On the assignment of 19F MAS NMR spectra of fluoroaluminates using through-space spectral edition of 19F–27Al and 19F–19F connectivities

Abstract

Advantages and limitations of ¹⁹F–²⁷Al and ¹⁹F–¹⁹F dipolar-based 2D NMR experiments for ¹⁹F MAS spectra assignments of fluoroaluminates are presented. In β-BaAlF₅, combination of 2D MAS ¹⁹F–²⁷Al CP-HETCOR and ¹⁹F–¹⁹F DQ-SQ NMR correlation experiments allows complete unambiguous assignment of the ten poorly resolved resonances of same relative intensities of the ¹⁹F MAS NMR spectrum. The gain in resolution of the ¹⁹F MAS 2D spectrum compared to a 1D spectrum is evidenced, allowing distinction of the two shared-fluorine resonances of Ba₃Al₂F₁₂. Limitations of the ¹⁹F MAS DQ-SQ NMR experiment are shown for Ba₃Al₂F₁₂ and for α-CaAlF₅. For β-BaAlF₅ and Ba₃Al₂F₁₂, final line assignments question those previously established from ¹⁹F isotropic chemical shift calculations, which demonstrate that such experiments are essential for correct line assignments and assessment of calculation results.