On the assignment of¹⁹F MAS NMR spectra of fluoroaluminates using through-space spectral edition of¹⁹F–²⁷Al and¹⁹F–¹⁹F connectivities

Abstract

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 beta-BaAlF(5), 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 Ba(3)Al(2)F(12). Limitations of the (19)F MAS DQ-SQ NMR experiment are shown for Ba(3)Al(2)F(12) and for alpha-CaAlF(5). For beta-BaAlF(5) and Ba(3)Al(2)F(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.

Origin of NMR shielding in fluorides

“…3 However, for other (heavier) nuclei, or large molecules and solids, the response depends on the details of the electronic structure in a rather complicated way, [4][5][6][7][8] therefore the interpretation of the experimental data is a rather difficult task. [9][10][11] Even by means of ab initio calculations, which are undoubtedly helpful, one can at best reproduce the experimental spectra allowing an assignment to specific atomic sites, 12,13 but so far it is basically not understood why the NMR shielding is smaller or larger in one compound than in another.…”

Assessment of DFT functionals with NMR chemical shifts

“…2 However, for other (heavier) nuclei, or larger molecules and in particular for solids such rules are more difficult to build [3][4][5][6][7][8] and the interpretation of the experimental data is more complicated task. [9][10][11] The interpretation procedure can be aided by ab-initio calculations provided that the computed spectra can properly reproduce the experimental results.…”

Homonuclear dipolar recoupling under ultra-fast magic-angle spinning: Probing 19F−19F proximities by solid-state NMR

“…We have used the same notation as that used in Ref. [26] to define the fluorine species (noted F). The assignment of the lines has been done for the three samples by simultaneous NMR experiments and DFT calculations [22,26,39].…”

“…[26] to define the fluorine species (noted F). The assignment of the lines has been done for the three samples by simultaneous NMR experiments and DFT calculations [22,26,39]. All inter-nuclear fluorine-fluorine distances up to 375, 473 and 466 pm have been given for a-CaAlF 5 , b-BaAlF 5 and Ba 3 Al 2 F 12 , respectively, in the SI of Ref [22].…”

“…The Back-to-Back (BABA) scheme, which emerged in the mid-1990s [8,9], is presently one of the most popular DQ homonuclear dipolar recoupling sequences under fast and ultra-fast MAS (m R > 30 kHz). This technique has been used mainly to observe 1 HÀ 1 H and 19 FÀ 19 F correlations [8,9,[24][25][26]. Nevertheless, it suffers from two major drawbacks.…”