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

Abstract

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 C2Fx intercalated with acetonitrile. Two major chemical states of carbon, namely the atoms covalently bound to fluorine and the bare atoms, are detected by 13C 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 C2Fx matrix and acetonitrile molecules is of van der Waals character. Decomposition of the 19F MAS NMR spectra reveals occurrence of at least six fluorine environments in each sample. By DFT calculations distinct 19F 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 19F RFDR, which is sensitive to dipolar coupling. Analyzing the data for C2Fx samples with different degrees of fluorination x, an evolution of the fluorine pattern is proposed. The reported calculated 19F NMR shielding parameters provide classification criteria for assignment of 19F NMR chemical shifts in fluorinated carbon materials.