doi:10.1016/S0009-2614(02)00307-X 
Copyright © 2002 Elsevier Science B.V. All rights reserved.
Aromaticity and hydrogenation patterns in highly strained fullerenes
Jeffrey C. Grossman
, 
, a, Michael E. Colvinb, Ngoc L. Tranb, Steven G. Louiec and Marvin L. Cohenc
a Physics Directorate, Mailstop L-415, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
b Computational Biochemistry Group, Mailstop L-448, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
c Department of Physics, University of California, Berkeley, CA 94720, USA
Received 16 October 2001;
revised 25 January 2002.
Available online 17 March 2002.
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Abstract
Gradient corrected density functional theory is applied to evaluate the structure and energetics of hydrogenation patterns in the C36 molecule and its component fragments. Overall strain and resonance energies for these compounds are determined using homodesmotic reactions that connect C36 and its constituent chemical components to simpler non-aromatic, unstrained compounds. Our calculations indicate that the dramatic difference in energetic stability between two similar solid C36 structures is due to the number of disrupted aromatic rings rather than to differential strain.
Fig. 1. Hypothetical dissociation of C36 into ‘pre-strained' component fragments.
Fig. 2. Symmetry unique bonds of C36 showing bond lengths and hybridizations.
Fig. 4. Hexa-hydrogenated C36 and cyclohexacene in two structures: (a) D3d symmetry, in which all six aromatic rings in the belt are disrupted, and (b) D3h symmetry, in which alternating aromatic rings in the belt are disrupted.
Fig. 3. Three of the linear acyclic and cyclic polyacenes studied: (a) hexacene, (b) octacene, and (c) decacene. (d)–(e) correspond to the cyclic forms of these linear molecules. In (a) two possible ways to hydrogenate the molecule are illustrated: one which disrupts alternate aromatic rings (C2v), and one which disrupts all aromatic rings (C2).
Fig. 5. Structures and energies of hydrogenated C36.
Table 1. Homodesmotic stabilization energies (kcal/mol) for both the linear and cyclic of polyacenes with 6, 8, 10, 12, and 14 fused phenyl rings
For the cyclic forms, the energy to dissociate the compound into
n-distorted benzenes (
Edissociation), and the energy to relax these distorted benzenes to planar benzene (
Estrain) are also listed.
Table 2. Hydrogenated forms of C36 and its components
Energies are in kcal/mol and given relative to the relevant lower-energy hydrogenated structure. All results are based on B3LYP/6-31G* energies of the B3LYP/6-31G* optimized geometry.
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