All-Nitrogen Cages and Molecular Crystals: Topological Rules, Stability, and Pyrolysis Paths

Konstantin P Katin,Alexey I Kochaev,Mikhail M Maslov,Savas Kaya,Valeriy B Merinov

doi:10.3390/computation8040091

Konstantin P Katin, Alexey I Kochaev + Show 3 more

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https://doi.org/10.3390/computation8040091

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Abstract

We combined ab initio molecular dynamics with the intrinsic reaction coordinate in order to investigate the mechanisms of stability and pyrolysis of N4 ÷ N120 fullerene-like nitrogen cages. The stability of the cages was evaluated in terms of the activation barriers and the activation Gibbs energies of their thermal-induced breaking. We found that binding energies, bond lengths, and quantum-mechanical descriptors failed to predict the stability of the cages. However, we derived a simple topological rule that adjacent hexagons on the cage surface resulted in its instability. For this reason, the number of stable nitrogen cages is significantly restricted in comparison with their carbon counterparts. As a rule, smaller clusters are more stable, whereas the earlier proposed large cages collapse at room temperature. The most stable all-nitrogen cages are the N4 and N6 clusters, which can form the van der Waals crystals with densities of 1.23 and 1.36 g/cm3, respectively. The examination of their band structures and densities of electronic states shows that they are both insulators. Their power and sensitivity are not inferior to the modern advanced high-energy nanosystems.

Highlights

The formation of the N2 molecule with the triple N≡N bond from two isolated nitrogen atoms results in the release of a large amount of energy (≈10 eV [1])
We focus on the pyrolysis mechanisms and the corresponding activation barriers, rather than on the relative formation energies of the structures under consideration
We constructed N10 H8, N9 H7, N8 H6, N7 H5, and N6 H4 naphthalene-like molecules with hexagon/hexagon, hexagon/pentagon, pentagon/pentagon, pentagon/square, and square/square interfaces, respectively. These molecules do not remain flat during geometry optimization due to the sp3 -hybridization of nitrogen, which differs from carbon bicycles in which sp2 hybridization occurs

Summary

Introduction

The formation of the N2 molecule with the triple N≡N bond from two isolated nitrogen atoms results in the release of a large amount of energy (≈10 eV [1]). Nitrogen is considered as the primary component of the most high-energy compounds. Such compounds usually contain carbon atoms that provide stability of the whole molecule framework decorated by oxygen-containing groups or other oxidants. RDX, HMX, and CL-20 are well-known examples of traditional high-energy structures with a carbon–nitrogen frame. More advanced energy materials have been proposed with a similar carbon–nitrogen architecture [2,3,4]. Compared to carbon–nitrogen compounds, pristine nitrogen systems are more powerful and safer for the environment. Closed full-nitrogen Nn cages with three-coordinated atoms and single N–N bonds are very attractive and environmentally friendly

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