Chemistry Nexus

Everyone knows that elemental nitrogen exists in the atmosphere as dinitrogen, N₂. There is a triple bond between the two nitrogen atoms. This is true – but under certain conditions, a fascinating N-N single bonded phase has been characterised.¹

In 1985 it was predicted that at high pressure, nitrogen would transform to a solid with a single-bonded crystalline structure called polymeric nitrogen. Later, it was proposed that it whould have a cubic gauche (cg-N) structure. Experimental evidence was scant however until 2004 when a team of scientists from Germany and Russia managed to make the compound directly from molecular nitrogen at temperatures above 2000 K and pressures above 110 GPa using a laser-heated diamond cell. The material was characterized by X-ray and Raman scattering methods we have identified this as the polymeric nitrogen (cg-N).

The phase is a stiff with a bulk modulus ≥300 GPa. This is characteristic of strong covalent solids. The polymeric nitrogen is metastable. The structure of N is polymeric with each nitrogen bound to three other nitrogen atoms. At a pressure of 115 GPa, each N-N bond length is 1.346 ± 0.004 Å. The N-N-N angles are all about 108.8°, very close to the ideal tetrahedral angle of just over 109°.

It did not prove possible to recover the polymeric nitrogen by releasing the pressure – in other words the polymer reverts to normal dinitrogen. The authors speculate that this form of nitrogen is a new class of single-bonded nitrogen materials that may have unique energy capacity properties (more than five times that of the most powerful energetic materials).

Abstract¹: Nitrogen usually consists of molecules where two atoms are strongly triple-bonded. Here, we report on an allotropic form of nitrogen where all atoms are connected with single covalent bonds, similar to carbon atoms in diamond. The compound was synthesized directly from molecular nitrogen at temperatures above 2,000 K and pressures above 110 GPa using a laser-heated diamond cell.

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