Synthesis of High-Nitrogen Energetic Material

Abstract

Pure nitrogen can be considered as a material with optimized storage of chemical energy because of the huge difference in energy between triply bonded di-nitrogen and singly bonded nitrogen. N ≡ N bond is one of the strongest chemical bonds known, containing 4.94eV atom−1 while the N–N bond is much weaker with −0.83eV atom−1 [1]. Therefore when transforming from singly bonded nitrogen to diatomic triply bonded molecular nitrogen, a large amount of energy should be released: about 2.3eV atom−1. Or, in other words, this chemical energy can be ideally stored by transforming a triple bond into three single bonds. Thus, nitrogen may form a high-energy density material with energy content higher than that of any known nonnuclear material. The greatest utility of fully single-bonded nitrogen would be as high explosives. Here, a tenfold improvement in detonation pressure over HMX (one the more powerful high explosives) seems possible [2]. A chemical approach for synthesis of nitrogen-energetic materials is creating large all-nitrogen molecules or clusters bound by single (N–N) or single and double (N=N) bonds. Calculations predict different polynitrogen molecules or clusters [3] such as, for instance, N4,N8,N20, or even nitrogen fullerene N60 (see for review Refs [3–5]) with high-energy-storage capacity. However, none of them has yet been synthesized with exception of N4(TdN4) [6], albeit with a very short lifetime of ∼1μs [6]. Synthesis of compounds having several nitrogen atoms consecutively is difficult because the single bond in nitrogen is relatively weak. It has been achieved only in compounds with other atoms. For instance, HN3 and other azides with linearN3 radical have been synthesized by Curtius in 1890 [7]. Only recently N5 was synthesized by Christe and coworkers [8]. On the basis of the N3− and N5 species nearly all nitrogen compounds were synthesized by attaching these radicals to a central atom of Te, B, and P such as N5P(N3)6,N5B(N3)4,Te(N3)4, and others (see for review Refs [2, 9–11]).