Nitric oxide

The ground state electronic configuration of NO is, in united atom notation:[10]

${\displaystyle (1\sigma )^{2}(2\sigma )^{2}(3\sigma )^{2}(4\sigma ^{*})^{2}(5\sigma )^{2}(1\pi )^{4}(2\pi ^{*})^{1}}$

The first two orbitals are actually pure atomic 1s_O and 1s_N from oxygen and nitrogen respectively and therefore are usually not noted in the united atom notation. Orbitals noted with an asterisk are antibonding. The ordering of 5σ and 1π according to their binding energies is subject to discussion. Removal of a 1π electron leads to 6 states whose energies span over a range starting at a lower level than a 5σ electron an extending to a higher level. This is due to the different orbital momentum couplings between a 1π and a 2π electron.

The lone electron in the 2π orbital makes NO a doublet (X ²Π) in its ground state whose degeneracy is split in the fine structure from spin-orbit coupling with a total momentum J=3/2 or J=1/2.

The dipole of NO has been measured experimentally to 0.15740 D and is oriented from O to N (⁻NO⁺) due to the transfer of negative electronic charge from oxygen to nitrogen. [11]

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.[6]

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.[12]

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:[6]

NO + NO₂ ⇌ ON−NO₂

The addition of a nitric oxide moiety to another molecule is often referred to as nitrosylation. The Traube reaction[13] is the addition of a two equivalents of nitric oxide onto an enolate, giving a diazeniumdiolate (also called a nitrosohydroxylamine).[14] The product can undergo a subsequent retro-aldol reaction, giving an overall process similar to the haloform reaction. For example, nitric oxide reacts with acetone and an alkoxide to form a diazeniumdiolate on each α position, with subsequent loss of methyl acetate as a by-product:[15]

This reaction, which was discovered around 1898, remains of interest in nitric oxide prodrug research. Nitric oxide can also react directly with sodium methoxide, ultimately forming sodium formate and nitrous oxide by way of an N-methoxydiazeniumdiolate.[16]

Nitric oxide reacts with transition metals to give complexes called metal nitrosyls. The most common bonding mode of nitric oxide is the terminal linear type (M−NO).[6] Alternatively, nitric oxide can serve as a one-electron pseudohalide. In such complexes, the M−N−O group is characterized by an angle between 120° and 140°. The NO group can also bridge between metal centers through the nitrogen atom in a variety of geometries.

In the laboratory, nitric oxide is conveniently generated by reduction of dilute nitric acid with copper:

8 HNO₃ + 3 Cu → 3 Cu(NO₃)₂ + 4 H₂O + 2 NO

An alternative route involves the reduction of nitrous acid in the form of sodium nitrite or potassium nitrite:

2 NaNO₂ + 2 NaI + 2 H₂SO₄ → I₂ + 2 Na₂SO₄ + 2 H₂O + 2 NO

2 NaNO₂ + 2 FeSO₄ + 3 H₂SO₄ → Fe₂(SO₄)₃ + 2 NaHSO₄ + 2 H₂O + 2 NO

3 KNO₂ + KNO₃ + Cr₂O₃ → 2 K₂CrO₄ + 4 NO

The iron(II) sulfate route is simple and has been used in undergraduate laboratory experiments. So-called NONOate compounds are also used for nitric oxide generation.

Nitric oxide reacts with the hydroperoxyl radical (HO^•
₂) to form nitrogen dioxide (NO₂), which then can react with a hydroxyl radical (^•OH) to produce nitric acid (HNO₃):

^•NO + HO^•
₂ → ^•NO₂ + ^•OH

^•NO₂ + ^•OH → HNO₃

Nitric acid, along with sulfuric acid, contributes to acid rain deposition.

^•NO participates in ozone layer depletion. Nitric oxide reacts with stratospheric ozone to form O₂ and nitrogen dioxide:

^•NO + O₃ → NO₂ + O₂

This reaction is also utilized to measure concentrations of ^•NO in control volumes.

As seen in the acid deposition section, nitric oxide can transform into nitrogen dioxide (this can happen with the hydroperoxy radical, HO^•
₂, or diatomic oxygen, O₂). Symptoms of short-term nitrogen dioxide exposure include nausea, dyspnea and headache. Long-term effects could include impaired immune and respiratory function.[21]