Thermal stability of the endohedral fullerenes $\mathrm{Na}{\mathrm{C}}_{60},$ $\mathrm{Na}{\mathrm{C}}_{70},$ and $\mathrm{Pa}{\mathrm{C}}_{60}$

Nitrogen and phosphorus atoms enclosed in fullerenes keep their atomic ground-state configuration, and no charge transfer or covalent bonding to the cage occurs. These systems can be dissolved in organic solvents and reacted with addends without losing the atomic character of the guest atom. In this paper, we present a detailed study of the annealing behavior of ${\mathrm{NaC}}_{60},$ ${\mathrm{NaC}}_{70},$ ${\mathrm{PaC}}_{60},$ and of some ${\mathrm{NaC}}_{60}$ adducts. The disintegration of these systems, as recorded by electron paramagnetic resonance measurements, occurs in the temperature range between 400 K and 600 K. The results are in qualitative agreement with potential energy calculations and confirm the proposed escape mechanism that proceeds via bond formation of the trapped atom with the cage. It is found that other combinations of group-V elements with fullerenes, e.g., ${\mathrm{NaC}}_{84},$ ${\mathrm{PaC}}_{70},$ and ${\mathrm{AsaC}}_{60}$ are not stable at room temperature. The escape mechanism and the possibility of disabling this path are discussed.