Summary
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula NO. It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its chemical formula (•N=O or •NO). Nitric oxide is also a heteronuclear diatomic molecule, a class of molecules whose study spawned early modern theories of chemical bonding. An important intermediate in industrial chemistry, nitric oxide forms in combustion systems and can be generated by lightning in thunderstorms. In mammals, including humans, nitric oxide is a signaling molecule in many physiological and pathological processes. It was proclaimed the "Molecule of the Year" in 1992. The 1998 Nobel Prize in Physiology or Medicine was awarded for discovering nitric oxide's role as a cardiovascular signalling molecule. Nitric oxide should not be confused with nitrogen dioxide (NO2), a brown gas and major air pollutant, or with nitrous oxide (N2O), an anesthetic gas. Upon condensing to a liquid, nitric oxide dimerizes to dinitrogen dioxide, but the association is weak and reversible. The N–N distance in crystalline NO is 218 pm, nearly twice the N–O distance. Since the heat of formation of •NO is endothermic, NO can be decomposed to the elements. Catalytic converters in cars exploit this reaction: 2 NO → O2 + N2 When exposed to oxygen, nitric oxide converts into nitrogen dioxide: 2 NO + O2 → 2 NO2 This reaction is thought to occur via the intermediates ONOO• and the red compound ONOONO. In water, nitric oxide reacts with oxygen to form nitrous acid (HNO2). The reaction is thought to proceed via the following stoichiometry: 4 NO + O2 + 2 H2O → 4 HNO2 Nitric oxide reacts with fluorine, chlorine, and bromine to form the nitrosyl halides, such as nitrosyl chloride: 2 NO + Cl2 → 2 NOCl With NO2, also a radical, NO combines to form the intensely blue dinitrogen trioxide: NO + NO2 ON−NO2 The addition of a nitric oxide moiety to another molecule is often referred to as nitrosylation.
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