Superposition model for F-19 isotropic chemical shift in ionic fluorides: from basic metal fluorides to transition metal fluoride glasses

Experimental measurements of F-19 isotropic chemical shifts on a large set of mainly ionic fluoride compounds (from simple metal fluorides to transiti on metal fluoride glasses) obtained by MAS NMR at 15 kHz spinning rate are simultaneously investigated. First, Ramsey's theory of die chemical shift w ith molecular orbitals obtained by Lodwin's orthogonalisation method is use d to evaluate the isotropic part of the F-19 chemical shift in ionic fluori des for which the crystallographic structure and the atomic radial wavefunc tions are known. Then, assuming that the paramagnetic part of the F-19 shie lding in a given material is simply the summation of the paramagnetic contr ibutions due to all the cations in the neighbourhood of the considered fluo rine, a superposition model of the F-19 isotropic chemical shift is develop ed. Finally, this empirical approach is applied to complex fluoride compoun ds of unknown structure and it is demonstrated that it allows to obtain rel iable structural informations. More generally, it can be applied to all the ionic fluorides. (C) 1999 Elsevier Science B.V. All rights reserved.