Assignment of the He@C84 isomers in experimental NMR spectra using density functional calculations

The He-3 chemical shifts were calculated for He-n@C-84 (n = 1, 2) fullerenes to obtain characteristic NMR patterns for distinguishing their isomers in a mixture. The density functional methods were calibrated on experimental data. Accuracy within 1 ppm could be reached without further fitting of individual shifts. Such precision allows for a semi-quantitative assignment of He-3 NMR spectra. Additional criteria in the identification are discussed, such as the relative energies of the isomers, positions of the satellite di-helium peaks, and the differential 3He shifts in the fullerenes reduced to anions. Endohedral 3He shifts are predicted for so far experimentally unknown He@C-84 and He@(6-)(84) isomers. (C) 2010 Elsevier B.V. All rights reserved.