Pocket and antipocket conformations for the CH₄@C₈₄ endohedral fullerene

The endohedral fullerene CH₄@C₈₄ has been studied using density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). In addition to the structure with a C-H bond of CH₄ in a tetrahedral pocket conformation, we find an alternative minimum, very close in energy (6.3-9.5 kJ/mol higher according to the level of theory), with the methane inverted, which we call the antipocket conformation. Computed IR spectra are reported for CH₄@C₈₄ and also for the reference system CH₄@C₆₀. The calculated vibrational levels, in a harmonic approximation, reveal close-lying translational, librational, and shell-vibrational modes. The results are also presented for the isoelectronic species NH₄⁺@C₆₀.