A rigorous theoretical study was made of the dynamics of the coupled translational and rotational motions of H2 molecules in C70 and C60, which were highly quantum mechanical. Diffusion Monte Carlo calculations were performed for up to three para-H2 (p-H2) molecules encapsulated in C70 and for one and two p-H2 molecules inside C60. These calculations provide a quantitative description of the ground-state properties, energetics, and the translation-rotation zero-point energies of the nanoconfined p-H2 molecules and of the spatial distribution of two p-H2 molecules in the cavity of C70. The energy of the global minimum on the intermolecular potential energy surface was negative for one and two H2 molecules in C70 but has a high positive value when the third H2 was added, implying that at most two H2 molecules could be stabilized inside C70. By the same criterion, in the case of C60, only the endohedral complex with one H2 molecule was energetically stable. The results were consistent with the fact that recently both (H2)nC70 (n = 1, 2) and H2C60 were prepared, but not (H2)3C70 or (H2)2C60. The zero-point energy of the coupled translation-rotation motions, from the DMC calculations, grew rapidly with the number of caged p-H2 molecules and was a significant fraction of the well depth of the intermolecular potential energy surface, 11% in the case of p-H2C70 and 52% for (p-H2)2C70. Consequently, the translation-rotation zero-point energy represents a major component of the energetics of the encapsulated H2 molecules. The inclusion of the zero-point energy nearly doubles the energy by which (p-H2)3C70 was destabilized and increased by 66% the energetic destabilization of (p-H2)2C60. For these reasons, the translation-rotation zero-point energy has to be calculated accurately and taken into account for reliable theoretical predictions regarding the stability of the endohedral fullerene complexes with hydrogen molecules and their maximum H2 content.

Hydrogen Molecules inside Fullerene C70: Quantum Dynamics, Energetics, Maximum Occupancy, and Comparison with C60. Sebastianelli, F., Xu, M., Baĉić, Z., Lawler, R., Turro, N.J.: Journal of the American Chemical Society, 2010, 132[28], 9826-32