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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. February 2003, Volume 2, Number 2, 86-95 |
Partial Structures of C60 Responsible for Its Lowest Electronic
Absorption Band: Corannulene or Triphenylene?
Tai-ichi Shibuya, Susumu Narita, Yasushi Nomura, and Tetsuo Morikawa
Internet Electron. J. Mol. Des. 2003, 2, 86-95
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Abstract:
Corannulene is widely accepted as a particularly important
fragment of the fullerene C60. We consider that corannulene
alone cannot explain the origin of the lowest-allowed electronic
absorption band of C60. Here we clarify what else is needed other
than corannulene, and propose triphenylene as a more important
partial structure of C60 for the electronic absorption. Full SECI
calculations are made in the semiempirical CNDO/S
approximation. We first observe how the degenerated
MO-energy levels of C60 split as the molecular symmetry Ih breaks
into two different elongated shapes, D5d with its C5-axis through
a pair of corannulene parts and D3d with its C3-axis through a
pair of triphenylene parts. Components of the molecular orbitals
of C60 that play principal roles in the lowest-allowed electronic
absorption are all found in the vicinity of the HOMO-LUMO gap
of triphenylene, whereas some part is missing in corannulene.
The lowest-allowed electronic transitions of larger triphenylene
derivatives tend to converge to the lowest absorption band of C60.
A pair of triphenylene molecules separated by the diameter of
C60 in the staggered conformation (D3d) makes a good model
system to explain the origin of the lowest-allowed absorption
band of C60. As the C60 molecule has 20 equivalent 6-membered
rings, there are ten choices for such a pair. On the other hand, a
pair of corannulene molecules in the staggered conformation
(D5d) lacks one component of the HOMO of C60, which we have
found in the encircled cis-polyene of ten ethylene units at the
equatorial belt of C60.
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