Nitro­sonium nitrate (NO(+)NO(3) (−)) structure solution using in situ single-crystal X-ray diffraction in a diamond anvil cell

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(+)NO(3) (−)), 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 N(2)O(4), N(2)O and N(2)–O(2) 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 (P2(1)/m space group) contains the triple-bonded NO(+) cation and the NO(3) (−) sp (2)-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.