a = 0.14 kJ mol^–1 for the average Arrhenius activation energy associated with the reorientation of the low polarity guest. Except for H₂S this represents the fastest reorienting polar guest known among the clathrate hydrates. The low temperature dielectric absorption curves can best be fitted with a Cole-Davidson asymmetric distribution of relaxation times and activation energies (with = 0.06 at 4 × 10⁶ Hz), which at 10⁷ Hz has been resolved into a double symmetric distribution of discrete relaxation times for CO in the small and large cages. The cross-polarization magic angle spinning¹³C NMR spectra indicate identical chemical shifts for CO in the small and large cages, in contrast to other hydrates. The static spectra show that the CO molecules undergo anisotropic reorientation in the large cages and that there is still considerable mobility at 77 K. One possible model for the anisotropic motion has the CO rapidly moving among sites over each of the 14 faces of the cage with the CO axis orientated towards the cage centre. The cage occupancy ratio at 220 K, _s/ _L = 1.11, indicates slightly greater preference of CO for the small cage.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.