At high pressures, autoionization – along with polymerization and metallization – is one of the responses of simple molecular systems to a rise in electron density. Nitro­sonium nitrate (NO+NO3), known for this property, has attracted a large interest in recent decades and was reported to be synthesized at high pressure and high temperature from a variety of nitro­gen–oxygen precursors, such as N2O4, N2O and N2–O2 mixtures. However, its structure has not been determined unambiguously. Here, we present the first structure solution and refinement for nitro­sonium nitrate on the basis of single-crystal X-ray diffraction at 7.0 and 37.0 GPa. The structure model (P21/m space group) contains the triple-bonded NO+ cation and the NO3 sp2-trigonal planar anion. Remarkably, crystal-chemical considerations and accompanying density-functional-theory calculations show that the oxygen atom of the NO+ unit is positively charged – a rare occurrence when in the presence of a less-electronegative element.

Laniel, D., Winkler, B., Koemets, E., Fedotenko, T., Chariton, S., Milman, V., Glazyrin, K., Prakapenka, V., Dubrovinsky, L. & Dubrovinskaia, N., Nitro­sonium nitrate (NO+NO3) structure solution using in situ single-crystal X-ray diffraction in a diamond anvil cell, (2021). IUCrJ 8.   abstract