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The nitronium ion (also called the nitryl ion), 2+ is a generally unstable cation created by the removal of an electron from the paramagnetic nitrogen dioxide molecule, or the protonation of nitric acid. It is not stable 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 sulfuric acid and nitric acid according to the equilibrium: H2SO4 + HNO3 & The nitronium ion also exists in the solid form of dinitrogen pentoxide, 25, 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 nitronium salts with anions of low nucleophilic power, such as nitronium perchlorate, NO2+ClO4− can be isolated but are extremely reactive. The nitronium ion is isoelectronic with carbon dioxide and like that compound has a linear structure with a O = N+ = O bond angle of 180°. It occurs in compounds such as nitryl fluoride, NO2F. As a discrete molecule, it is called the nitryl radical, ·NO2; in fact, this is fairly stable and known as the compound nitrogen dioxide. The related negatively charged species is NO2−, the nitrite ion.
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