Single-walled carbon nanotubes (SWCNTs) show a new facet of their extremely diversified reactivity through the possibility of becoming spontaneously efficiently functionalized in the presence of gaseous fluorine at room temperature, in sharp contrast with any other carbon allotrope. The synthesis route employed here optimizes the formation of an unusual weak bonding mode of fluorine to such a carbon frame. This particular C/F interaction could then be studied independently using solid state 19F NMR spectroscopy and the abnormally high chemical shift measured shows a reduced covalence of the bond between carbon and fluorine. In compliance with its weakness, the C-F bond is elongated to about 0.140 nm. It is further suggested that a metastable endo-addition pattern may form in such experimental conditions.