Structural and Electron Aspects of Nitrogen‐Nitrogen Bond
Introduction Nitrogen, like many other chemical elements, has catenation ability. The variety and multiplicity of molecular structures with nitrogen-nitrogen bond is significantly smaller than for carbon. Such compounds are of particular interest as potential high-energy density materials (HEDM) [1], because they can undergo exothermic reaction Nx → (x/2)N2 releasing 50 kcal/mol or more per one nitrogen atom [2]. However, their practical application is limited by their stability. Known are chain, cyclic as well as cage systems containing nitrogennitrogen bond. The experimental studies have proven that it is possible to obtain ions N3 − to N5 + in laboratory conditions [3÷5] as well as polymer networks [6] but only under high pressure (110 GPa). Based on the crystallographic resources of Cambridge Structural Database (CSD) [7], one can conclude that the largest family of chemical compounds involves systems with two nitrogen atoms connected to each other (approx. 85% of all structures of type Nx, where x ≥ 2). Among these system, the largest group are hydrazo, azo, azoxy and azodioxy compounds and compounds containing nitramine group. The first four groups form oxidation reduction series in relation to each other. Hydrazo functional group contains two nitrogen atoms sp3 connected by σ bond and each of them forms additional chemical bond with hydrogen atom. While two nitrogen atoms of hybridization sp2 each of which has nonbonding electron pair and which are doublebounded to each other form bivalent nitrogen group. The molecules of chemical compounds containing this functional group may exist as two geometric isomers E and Z. Azoxy functional group is formed as a result of formation of chemical bond between oxygen atom and free electron pair of one of azo group nitrogen atoms. Azoxy compounds, called also diazene N-oxides also show geometric isomerism Z and E with the respect to nitrogen-nitrogen bond. If both nitrogen atoms of azo group form chemical bond with oxygen atom (>N2O2), such group is called azodioxy. It is the most oxidized form of nitrogennitrogen bond [8–14]. The different order of connecting oxygen and nitrogen atoms within the group of >N2O2 (N–NO2) can be found in family of compounds called nitramines. Nitramine group forms in these compound four-core π-electron system that may interact with substituents of amide nitrogen atom. The studies of properties of N-N bond and effect of nitramine group on ring aromaticity in family of nitramine phenyl derivatives are also motivated by the desire to explain the mechanism of so called nitramine rearrangement.