N2 is only known neutral and stable structural form (allotrope) of nitrogen. Synthesis of neutral N3 and N4 were reported earlier but could not be isolated due to extreme instability. Researchers have now succeeded in room temperature synthesis of hexanitrogen (N6), a new neutral allotrope of nitrogen which they could trap in argon matrices at 10K. The synthesis of N6 was confirmed spectroscopically and the new allotrope was demonstrated to be stable. The decomposition reaction was exothermic with release of a large amount of energy suggesting possible application as an energy storage technology subject to requirement of liquid nitrogen temperatures. Notwithstanding this, preparation of a new stable, neutral allotrope of nitrogen is a significant development in chemistry.
Allotropes are different structural forms of an element resulting from different manner of bonding of atoms of the same element. They have different properties based on their structure and are charge-neutral existing in nonradical forms. For example, diamond, graphite, and graphene are neutral, stable allotropes of carbon. So are O2 and O3 (ozone) allotropes of oxygen.
How about neutral, stable structural forms of nitrogen? The good old N2 is only known stable allotrope of nitrogen. Two other neutral allotropes N3 and N4 were reported earlier in 1956 and 2002 respectively but but could not be isolated due to extreme instability.
Chemists have now successfully synthesised C2h-symmetric hexanitrogen (C2h-N6) at the room-temperature. [C2h symmetry is a common form of symmetry observed in the chemical world. A C2h-symmetric molecule possesses a two-fold rotation axis (C2) and a horizontal mirror plane (σh)].
Hexanitrogen (N6), the new neutral allotrope of nitrogen was synthesised at room temperature through the gas-phase reaction of chlorine (Cl2) or bromine (Br2) with silver azide (AgN3) under reduced pressure. This was followed by cryogenic trapping in argon matrices at 10 Kelvin. The researchers were also able to prepare hexanitrogen in pure form as a film at 77 Kelvin (the boiling point of liquid nitrogen).
Hexanitrogen (N6) thus prepared in the laboratory was characterised spectroscopically and was demonstrated to be stable.
On decomposition, hexanitrogen (N6) breaks into three N2 molecules. The reaction is exothermic with a release of 185.2 kcal per mole of energy, which is 2.2 and 1.9 times higher than the energy released on decomposition of TNT and HMX by weight. Because of such a large energy release on decomposition, hexanitrogen (N6) may be a promising clean energy-storage material, however that would require keeping hexanitrogen at liquid nitrogen temperatures below 77K which may not work out well for an energy storage-technology.
Notwithstanding any future application, room temperature preparation of this new neutral molecular allotrope of nitrogen which could be trapped in a cryogenic setting is a significant development in chemistry.
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References:
- Qian, W., Mardyukov, A. & Schreiner, P.R. Preparation of a neutral nitrogen allotrope hexanitrogen C2h-N6 . Nature 642, 356–360 (2025). Published: 11 June 2025. DOI: https://doi.org/10.1038/s41586-025-09032-9
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