Local energy decomposition analysis and molecular properties of encapsulated methane in fullerene (CH4@C60)

Methane has been successfully encapsulated within cages of C-60 fullerene, which is an appropriate model system to study confinement effects. Its chemistry and physics are also relevant for theoretical model descriptions. Here we provide insights into intermolecular interactions and predicted spectroscopic responses of the CH4@C-60 complex and compared them with results from other methods and with data from the literature. Local energy decomposition analysis (LED) within the domain-based local pair natural orbital coupled cluster singles, doubles, and perturbative triples (DLPNO-CCSD(T)) framework was used, and an efficient protocol for studies of endohedral complexes of fullerenes is proposed. This approach allowed us to assess energies in relation to electronic and geometric preparation, electrostatics, exchange, and London dispersion for the CH4@C-60 endohedral complex. The calculated stabilization energy of CH4 inside the C-60 fullerene was -13.5 kcal mol(-1) and its magnitude was significantly larger than the latent heat of evaporation of CH4. Evaluation of vibrational frequencies and polarizabilities of the CH4@C-60 complex revealed that the infrared (IR) and Raman bands of the endohedral CH4 were essentially silent due to the dielectric screening effect of C-60, which acted as a molecular Faraday cage. Absorption spectra in the UV-vis domain and ionization potentials of C-60 and CH4@C-60 were predicted. They were almost identical. The calculated H-1/C-13 NMR shifts and spin-spin coupling constants were in very good agreement with experimental data. In addition, reference DLPNO-CCSD(T) interaction energies for complexes with noble gases (Ng@C-60; Ng = He, Ne, Ar, Kr) were calculated. The values were compared with those derived from supramolecular MP2/SCS-MP2 calculations and estimates with London-type formulas by Pyykko and coworkers [Phys. Chem. Chem. Phys., 2010, 12, 6187-6203], and with values derived from DFT-based symmetry-adapted perturbation theory (DFT-SAPT) by Hesselmann & Korona [Phys. Chem. Chem. Phys., 2011, 13, 732-743]. Selected points at the potential energy surface of the endohedral He-2@C-60 trimer were considered. In contrast to previous theoretical attempts with the DFT/MP2/SCS-MP2/DFT-SAPT methods, our calculations at the DLPNO-CCSD(T) level of theory predicted the He-2@C-60 trimer to be thermodynamically stable, which is in agreement with experimental observations.

Automated summary

The study investigates the confinement effects of methane encapsulated in C-60 fullerene, using local energy decomposition analysis to provide insights into intermolecular interactions. The calculated stabilization energy of methane inside the fullerene was significantly higher than its latent heat of evaporation. Vibrational frequencies of the endohedral complex were found to be essentially silent due to the dielectric screening effect of C-60.