Solid-state NMR analysis of fluorinated single-walled carbon nanotubes: Assessing the extent of fluorination

Fluorinated single-walled carbon nanotubes (F-SWNTs) have been characterized by magic angle spinning C-13 NMR spectroscopy and the results correlated with Raman, IR, and X-ray photoelectron spectroscopy measurements. The C-13 NMR shift for the sp(3) fluorine-substituted (CF) carbon atoms of the SWNT sidewall is observed at delta = 83.5 ppm. This apparently unusual shift compared to those of most other tertiary alkyl fluorides is confirmed to be due to the CF moieties from ab initio calculations on an 80-carbon fragment of the 5,5 (armchair) SWNT and is in good agreement with the predominance of 1,2-addition rather than 1,4-addition of fluorine. The lack of observable scalar C-13-F-19 coupling for the CF carbon signal over a wide range of spinning speeds and at two different field strengths apparently results from interaction between F-19-F-19 and C-13-F-19 dipolar couplings and from magnetization exchange between the C-13 doublet components caused by fluorine spin diffusion. The assignment of the 83.5 ppm peak is further confirmed by the correlation of its diminished intensity upon thermolysis of the F-SWNT (400, 450, and 550 degrees C) with the relative intensity of the D (disorder) band in Raman and the C:F ratio from X-ray photoelectron spectroscopy (XPS). On the basis of the XPS signal, it appears that the CF2 defect units decompose at a lower temperature than the CF sidewall moieties, suggesting that cutting chemistry precedes sidewall functional group removal. We propose that, where a comparison of samples with a high degree of functionalization is required, NMR provides a much better quantification than Raman. However, where a comparison between samples with low levels of functionalization or large differences in degree of functionalization is required, Raman provides a much better quantification than NMR.