Fluorine Patterning in Room-Temperature Fluorinated Graphite Determined by Solid-State NMR and DFT

摘要

Fluorination of graphite at room temperature allows producing graphite fluoride compounds with a controlled content of fluorine. Here we combine solid-state NMR spectroscopy and DFT calculations to study the structure and reveal the fluorine patterning in graphite fluorides C2F_x intercalated with acetonitrile. Two major chemical states of carbon, namely the atoms covalendy bound to fluorine and the bare atoms, are detected by ~(13)C MAS NMR irrespective of the degree of fluorination. The data indicate that although all graphene sheets were subjected to fluorination, the near-planar configuration is preserved. The interaction between host C2F_x matrix and acetonitrile molecules is of van der Waals character. Decomposition of the ~(19)F MAS NMR spectra reveals occurrence of at least six fluorine environments in each sample. By DFT calculations distinct ~(19)F chemical shifts are attributed to isolated, end chain, "linked" (which include midchain, cyclic, and branched) CF groups and infinite CF arrays. The assignment is confirmed by ~(19)F RFDR, which is sensitive to dipolar couphng. Analyzing the data for C2F_x samples with different degrees of fluorination x, an evolution of the fluorine pattern is proposed. The reported calculated ~(19)F NMR shielding parameters provide classification criteria for assignment of ~(19)F NMR chemical shifts in fluorinated carbon materials.