Solid state NMR study of nanodiamond surface chemistry.

Solid state NMR measurements using 13C, 1H and 19F nuclei (MAS, CP-MAS) underline the surface chemistry of nanodiamonds from different synthesis (detonation, high pressure high temperature and shock compression). The comparison of the spin-lattice relaxation times T1 and physicochemical characterization (spin densities of dangling bonds, specific surface area and Raman and infrared spectroscopies) for the various samples, as synthesized, chemically purified and fluorinated allows the nature and the location of the various groups, mainly C-OH, C-H and C-F to be investigated. C-OH groups are located only on the surface whereas C-H and dangling bonds seem to be distributed in the whole volume. Fluorination was studied as a chemical treatment for purification and change of the hydrophobicity through the conversion of the C-OH groups into covalent C-F bonds.

Direct fluorination of carbon nanocones and nanodiscs.

This work focuses on the reactivity of carbon nanodiscs and nanocones with respect to pure fluorine gas. The starting materials, as-synthesized without post-treatment, consist of a mixture of nanodiscs (approximately 70% w/w), nanocones (approximately 20% w/w) and amorphous carbons (approximately 10% w/w). In order to investigate their reactivity in pure F2 gas, two experiment sets have been performed: (i) in situ Thermo Gravimetric Analysis under diluted F2 and relative F2 pressure measurements, which highlight the temperature domain for an efficient fluorination, and then, allow the fluorination conditions to be optimized; (ii) the fluorination under pure F2 gas was performed at temperatures ranged between room temperature and 450 degrees C. Ex situ characterization was carried out using 13C and 19F solid state Nuclear Magnetic Resonance and Scanning Electron Microscopy. For the low reaction temperature (up to 300 degrees C), the chemical stability of these kinds of nanocarbons prevents from intensive fluorination. On the other hand, at temperature higher than 300 degrees C, the fluorination is important but competes with the material decomposition. The fluorination mechanism has been established taking into account NMR and SEM data.

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.

EPR and solid-state NMR studies of poly(dicarbon monofluoride) (C2F)n.

Poly(dicarbon monofluoride) (C2F)n was studied by electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR). The effects of physisorbed oxygen on the EPR and NMR relaxation were underlined and extrapolated to poly(carbon monofluoride) (CF)n and semi-covalent graphite fluoride prepared at room temperature. Physisorbed oxygen molecules are shown to be an important mechanism of both electronic and nuclear relaxations, resulting in apparent spin-lattice relaxation time and line width during NMR and EPR measurements, respectively. The effect of paramagnetic centers on the 19F spin-lattice relaxation was underlined in accordance with the high electron spin density determined by EPR. 19F magic angle spinning (MAS) NMR, 13C MAS NMR, and 13C MAS NMR with 19F to 13C cross polarization (CP) underline the presence of two types of carbon atoms, both sp3 hybridized: some covalently bonded to fluorine and the others linked exclusively to carbon atoms. Finally, a C-F bond length of 0.138 +/- 0.002 nm has been determined thanks to the re-introduction of dipolar coupling using cross polarization.

Solid-state NMR (19F and 13C) study of graphite monofluoride (CF)n: 19F spin-lattice magnetic relaxation and 19F/13C distance determination by Hartmann-Hahn cross polarization.

Graphite monofluoride (CF)(n) was studied by solid-state NMR. (19)F spin-lattice relaxation time T(1) and second moment measurements of the (19)F line are presented. A "chair" conformation structure is found to be compatible with the experimental data. Relaxation is shown to be mainly due to paramagnetic oxygen. The presence of a molecular motion with an activation energy of 1.685 kJ.mol(-1) (202.7 K) is also evidenced. (19)F magic angle spinning (MAS) NMR and (13)C MAS NMR with (19)F to (13)C cross-polarization allows the determination of CF and CF(2) groups. Reintroduction of dipolar coupling by cross-polarization is used for C-F bond length determination (0.138 +/- 0.001 nm).