Nitronium Ion

The nitronium ion, or sometimes the nitryl ion (incorrect because it is not a radical), NO+
2, is a generally reactive cation created by the removal of an electron from the paramagnetic nitrogen dioxide molecule, or the protonation of nitric acid.

It is reactive enough to exist in normal conditions, but it is used extensively as an electrophile in the nitration of other substances. The ion is generated in situ for this purpose by mixing concentrated sulfuric acid and concentrated nitric acid according to the equilibrium:

2 H₂SO₄ + HNO₃ → 2 HSO−
4 + NO+
2 + H₃O+

Historically, the nitronium ion was detected by Raman Spectroscopy, since its symmetric stretch is Raman active but Infrared inactive. (The nitronium ion is linear like carbon dioxide and their vibrational spectra are similar). The nitronium ion also exists in the solid form of dinitrogen pentoxide, N₂O₅, which is an ionic solid formed from nitronium and nitrate ions. Its liquid and gaseous forms, however, are molecular and do not contain nitronium ions.

A few stable nitronium salts with anions of weak nucleophilicity can be isolated. These include nitronium perchlorate (NO+
2ClO−
4), nitronium tetrafluoroborate (NO+
2BF−
4), nitronium hexafluorophosphate (NO+
2PF−
6), nitronium hexafluoroarsenate (NO+
2AsF−
6), and nitronium hexafluoroantimonate (NO+
2SbF−
6). These are all very hygroscopic compounds.

The nitronium ion is isoelectronic with carbon dioxide and nitrous oxide, and like those molecules has a linear structure with an ONO bond angle of 180°. For this reason it has a similar vibrational spectrum to carbon dioxide: the Raman active symmetrical stretch was first used to identify the ion in nitrating mixtures.