19F-NMR solid state investigations of monovalent alkali metal fluorides and tetra-alkylammonium fluorides

The F-19-NMR chemical shifts of the alkali metal fluorides and of monovalent tetra-alkylammonium fluorides were measured under strictly anhydrous conditions. For reducing the line widths of the broad resonance signals, conventional and ultrafast MAS at 32 kHz rotational frequency as well as CRAMPS (combined rotation and multiple-pulse spectroscopy) techniques were used. Averaging the strong homonuclear dipolar interactions of fluorine by MAS, the dependence of line width on the frequency of sample rotation is given for (CH3)(4)N+ F-. The data obtained are compared with CRAMPS results. The broad distribution in shielding of the fluorine nucleus in the cubic alkali fluorides in the column from LiF to CsF is discussed in terms of cation-anion polarization effects, creating a partial covalent bond. The experimental chemical shift values were correlated with the Allred-Rochow electronegativities and with the polarizabilities of ions in the alkali fluoride crystals. The linear fit of both gave R-values better than 0.96. From the latter, a chemical shift of the "free F-ion" at -293 ppm, referred to CCl3F, was extrapolated. (C) 2002 Elsevier Science B.V. All rights reserved.