**4. Conclusions**

its electronic density more easily, so that it represents the most reactive molecule. Previous results for C60 encapsulation [53] showed a reduced HOMO-LUMO gap, as the number of nitrogens increased: 2, 4, 6, 8, 10. This is somewhat similar to the results for hardness, as one of the approximations to this parameter is precisely the HOMO-LUMO gap (**Figure 4**).

**Figure 4.** Frontier orbitals for Nn@C70, where *n* = 3–10, endohedral fullerenes, calculated using the B3LYP/6-311G method.

Previous studies have mentioned that from 2 to 10 nitrogen atoms, there would be a charge transfer from the C60 cage to the nitrogen polymer, explained as resulting from the greater ionization and electronegativity potential of N compared to C. For more than 10 nitrogen atoms, there would be a reverse transfer from the nitrogen to the carbon atoms. This was attributed to reduced space availability and overlapping of orbitals [53]. In this work, the frontier orbitals of the most stable structures were analyzed, showing that there is an appreciable contribution from frontier orbitals to the nitrogen atoms. This implies that there must be areas susceptible to receiving charge density. It is especially interesting that when the number of N atoms within the fullerene increases, the contribution of Nitrogen-

tems, a very considerable contribution of HOMO is observed, and a sure indication that possible interactions with the cage are initiating. This may also relate to the suggestion that greater interaction between the cage and the Nn takes place, as the amount of nitrogen atoms increases. The reaction energy may also result from this, particularly as it is apparent

there is a more or less constant increase of 150 kcal/mol in stabilization

and higher, suggesting greater participation

and higher sys-

centered orbitals for LUMO is even greater. Particularly, in the case of N<sup>8</sup>

**3.3. Analysis of frontier orbitals**

40 Fullerenes and Relative Materials - Properties and Applications

that from N3

to N7

energy. This reaction energy changes with N8

on the part of the cage and its interaction with Nn.

The C70 fullerene is viable for storage and stabilization of nitrogen aggregates of at least 3–10 atoms, without presenting any concurrent structural deformation in the carbon network or obvious interaction (bond-like) between the nitrogen atoms and the carbon of the fullerene.

Evidently, the formation energy from the isolated nitrogen atoms is favored in all cases and from N3 to N8 there is a progressive increase, indicating the role of the cage as an encapsulator and stabilizer of polynitrogenous structures. When we propose the formation of endohedral fullerenes, initiating with materials that can be found under laboratory conditions, we predict that metastable structures will form, a fact that may be of interest in the potential use of these materials as HEDMs. The stabilizing contribution of the cage becomes more evident, as the number of nitrogen atoms within it increases, manifested in a considerable contribution on the part of frontier orbitals as potential charge stabilizers.

As C70 fullerene is a large molecule of approximately 1 nm diameter, nitrogen, polymers of 3–7 nitrogen atoms prefer to remain as nitrogen molecules or as azides. However, from 8 atoms onwards, the fullerene cage begins to have greater interaction with the nitrogen polymers, reflected in the fact that these begin to compact and create more complex nitrogen polymers (in the form of rings).

According to indexes of global chemical reactivity, endohedral fullerenes present an odd/ even behavior that corresponds to that expected for open layer species (odd number of electrons: lower ionization potential and hardness, greater electronic affinity and chemical potential) and closed layer (even number of electrons: greater ionization potential and hardness, lower electronic affinity and chemical potential). However, for 8, 9 and 10 encapsulated nitrogen atoms, there is a change in behavior. This coincides with the change in formation energies and the analysis of frontier orbitals, reflecting greater participation of the cage in fullerene behavior. The fact that smaller fullerenes have been able to encapsulate more atoms is an indication that the point of saturation in this structure has not yet been reached, providing an incentive to find new structures with greater complexity, as the degree of encapsulation progresses. These would thus constitute candidates for energetic materials and studies are ongoing.
