Cluster models and a initio calculations of 19F NMR isotropic chemical shifts for inorganic fluorides

F-19 NMR isotropic chemical shift (delta(iso)) calculations are performed in crystallized compounds using the GIAO method with the B3LYP hybrid functional at DFT level. Clusters centered on the studied fluorine atoms mimic the crystalline structures. The 6-311 + G(d) basis set is chosen for the central fluorine atom, and the LanL2DZ basis set for the others. The metal atoms are described by the 3-21G(2d) basis set or, when not available, by the CRENBL basis set with the corresponding ECP, and augmented with 2d polarization functions when existing. First, for high-symmetry systems (MF, MF2, and MF3 compounds), a systematization of the cluster building up from coordination spheres is proposed, generalized to fluoroperovskites and fluoroaluminates KAlF4 and RbAIF(4). When applied to rather low symmetry systems such as barium fluorometalates BaMgF4, BaZnF4, and Ba2ZnF6, the definition of the coordination spheres is far from easy. Then, for structures built up from a MF6 octahedron network, we may define different starting clusters: [FM2F8] for the shared fluorine atoms, [FMF4] for the unshared ones, and [FBa4](7+) for the free ones. Analogous starting clusters are then tested on compounds from the NaF-AlF3, BaF2-AlF3, and CaF2-AlF3 binary systems and for alpha-BaCaAlF7 that are also built up from a MF6 Octahedron network. For each of these corresponding fluorine sites, delta(iso) values are calculated with the starting clusters and several larger clusters and compared to the experimental delta(iso) values. For the barium-containing clusters, the RMS deviation is equal to 51 ppm. It is suggested that this result may be related to the poor quality of the barium basis sets for which no polarization functions are available for the moment. In total, chemical shifts were calculated for 122 fluorine sites, in a various range of compounds. For the clusters without barium, the ab initio method leads to a RMS equal to 22 ppm, which is a quite nice result keeping in mind that the 19(F) chemical shift range is larger than 200 ppm.