Calculation of 19F and 27Al NMR parameters for rosenbergite, AlF[F0.5(H2O)0.5]4.H2O, a possible model for Al hydroxyfluorides in solution.
(19)F and (27)Al NMR chemical shifts are calculated for the F and Al atoms of the mineral rosenbergite, AlF[F(0.5)(H(2)O)(0.5)](4).H(2)O The structure of rosenbergite consists of infinite chains of F-corner-sharing Al[F(4)(H(2)O)(2)] octahedra and isolated water molecules. An F-centered molecular cluster of composition Al(2)F(3)(OH(2))(8) (3+) was initially used to model the mineral, with geometries taken both from the two different available x-ray crystal structures and from equilibrium geometries calculated at the 6-31G* B3LYP level (both with and without polarizable continuum solvation). Related Al(F)F(n) em leader clusters, with additional F(-) replacing H(2)O, were also studied. A larger Al-centered cluster model Al(3)F(4)(OH(2))(12) (5+) was also generated from one of the x-ray geometries and produced very similar bridging F shieldings but slightly different Al shieldings. The NMR shieldings were calculated using both HF and B3LYP GIAO methods, with 6-311+G(2df,p) basis sets, and the HF and B3LYP results averaged for the F shieldings as described in previous work. Calculated (19)F NMR shifts (relative to CCl(3)F) using this procedure were within a few ppm of experiment when either set of x-ray crystal structure coordinates was used, but differed by as much as 20 ppm for the energy-optimized geometries. Rosenbergite-like fragments with geometries optimized in water, simulated by a PCM, were used to model Al hydroxyfluoride species in solution. The (19)F NMR shifts for the bridging F atoms in several such model complexes are very similar to those usually attributed to monomeric species such as Al(OH(2))(5)F(2+) in solution, suggesting that the solution species are actually corner bridging oligomers. The F in the monomeric Al(OH(2))(5)F(2+) solution species is too strongly shielded by about 20 ppm to match the experimental peak usually assigned to it.
Tossell
J A
JA
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA. tossell@chem.umd.edu
Liu
Yun
Y