Subject Number : S-16-MS-0030
Support Type : Collaborative research
Proposal Title (English) : Quantum Chemical study of Diels-Alder reactions of fullerenes at extreme high pressure
Username (English) : Roberto Cammi
Affiliation (English) : Dipartimento di Chimica, Università degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy

1. Summary
High-pressure Diels-Alder cycloaddition reaction of fullerenes is an important synthetic method for the thermally stable cycloadducts. The effects of high pressure on the potential energy surfaces of Diels-Alder cycloaddition of cyclopentadiene and C60 were studied with a recently developed approach, the polarizable continuum model for extreme pressure (XP-PCM). The high pressure reduces the activation energies and increases reaction energies drastically, making the DA reaction more favorable. The pressure effects on the reaction energetics may be divided into the cavitation and electronic contributions. For the activation energy, the cavitation contribution is significant in comparison with the electronic contribution. To assist future experiments, the activation volume and reaction volume were computed based on the relationship between activation energy or reaction energy with the pressure as a consequence of the fitting linear correlation between activation energy or reaction energy with the pressure.
2. Computational method
All the DFT calculations were performed using the M06-2X functional with 6-31G(d) in Gaussian09 program B.01. Subsequent frequency calculations at the same level of theory confirmed those molecular structures are local minima or transition states (TSs). Argon (ε0=1.430, ρ=1.3954 g/cm3 at standard thermodynamic conditions) was used as the external medium. The dispersion effect is important and a stable reactant complex forms between C60 and CP before TS. In cavitation, specifically, we avoided the unnecessary external medium in the center of C60 fullerene, which is generated in the standard model.
3. Results and Discussion
By using the XP-PCM method, the effects of high pressure (1~16 GPa) on the PES of DA cycloaddition reaction of cyclopentadiene and both [5,6] and [6,6] bonds of fullerene C60 have been studied for the first time. Although the [6,6] bond is always preferred over the [5,6] bond thermodynamically and kinetically, the high pressure leads to the large reduction of the activation energy and increase of the exothermicity, making the DA reaction more favorable. More interestingly, under extreme high pressure conditions, three cycloaddition reactions become barrier-less, suggesting the possibility of the synthesis of both 66- and 56-cycloadducts.

Figure: (Left) Potential energy profile along the intrinsic reaction coordinate (IRC) of three DA cycloadditions of CP and C60. (Right) Correlation between the activation energy and the pressure.
4. Others
N/A
5. Publication/Presentation
T. Yang, R. Fukuda, R. Cammi, M. Ehara, submitted for publication.
6. Patent
N/A