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

摘要

Advantages and limitations of ~(19)F-~(27)Al and ~(19)F-~(19)F dipolar-based 2D NMR experiments for ~(19)F MAS spectra assignments of fluoroaluminates are presented. In β-BaAlF5, combination of 2D MAS ~(19)F-~(27)Al CP-HETCOR and ~(19)F-~(19)F DQ-SQ NMR correlation experiments allows complete unambiguous assignment of the ten poorly resolved resonances of same relative intensities of the ~(19)F MAS NMR spectrum. The gain in resolution of the ~(19)F MAS 2D spectrum compared to a 1D spectrum is evidenced, allowing distinction of the two shared-fluorine resonances of Ba3Al2F_(12). Limitations of the ~(19)F MAS DQ-SQ NMR experiment are shown for Ba3Al2F_(12) and for α-CaAlF5. For β-BaAlF5 and Ba3Al2F_(12), final line assignments question those previously established from ~(19)F isotropic chemical shift calculations, which demonstrate that such experiments are essential for correct line assignments and assessment of calculation results.