Nitrogen triiodide

Nitrogen triiodide, also called nitrogen iodide, is the chemical compound with the formula NI3. It is a 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.

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. NI3 is pyramidal (C3v molecular symmetry), as are the other nitrogen trihalides as well as ammonia.

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. 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:


 * 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". 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.
 * 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.