Nitro-sonium nitrate (NO⁺NO₃ ^-) structure solution using in situ single-crystal X-ray diffraction in a diamond anvil...

At high pressures, autoionization - along with polymerization and metallization - is one of the responses of simple molecular systems to a rise in electron density. Nitro-sonium 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 nitro-gen-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 nitro-sonium 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 ^- 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.