SOLID-STATE NMR-STUDIES OF THE MOLECULAR-MOTION IN THE KAOLINITE-DMSO INTERCALATE

The structure and motion of the guest molecule in the interlayer space of the kaolinite:DMSO intercalation compound have been studied by H-2 NMR spectroscopy of static samples, C-13, Al-27, and Si-29 high-resolution solid-state NMR spectroscopies, variable-temperature powder X-ray diffraction, and Fourier transform infrared spectroscopy. The guest DMSO molecules are hydrogen-bonded via the oxygen atom to the kaolinite hydroxyls which face the interlayer space, and there is a degree of interaction between the sulfur atom of the DMSO molecule and the siliceous matrix of kaolinite. There are two independent sites for the DMSO methyl groups with effective H-2 quadrupole coupling constants of 59 and 67 kHz, and the population of at least one site is in motion. The difference in these couplings probably reflects different geometries of the methyl groups in the two sites, caused by their partial "keying" into the siliceous matrix. Three motional models produce simulations which are in acceptable agreement with the experimental spectra. One fit is consistent with the keying of one methyl group of a DMSO molecule into the kaolinite layer and raises an interesting point concerning the packing of the DMSO molecules in the interlayer spacing. Above ca. 339 K further exchange processes must be taken into account in H-2 NMR line shape simulations and may trigger structural transformations of the intercalate.