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.