Recent synthesis of the endohedral complexes of C70 and its open-cage derivative with one and two H2 molecules has opened the path for experimental and theoretical investigations of the unique dynamic, spectroscopic, and other properties of systems with multiple hydrogen molecules confined inside a nanoscale cavity. Here we report a rigorous theoretical study of the dynamics of the coupled translational and rotational motions of H 2 molecules in C70 and C60, which are highly quantum mechanical. Diffusion Monte Carlo (DMC) calculations were performed for up to three para-H2 (p-H2) molecules encapsulated in C70 and for one and two p-H2 molecules inside C 60. These calculations provide a quantitative description of the ground-state properties, energetics, and the translation-rotation (T-R) zero-point energies (ZPEs) 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 (PES) is negative for one and two H2 molecules in C70 but has a high positive value when the third H2 is added, implying that at most two H2 molecules can be stabilized inside C70. By the same criterion, in the case of C60, only the endohedral complex with one H2 molecule is energetically stable. Our results are consistent with the fact that recently both (H 2)n@C70 (n = 1, 2) and H2@C 60 were prepared, but not (H2)3@C70 or (H2)2@C60. The ZPE of the coupled T-R motions, from the DMC calculations, grows rapidly with the number of caged p-H2 molecules and is a significant fraction of the well depth of the intermolecular PES, 11% in the case of p-H2@C70 and 52% for (p-H2)2@C70. Consequently, the T-R ZPE represents a major component of the energetics of the encapsulated H2 molecules. The inclusion of the ZPE nearly doubles the energy by which (p-H2)3@C70 is destabilized and increases by 66% the energetic destabilization of (p-H2)2@C 60. For these reasons, the T-R ZPE 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry