Methane has been successfully encapsulated within cages of C<sub>60</sub> fullerene, and it is an appropriate model system to study confinement effects. Its chemistry and physics is also relevant for theoretical model descriptions. Here we provided insights into intermolecular interactions and predicted spectroscopic responses of the CH<sub>4</sub>@C<sub>60</sub> complex and compared with results from other methods and with literature data. 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 CH<sub>4</sub>@C<sub>60</sub> endohedral complex. The calculated stabilization energy of CH<sub>4</sub> inside the C<sub>60</sub> fullerene was −13.5 kcal/mol and its magnitude was significantly larger than the latent heat of evaporation of CH<sub>4</sub>. Evaluation of vibrational frequencies and polarizabilities of the CH<sub>4</sub>@C<sub>60</sub> complex revealed that the infrared (IR) and Raman bands of the endohedral CH<sub>4</sub> were essentially “silent” due to dielectric screening effect of the C<sub>60</sub>, which acted as a molecular Faraday cage. Absorption spectra in the UV-Vis domain and ionization potentials of the C<sub>60</sub> and CH<sub>4</sub>@C<sub>60</sub> were predicted. They were almost identical. The calculated <sup>1</sup>H/<sup>13</sup>C 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<br>(Ng@C60 ; Ng = He, Ne, Ar, Kr) were calculated. The values were compared with those derived from supermolecular MP2/SCS-MP2 calculations and estimates with London-type formulas by Pyykkö and coworkers [Phys. Chem. Chem. Phys., 2010, 12, 6187-6203], and with values derived from<br>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<sub>2</sub>@C<sub>60</sub> 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<sub>2</sub>@C<sub>60</sub> trimer to be thermodynamically stable, which is in agreement with experimental observations.
London dispersion effects in the coordination and activation of alkanes in σ-complexes: a local energy decomposition study
