The ~3He NMR spectra of C_(60)F_(18) and C_(60)F_(36); the parallel between hydrogenation and fluorination

摘要

Both i~3HeC_(60)F_(18) and i~3HeC_(60)F_(36) have been prepared by fluorinating i~3HeC_(60). The ~3He NMR spectrum of i~3HeC_(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~3HeC_(60)F_(18). 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~3HeC_(60)F_(36) consists of two almost coincident lines (intensity ration 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 C_(60)F_(36). The spectrum is similar to that (lines at -7.8 and -7.9 ppm in a similar ratio) recorded previously for i~3HeC_(60)H_(36) and provides compelling evidence that C_(60)H_(36) also consists of a mixture of C_3 (major) and T (minor) isomers. The observed ca. 2-3 ppm upfield shift of the spectral lines for the C_(60)F_(36) isomers compared to those for the C_(60)F_(36) isomers is reproduced by both density functional and SCF calculations. The C_3 isomer involved has been identified by comparison of its 2D ~(19)F 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.