Subject Number: S-15-MS-0041
Support Type: Collaborative research
Proposal Title: Theoretical Studies on the Reaction Mechanism of Fullerenes and Metallofullerenes
Username: Xiang Zhao
Affiliation: Institute for Chemical Physics, Xi’an Jiaotong University, China

1. Summary
Endohedral scandium fullerenes have attracted substantial interests since they were synthesized and isolated in 1992. Sc2C74 series including both Sc2@C74 and Sc2C2@C72 forms were thoroughly investigated with density functional theory methods including B3LYP, CAM-B3LYP, M06-2X, wB97XD and some other DFT methods combined with statistical mechanics in the present work. Among all the Sc2C74 isomers, Sc2C2@Cs(10528)-C72 is found to be the most thermodynamically stable one, and it is overwhelming at the temperature region of fullerene formation, which is well consistent with experiment. A deviation that B3LYP method as well as other DFT methods without long range corrections tends to overestimate energies of Sc2C2@C72 series was exposed for the first time to our best knowledge, and this deviation may not only be restricted to Sc2C2@C72 series but also apply to other scandium carbide fullerenes Sc2C2@C2n (2n≠72) and even other metal carbide fullerenes. Misleading conclusions will be drawn on the basis of inaccurate energies calculated with B3LYP method and other DFT methods without long range corrections. As B3LYP, BP86, PBE, etc. are fairly widely-used methods in theoretical studies of endohedral metallofullerenes, our work is an instructive warning to these studies.
2. Computational Method
In electronic structure calculations, all Full geometry optimizations of all relevant structures are performed by using the dispersion-corrected density functional theory approach M06-2X with basis sets of 6-31G(d) for H, C, N and O atoms and Lanl2dz with the corresponding effective core potential for La and Yb atoms. Vibrational analyses upon the optimized structures are performed at the same level of theory to demonstrate the stationary points to be minimal for reactants, intermediates and adducts or saddle points for transition states. Intrinsic reaction coordinate (IRC) analyses are employed to confirm the transition states. Solvent effects have been considered by the polarizable continuum model (PCM)..
3. Results and Discussion
The Sc2C74 series including Sc2@C74 and Sc2C2@C72 series were thoroughly investigated with four DFT methods, namely, B3LYP, CAM-B3LYP, M06-2X and wB97XD, in conjunction with statistical mechanics, and some of the Sc2C74 isomers were further investigated with five other methods, i.e. BLYP, B3PW91, BP86, PBE and PBE0. Our results disclosed that Sc2C2@Cs(10528)-C72 whose cage does not obey the well-known isolated pentagon rule and has two pairs of pentagon adjacencies possesses the best thermodynamic stability at temperature region of fullerene formation, which is well in line with experiment. A speculation that to isolate other Sc2C74 isomers but Sc2C2@Cs(10528)-C72 are almost impossible was drawn on the basis of the fact that other Sc2C74 isomers possess very small populations at temperature region of fullerene formation. More importantly, we found that B3LYP method as well as other DFT methods without long range corrections, such as BLYP, B3PW91, BP86, PBE and PBE0, tends to overestimate energies of Sc2C2@C72 isomers whereas energies of Sc2@C74 isomers are relatively accurate, and erroneous conclusion will be come to in light of inaccurate energies, and this overestimation may also hold for other scandium carbide fullerenes and even other metal carbide fullerenes. It is an instructive warning to theoretical investigation on endohedral metallfullerenes that B3LYP and other DFT methods without long range corrections may overestimate energies of metal carbide fullerenes. Physical chemical properties of Sc2C2@Cs(10528)-C72 are also investigated. Both the highest occupied and lowest unoccupied molecular orbital are mainly distributed on the carbon cage, intimating that redox reactions will occur on the carbon cage. The valence state of Sc2C2@Cs(10528)-C72 cannot be simply described as (Sc2C2)4+@C724-, because the covalent interaction between Sc2C2 cluster and carbon cage indeed plays an important role. The IR spectrum was simulated and the signal at 1972 cm-1 corresponding to stretching vibration of C-C bond of the Sc2C2 cluster is discovered to be an important signal to distinguish metal carbide fullerenes from dimetallofullerenes.

Figure 1. (a) Front and (b) side views of Sc2C2@Cs(10528)-C72 (The two pentalene motifs of Cs(10528)-C72 cage are highlighted in red. Two Sc are labeled with “75” and “76” and C atoms of the encapsulated cluster are labeled with “73” and “74”)

4. Others
NA
5. Publication/Presentation
(1) R.S. Zhao, Y.J. Guo, P. Zhao, M. Ehara, S. Nagase, X. Zhao, J. Phys. Chem. C, 2016, 120, 1275-1283.
(2) P. Zhao, M. Ehara, X. Zhao, to be published in PCCP.
6. Patent
N/A