Accurate calculations of bond-breaking energies in C60 using the three-layered ONIOM method

The three-layered ONIOM method was used to calculate the adiabatic singlet-triplet (S-0 --> T-1) and singlet-quintet (S-0 --> Q) energy splittings in C-60 fullerene. The ONIOM(G2MS:ROMP2/6-31G(d):ROHF/6-31G) calculation gives a value of 35.1 for the S-0 --> T-1 splitting, in good agreement with the experimental value of 36.1 kcal/mol. For accurate bond energies, for which typically a coupled-cluster level of theory is required, the ONIOM appears to be an ideal method for large molecules. The S-0 --> Q splitting, corresponding to the complete breaking of a double bond in C-60, is predicted to be 252 kcal/mol, a large value due to the severe strain and the disruption of conjugation in the product. (C) 1999 Elsevier Science B.V. All rights reserved.