Probing Local Environments in Paramagnetic Europium-Substituted Keggin Solids by 31P Magic Angle Spinning NMR Spectroscopy
Wenlin Huang, Mark Schopfer, Cheng Zhang, Robertha C. Howell, Becky A. Gee, Lynn C. Francesconi,* and Tatyana Polenova*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, Department of Chemistry, City University of New York-Hunter College, 695 Park Avenue, New York, New York 10021, and Department of Chemistry and Biochemistry, Long Island University-Brooklyn Campus, 1 University Plaza, Brooklyn, New York 11201
Paramagnetic Eu-substituted Keggin oxopolytungstates crystallize in different forms, determined by the nature of the counterions. The crystal packing is in turn responsible for the variations in the geometry of paramagnetic Eu sites with respect to the anion core. We probed the paramagnetic environments in a series of Eu-substituted Keggin solids, by 31P magic angle spinning NMR spectroscopy. 31P spinning sideband envelopes are dominated by the electron-nuclear dipolar interaction. For the compounds under investigation, both the magnitude and the asymmetry parameter of the electron-nuclear dipolar coupling tensor are sensitive to the mutual arrangements of paramagnetic Eu sites in the crystal lattice. and also report on the stoichiometry of the anion. The electron-nuclear dipolar coupling tensors were calculated from the crystallographic coordinates and the experimentally determined effective magnetic moments, assuming a point dipole approximation. The computed tensors are in very good agreement with the experimental spectra. Furthermore, the P-Eu distance estimates, accurate to within 0.06-0.12 Å, can be obtained directly from the magnitude of the electron-nuclear dipolar coupling. This work demonstrates that 31P MAS NMR spectroscopy is a useful probe for investigating local environments in paramagnetic Keggin solids.