Nitro-sonium nitrate (NO+NO3 -) 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. 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.