Solid-state 137Ba NMR spectroscopy: an experimental and theoretical investigation of 137Ba electric field gradient tensors and their relation to structure and symmetry.

Ultrawideline (137)Ba 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 (137)Ba 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 (137)Ba SSNMR spectra at both fields yield the quadrupolar parameters, and in select cases the barium chemical shift anisotropies (CSAs). Quadrupolar interactions dominate the (137)Ba powder patterns, with quadrupolar coupling constants, C(Q)((137)Ba), ranging from 7.0 to 28.8 MHz. The (137)Ba 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 (137)Ba EFG tensors performed using CASTEP software. The rapidity with which (137)Ba SSNMR spectra can be acquired using the WURST pulse sequence and/or at ultrahigh magnetic fields and the sensitivity of the (137)Ba EFG tensor parameters to the changes in the barium environment suggest that (137)Ba SSNMR has great potential for structural characterization of a variety of barium-containing materials.