3He NMR: A Powerful New Tool for Following Fullerene Chemistry

Abstract

The fullerenes react readily with a variety of reagents. Following their reactions with NMR spectroscopy has not been easy. If several products are formed or, as is common, bis-adducts are produced, assigning the carbon peaks is difficult or impossible. Continuing our studies of endohedral fullerene compounds of noble gases, we have recently used high pressure and heating to introduce [sup 3]He into C[sub 60] and C[sub 70], increasing incorporation levels to around 0.15%. We have used this material to obtain the first [sup 3]He NMR spectra of helium compounds. Each helium-labeled fullerene gives a single sharp peak. The large chemical shifts (C[sub 60], [minus]6.3 ppm; C[sub 70], [minus]28.8 ppm from dissolved [sup 3]He as reference) show that there are substantial aromatic ring currents modifying the magnetic field experienced by the helium nucleus in the center of the fullerenes. We expected that altering the [pi]-bonding structure of the fullerene through reaction would produce substantial shifts in the [sup 3]He peak. We have now verified this expectation. We have subjected a [sup 3]He-labeled fullerene mixture (about 70% C[sub 60], 30% C[sub 70]) to the reaction conditions recently reported by Maggini, Scorrano, and Prato for the azomethine ylide addition to fullerenes to formmore » N-methylpyrrolidines. It is striking that addition to just one of the 30 double bonds of [sup 3]He C[sub 60]produced a 3 ppm shift of the helium upfield. At this point, even qualitative rationalization of the direction of these shifts would be speculative. It is interesting that the adducts of C[sub 70] shift downfield. 6 refs., 2 figs.« less