Nitrate radical
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Like nitrogen dioxide NO
2, it is a radical (a molecule with an unpaired valence electron), which makes it paramagnetic. It is the uncharged counterpart of the nitrate anion NO−
3 and an isomer of the peroxynitrite radical OONO.
Nitrogen trioxide is an important intermediate in reactions between atmospheric components, including the destruction of ozone.
History
The existence of the NO
3 radical was postulated in 1881-1882 by Hautefeuille and Chappuis to explain the absorption spectrum of air subjected to a silent electrical discharge.
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Structure and properties
The neutral NO
3 molecule appears to be planar, with three-fold rotational symmetry (symmetry group D3h); or possibly a resonance between three Y-shaped molecules.
The NO
3 radical does not react directly with water, and is relatively unreactive towards closed-shell molecules, as opposed to isolated atoms and other radicals. It is decomposed by light of certain wavelengths into nitric oxide NO and oxygen O
2.
The absorption spectrum of NO
3 has a broad band for light with wavelengths from about 500 to 680 nm, with three salient peaks in the visible at 590, 662, and 623 nm. Absorption in the range 640–680 nm does not lead to dissociation but to fluorescence: specifically, from about 605 to 800 nm following excitation at 604.4 nm, and from about 662 to 800 nm following excitation at 661.8 nm. In water solution, another absorption band appears at about 330 nm (ultraviolet). An excited state NO*
3 can be achieved by photons of wavelength less than 595 nm.
Preparation
Nitrogen trioxide can be prepared in the gas phase by mixing nitrogen dioxide and ozone:
- NO
2 + O
3 → NO
3 + O
2
This reaction can be performed also in the solid phase or water solutions, by irradiating frozen gas mixtures, flash photolysis and radiolysis of nitrate salts and nitric acid, and several other methods.
Nitrogen trioxide is a product of the photolysis of dinitrogen pentoxide N
2O
5, chlorine nitrate ClONO
2, and peroxynitric acid HO
2NO
2 and its salts.
- N2O5 → NO2 + NO3
- 2 ClONO2 → Cl2 + 2 NO3