Solid-state NMR study of the post-fluorination of (C2.5F)n fluorine-GIC.

The conversion of (C2.5F)n fluorine-graphite intercalation compounds (GIC) into covalent graphite fluoride during a post-treatment in pure F2 gas is studied by solid-state NMR. First, a careful characterization of the starting material is performed; in particular, for the first time for fluorinated carbons, two-dimensional 19F--> 13C cross-polarization wide-line separation (CP-WISE) experiments were carried out. This completes the classical NMR data such as 19F and 13C chemical shifts, quantitative 13C solid echo, and C-F bond length measurements, which were estimated by dipolar recoupling using inverse CP MAS. The data of the raw (C2.5F)n and of the samples post-fluorinated at 350, 450, and 550 degrees C were compared to investigate the C-F bonding as a function of the treatment temperature. The C-F bonding is discussed taking into account a hyperconjugation of the C-F bonds with neighboring unfluorinated carbon atoms.

Heteronuclear dipolar recoupling using Hartmann-Hahn cross polarization: a probe for 19F-13C distance determination of fluorinated carbon materials.

A NMR determination of the C-F bond length in fluorinated carbon materials using dipolar recoupling is described. To this end Hartmann-Hahn cross polarization with magic angle spinning (inverse cross polarization sequence) is used. A description of the corresponding 13C magnetization evolution as a function of the evolution time and its simulation for typical fluorinated samples are realized. The procedure is applied to 15 different materials having different bonding (semi-covalent or covalent) and from various carbon allotropic varieties. The distance evolves from 0.138+/-0.002 nm for covalent bonding to 0.1445+/-0.002 nm for semi-covalent bonding. Other parameters may affect the C-F bond length e.g. steric hindrance which leads for fluorinated fullerenes to an increase of this distance up to 0.146+/-0.002 nm.