| Résumé | Ultrawideline 137Ba SSNMR spectra of several barium-containing systems (barium nitrate, barium carbonate, barium chlorate monohydrate, barium chloride dihydrate, anhydrous barium chloride, and barium hydrogen phosphate) were acquired at two different magnetic field strengths (9.4 and 21.1 T) using frequency-stepped techniques. The recently reported WURST-QCPMG pulse sequence (O’Dell et al. Chem. Phys. Lett. 2008, 464, 97-102) is shown to be very useful for rapidly acquiring high signal-to-noise 137Ba SSNMR spectra. The breadths of the second-order quadrupolar-dominated spectra and experimental times are notably reduced for experiments conducted at 21.1 T. Analytical simulations of the 137Ba SSNMR spectra at both fields yield the quadrupolar parameters, and in select cases the barium chemical shift anisotropies (CSAs). Quadrupolar interactions dominate the 137Ba powder patterns, with quadrupolar coupling constants, CQ (137Ba), ranging from 7.0 to 28.8 MHz. The 137Ba electric field gradient (EFG) parameters extracted from these spectra are correlated to the local environments at the barium sites, via consideration of molecular symmetry and structure, and first principles calculations of 137Ba EFG tensors performed using CASTEP software. The rapidity with which 137Ba SSNMR spectra can be acquired using the WURST pulse sequence and/or at ultrahigh magnetic fields and the sensitivity of the 137Ba EFG tensor parameters to the changes in the barium environment suggest that 137Ba SSNMR has great potential for structural characterization of a variety of barium-containing materials. |
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