Variable-Temperature Magic-Angle-Spinning Technique for Studies of Mobile Species in Solid State NMR (Nuclear Magnetic Resonance).

摘要

We demonstrate the utility of variable-temperature (VT) magic-angle-spinning (MAS) Fourier transform nuclear magnetic resonance (FT-NMR) spectroscopy for the study of mobile species in solids, especially when quadrupolar nuclei are being observed. A new VT-MAS probe is described for use in the temperature range 150 - 400 K, using spectrometers with high-field superconducting magnets. It has been utilized in studies of the bonding and dynamics of alkali cations in hydrated zeolites at an applied field of 7.05 T. Results are presented for the cases of Cs/Na-A and Cs/Li-A zeolites, in which the Cs sup + ions exchange rapidly, at 293 K, between six-ring and eight-ring sites in the large cage, so that on the time scale of the NMR measurements the ions are indistinguishable and a single (133)Cs resonance is observed. Below 250 K, two well-resolved signals are observed in the Cs/Na-zeolite, 107 ppM apart. These are easily assigned to the Cs(+) ions in the two sites, the large difference in chemical shifts being attributed to the stronger bonding of Cs(+)ions at the six-ring sites. Three sites are distinguished in the Cs/Li-A zeolite, the third site being assigned to a position near to a four-ring in the large cage. 26 refs., 3 figs., 1 tab. (ERA citation 14:020115)