SEMIEMPIRICAL CALCULATIONS OF DIHYDROGENATED BUCKMINSTERFULLERENES, C60H2

The electronic structures of all possible C60H2 isomers in the singlet state with hydrogens external to the surface have been calculated with the PM-3 parameterization of the MNDO Hamiltonian. A localized polyene structure similar to that of C60 is predicted for all of the C60H2 isomers in the lowest energy singlet state except for two. For these two singlet isomers, one is predicted to have partial charge-transfer character and the other to have a highly delocalized pi-system. The DELTA-H(f)-degrees of the closed-shell singlet isomers are predicted to fall in the range of 776-836 kcal/mol as compared to a calculated value of 812 kcal/mol for C60. The DELTA-H(f)-degrees are predicted to increase by 8.5 kcal/mol for each double bond placed in a five-membered ring on hydrogenation. The calculated DELTA-H(f)-degrees for the open-shell singlets and triplets arc approximately 815 kcal/mol and are essentially independent of the position of hydrogen attachment. If the number of double bonds in five-membered rings is less than 5, the closed-shell singlet should be favored, whereas an open-shell structure is predicted for the other isomers. Only a few isomers are predicted to form exothermically by addition of H-2 to C60 or by extraction of two hydrogens from alkanes. The trends in the HOMO and LUMO energies follow those of the DELTA-H(f)-degrees. The values mu, alpha, beta, and gamma for the closed-shell singlet state of the C60H2 isomers were calculated with a finite-field method. The values of alpha and gamma increases as DELTA-H(f)-degrees increases.