Nitrogen triiodide

Nitrogen triiodide, also called nitrogen iodide, is the chemical compound with the formula NI₃. It is an extremely sensitive contact explosive: small quantities explode with a gunpowder-like snap when touched even lightly, releasing a purple cloud of iodine vapor. NI₃ has a complex structural chemistry that has required relatively heroic efforts to elucidate because of the instability of the derivatives.

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Decomposition

The decomposition of NI₃ proceeds via the following reaction:

2NI₃(s) → N₂(g) + 3I₂(g) ΔH = –290 kJ/mol

Structure of NI₃ and its derivatives

Nitrogen triiodide is a dark red compound, first characterized by X-ray crystallography in 1990, when it was prepared by an ammonia-free route. Boron nitride reacts with iodine fluoride in trichlorofluoromethane at -30 °C to produce pure NI₃ in low yield.^[1] NI₃ is pyramidal (C_3v molecular symmetry), as are the other nitrogen trihalides as well as ammonia.^[2]

The material that is usually called "nitrogen triiodide" is prepared by the reaction of iodine with ammonia. When this reaction is conducted at low temperatures in anhydrous ammonia, the initial product is NI₃·(NH₃)₅, but this material loses some ammonia upon warming to give the 1:1 adduct NI₃·(NH₃). This adduct was first reported by Bernard Courtois in 1812, and its formula was finally determined in 1905 by Silberrad.^[3] Its solid state structure consists of chains of -NI₂-I-NI₂-I-NI₂-I-... Ammonia molecules are situated between the chains. In the dark and kept cold and damp with ammonia, NI₃·(NH₃) is stable. The dry material is, however a contact explosive decomposing according to the following equation:^[2]

8NI₃NH₃ → 5 N₂ + 6 NH₄I + 9 I₂

The instability of NI₃ itself or NI₃NH₃ can be attributed to the great stability of N₂.

Nitrogen triiodide in classroom demonstrations and popular culture

• Small amounts of nitrogen triiodide are sometimes synthesized as a demonstration to high school chemistry students or as an act of "chemical magic".^[4] To highlight the sensitivity of the compound, it is usually detonated by touching it with a feather but even the slightest air current or other movement can cause detonation. Nitrogen triiodide is also notable for being the only known explosive that detonates when exposed to alpha particles and nuclear fission products.^[5]
• NI₃·NH₃ explosions leave orange-to-purple iodine stains that can be removed by sodium thiosulfate solution.
• Nitrogen triiodide is the contact explosive used in Brainiac: Science Abuse, named "Peter Logan's Exploding Paste" on the show, although the details of its production are not mentioned for safety purposes.
• In Robert Heinlein's novel Farnham's Freehold, the eponymous Hugh Farnham uses nitrogen triiodide (made from ammonia and iodine) as a blasting powder.

References

1. ^ Tornieporth-Oetting, I.; Klapötke, T. Angewandte Chemie International Edition in English," 1990, volume 29, pages 677-679. [1]
2. ^ ^{a b} Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
3. ^ Silberrad, O. "On the Constitution of Nitrogen Triiodide" Journal of the Chemistry Society 1905, volume 87, pages 55-66. DOI: 10.1039/CT9058700055
4. ^ Ford, L. A. and Grundmeier, E. W. Chemical Magic. Dover, 1993, p. 76. ISBN 0486676285
5. ^ Bowden, F. P. Initiation of explosion by neutrons, α-particles, and fission products. Proc. Roy. Soc. (London) 1958, A246, 216-19.