**3. Results**

polynitrogens may exist such as N<sup>7</sup>

32 Fullerenes and Relative Materials - Properties and Applications

**2. Methodology**

examples have been studies of N@C60, N@C70 or even N2

not necessarily related to high temperatures and pressure.

species, contributing to the development of new materials in the future.

, N12, N18, N20 and a great etcetera [36–42]. Considering

@C60 [48–52], which have extended

larger structures, there is no doubt that polyhedral or fullerene-type structures have provided inspiration, as they are considered as alternative polynitrogenous arrangements with a greater number of nitrogen atoms [18, 19, 21, 41, 43–47]. In this sense, a hollow structure could in principle be useful for storing internal molecules, which is why it is common to find hollow carbon structures as candidates for confining polynitrogenous species. Currently, the most notable

the use of the C60 cage as a way of confining up to 16 nitrogen atoms [53, 54]. Also recently, the encapsulation of a polymer nitrogen chain in a carbon nanotube was conceived, as in principle this could be stable up to room temperature [55]. This contrasts with other proposed polynitrogenated phases of nitrogen that have been inspired by the analogy to phosphorus and arsenic, all superior in energy to the "cubic gauche" (CG). These have been proposed for extreme conditions and some experimental evidence has even been found for them [56–61]. Therefore, confinement represents an alternative for the stabilization of polynitrogenated species that are

As our work group has studied fullerenes and their reactivity [7, 16, 62–65] and it has been proposed that C60 might be an ideal candidate for trapping nitrogen by a polymer [53, 54], the intention now is to show that C70 might also be an alternative way of storing polynitrogenous

Initiating with considerable structural diversity, we calculated and modeled endohedral fullerenes; Nn@C70 (*n* = 3–10). For this, we used the C70 structure that corresponds to the isomer from this composition and complies with the isolated pentagon rule, IPR. The Avogadro visualizer was used as an auxiliary [66]. Given the considerable number of possible isomers, we determined the minimum energy structures in two stages. Initially, we obtained the geometry optimizations for all molecules, using the PM6 method [67]. To ensure that the global minimum for each composition has been identified, the use of search algorithms, such as those inspired by genetic algorithms, is essential, but this is currently beyond the scope of this type of system. However, the diversity of proven structures inspires confidence in our determination of the most important and representative species. Subsequently, both the geometry and the electronic structure were refined for the lower energy isomers of each composition. Likewise, the calculation of vibrational frequencies was undertaken in order to corroborate that the stationary points located on the potential energy surface correspond to a minimum (NImag = 0). All this was undertaken using the hybrid functional B3LYP with the base set 6-311G [68, 69]. As the structures of the polynitrogen species in free state may differ according to charge, this factor was also evaluated, determining the structures for the isomers neutral, cation and anion. All calculations were performed using the Gaussian09 program [70].

We also calculated ionization energies (IE) and electron affinities (EA). These were therefore calculated to reveal the following energy differences: IE = Ecation − Eneutral, EA = Eneutral − Eanion.

It was previously reported that the methodology consisted of an optimization at the PM6 level. As the refinement and electronic structure were performed at the B3LYP/6-311G level, they are presented exclusively in the following.

For the majority of the structures obtained, neutral and charged species correspond to the same arrangement. In cases where it is not observed, it will be mentioned in due course.
