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Calculation of 19 F and 27 Al NMR parameters for rosenbergite, AlF[F 0.5 (H 2 O) 0.5 ] 4 ·H 2 O, a possible model for Al hydroxyfluorides in solution
Author(s) -
Tossell J. A.,
Liu Yun
Publication year - 2004
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1434
Subject(s) - chemistry , octahedron , crystallography , solvation , chemical shift , crystal structure , molecule , basis set , molecular geometry , cluster (spacecraft) , computational chemistry , density functional theory , organic chemistry , computer science , programming language
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 … 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. Copyright © 2004 John Wiley & Sons, Ltd.