Wednesday, June 14, 2006

JACS: Ashbrook and Wimperis; 27Al MQMAS and STMAS as probes of dynamics in microporous solids

J. Am. Chem. Soc., 128 (24), 8054 -8062, 2006. 10.1021/ja057682g S0002-7863(05)07682-1

Dynamics on the Microsecond Timescale in Microporous Aluminophosphate AlPO-14 as Evidenced by 27Al MQMAS and STMAS NMR Spectroscopy

Sasa Antonijevic, Sharon E. Ashbrook, Silke Biedasek, Richard I. Walton,* Stephen Wimperis,*# and Huaixin Yang

Contribution from the Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland, School of Chemistry and EaStCHEM, University of St Andrews, St Andrews KY16 9ST, United Kingdom, Institut für Technische und Makromolekulare Chemie, Universität Hamburg, 20146 Hamburg, Germany, Department of Chemistry, The Open University, Milton Keynes MK7 6AA, United Kingdom, Department of Chemistry and WestCHEM, University of Glasgow, Glasgow G12 8QQ, United Kingdom, and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China;
Received November 11, 2005


Multiple-quantum magic angle spinning (MQMAS) and satellite-transition magic angle spinning (STMAS) are two well-known techniques for obtaining high-resolution, or "isotropic", NMR spectra of quadrupolar nuclei. It has recently been shown that dynamics-driven modulation of the quadrupolar interaction on the microsecond timescale results in linewidths in isotropic STMAS spectra that are strongly broadened, while, in contrast, the isotropic MQMAS linewidths remain narrow. Here, we use this novel methodology in an 27Al (I = 5/2) NMR study of the calcined-dehydrated aluminophosphate AlPO-14 and two forms of as-synthesized AlPO-14, one prepared with isopropylamine (C3H7NH2) as the template molecule and one with piperidine (C5H10NH). For completeness, the 31P and 13C (both I = 1/2) MAS NMR spectra are also presented. A comparison of the 27Al MQMAS and STMAS NMR results show that, although calcined AlPO-14 appears to have a rigid framework structure, the extent of motion in the two as-synthesized forms is significant, with clear evidence for dynamics on the microsecond timescale in the immediate environments of all four Al sites in each material. Variable-temperature 27Al STMAS NMR studies of the two as-synthesized AlPO forms reveal the dynamics to be complex, with the motions of both the guest water molecules and organic template molecules shown to be contributing. The sensitivity of the STMAS NMR experiment to the presence of microsecond timescale dynamics is such that it seems likely that this methodology will prove useful in NMR studies of host-guest interactions in a wide variety of framework materials.

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