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

Both i(3)HeC(60)F(18) and i(3)HeC(60)F(36) have been prepared by fluorinating i(3)HeC(60). The He-3 NMR spectrum of i(3)HeC(60)F(18) shows a single line at -16.66 ppm, very close to the value of -16.45 ppm, recorded previously for the isostructural i(3)HeC(60)H(18). Density functional calculations afford values of -15.0 and -16.2 ppm for the hydrogenated and fluorinated compounds, respectively. The He-3 NMR spectrum of i(3)HeC(60)F(36) consists of two almost coincident lines (intensity ratio of ca. 3:1), at -10.49 and -10.52 ppm, attributable respectively to the C-3 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(3)HeC(60)H(36), and provides compelling evidence that C60H36 also consists of a mixture of C-3 (major) and T (minor) isomers. The observed ca. 2-3 ppm upheld 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 C-3 isomer involved has;been identified by comparison of its 2D F-19 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.