The 3He NMR spectra of C60F18 and C60F36; the parallel between hydrogenation and fluorination

Abstract

Both i ³HeC₆₀F₁₈ and i ³HeC₆₀F₃₆ have been prepared by fluorinating i ³HeC₆₀. The ³He NMR spectrum of i ³HeC₆₀F₁₈ shows a single line at –16.66 ppm, very close to the value of –16.45 ppm, recorded previously for the isostructural i ³HeC₆₀H₁₈. Density functional calculations afford values of –15.0 and –16.2 ppm for the hydrogenated and fluorinated compounds, respectively. The ³He NMR spectrum of i ³HeC₆₀F₃₆ consists of two almost coincident lines (intensity ratio of ca. 3∶1), at –10.49 and –10.52 ppm, attributable respectively to the C₃ and T isomers shown previously to be the components (also in ca. 3∶1 ratio) of C₆₀F₃₆. The spectrum is similar to that (lines at –7.8 and –7.9 ppm in a similar ratio) recorded previously for i ³HeC₆₀H₃₆, and provides compelling evidence that C₆₀H₃₆ also consists of a mixture of C₃ (major) and T (minor) isomers. The observed ca. 2–3 ppm upfield shift of the spectral lines for the C₆₀F₃₆ isomers compared to those for the C₆₀H₃₆ isomers is reproduced by both density functional and SCF calculations. The C₃ isomer involved has been identified by comparison of its 2D ¹⁹F NMR spectrum with that for the T isomer. It is not the isomer calculated to be the most stable one, and its formation is believed to be favoured by contiguous activation of double bonds adjacent to those that have already undergone addition.