AB-INITIO HELIUM NMR CHEMICAL-SHIFTS OF ENDOHEDRAL FULLERENE COMPOUNDS HE-AT-C-N (N=32-180)

Helium NMR chemical shifts have been computed at the gauge including atomic orbitals (GIAO)-SCF/dz(C), tzp(He)//MNDO level for the endohedral fullerene compounds He@C-n (n = 32, 36, 50, 60, 70, 76, 78, 80, 82, 120, 180). The endohedral He shielding increases in the sequence C-60 < C-84 < C-82 < C-78 < C-76 < C-70, in accord with recent London calculations. For the smaller fullerenes (n < 60), large endohedral shieldings are computed, e.g. delta(He)= -30.3 ppm for He@C-50 (D-5h), while a nearly constant endohedral chemical shift of approximate to -17 ppm is predicted for the symmetric large fullerenes C-120 and C-180.