3P128

Highly Charged States of C60 Molecule: a DFT Study○ Riadh Sahnoun, Hirohiko Kono, Yuichi Fujimura

Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japanemail: sahnoun@mcl.chem.tohoku.ac.jp

Introduction
Since the elaboration of the parent fullerene molecule C60, tremendous interest in fullerene
derivatives, as well as in C60 itself, has been devoted. Singly and multiply charged fullerene
ions exhibit a wealth of chemistry in the gas phase at room temperature, ranging from electrontransfer between fullerene and the substituents, to ion-induced polymerization [1]. A
spectacular production of stable and highly charged C60, reaching up to C6012+, has recentlybeen appeared in the literature [2]. The authors, Bhardwaj and coworkers, claimed that they
were able to detect and identify the different ionized C60z+ in a laser field. No previous
theoretical work has been reported in the literature on such highly charged fullerene. This
prompts us to take the initiative to elucidate this kind of highly charged fullerenes. We report
here our first results on selected C60 cations, ranging from C60+ to C6018+. Using DFT method,
we clarify up to which cationic level the C60 can be ionized. We explain the unusual stabilityof those cations.
Computational details
All calculations have been carried out using the Gaussian 03 suites of program. Thecalculations were performed at the DFT level of theory. Geometry optimizations were carriedout using the B3LYP functional, which consists of a hybrid Becke+Hartree–Fock exchange
and Lee–Yang–Parr correlation functional with nonlocal corrections. For the icosahedral (Ih)point group we use the 6-31G(d) basis set implemented in Gaussian 03. For lower symmetries,
the 3-21G and the 3-21G(d) basis sets have been used.
Results
It is well known that the C60 has the Ih point group which is highly symmetric; however,removing of one electron yields drastic change in the structure. Except C6010+ and C6018+
cations which possess Ih point group, the symmetry for the other cations will be broken andthey adopt lower symmetry. The highest symmetry we could reach for C60+, for example, wasD5d. Successive ionization leads to further lowering the symmetry. Therefore, we have used
the C2 symmetry, for which the optimization of all cations (up to C6014+) could converge, inorder to evaluate the curve of the ionization potential for the neutral fullerene C60. For thiscalculation, and owing to computational limitation, we used the 3-21G basis set. The result isshown in Figure 1. As can be deduced from Figure 1, the ionization potential follows aquadratic curvature. Inspection of the geometric structures shows that the carbon-carbon bondlengths increase monotonically as the charge of the fullerene cation is increased, leading tothe expansion of the fullerene cage. Depending on the ionization, the structure of the fullerene
cation undergoes a prolate/oblate distortion, in agreement with the experiments done by
Bhardwaj et al. [2]. Only for C605+, C6010+ and C6014+, where the structure reflects the Ihsymmetry, the distortion can be characterized as a breathing mode. This has also
consequences on the charge distribution and the bond lengths for carbon atoms in equivalentposition. The reason for lowering of the symmetry, distortion of the structure in prolate/oblateforms, can be attributed to orbital relaxation.

Relative energy (eV)

500400300200100016111621

Charge state

Figure 1. Ionization potential curve of C60 calculated at B3LYP/3-21G level using C2symmetry.
In fact, in the Ih symmetry, there are 5 degenerated MOs for the HOMO succeeded by 4
degenerated MOs (by 0.048 eV) for HOMO-1 and 5 MOs (by 0.049 eV) for HOMO-2. When
all the electrons in the HOMO and/or in the HOMO-1 are removed, no breaking symmetry
occurs. The system conserves the Ih symmetry. However, for the other cases, orbital relaxation
occurs. As a consequence, a geometric relaxation takes palce in order to compensate the strain
in the system.
Identification of the stationary point for selected structures is in progress. More details will begiven in the symposium.
References[1] D. K. Bohme, O. V. Boltalina, P. Hvelplund in “Fullerenes: Chemistry, Physics, andTechnology” Ed. K. M. Kadish and R. S. Ruoff. John Wiley & Sons Inc. 2000, pp 481-529.
[2] V. R. Bhardwaj, P. B. Corkum, and D.M. Rayner, Phys. Rev. Lett., 91, 203004 (2003).