Nitric oxide

Nitric oxide

Names
IUPAC name Nitrogen monoxide[1]
Systematic IUPAC name Oxidonitrogen(•)[2] (additive)
Other names Nitrogen oxide Nitrogen(II) oxide Oxonitrogen Nitrogen monoxide
Identifiers
CAS Number	10102-43-9 Y
3D model ( JSmol )	Interactive image
3DMet	B00122
ChEBI	CHEBI:16480 Y
ChEMBL	ChEMBL1200689 N
ChemSpider	127983 Y
DrugBank	DB00435 Y
ECHA InfoCard	100.030.233
EC Number	233-271-0
Gmelin Reference	451
IUPHAR/BPS	2509
KEGG	D00074 Y
PubChem CID	145068
RTECS number	QX0525000
UNII	31C4KY9ESH Y
UN number	1660
CompTox Dashboard (EPA)	DTXSID1020938
InChI InChI=1S/NO/c1-2 Y Key: MWUXSHHQAYIFBG-UHFFFAOYSA-N Y InChI=1/NO/c1-2 Key: MWUXSHHQAYIFBG-UHFFFAOYAI
SMILES [N]=O
Properties
Chemical formula	NO
Molar mass	30.006 g·mol−1
Appearance	Colourless gas
Density	1.3402 g/L
Melting point	−164 °C (−263 °F; 109 K)
Boiling point	−152 °C (−242 °F; 121 K)
Solubility in water	0.0098 g / 100 ml (0 °C) 0.0056 g / 100 ml (20 °C)
Refractive index (nD)	1.0002697
Structure
Molecular shape	linear (point group C∞v)
Thermochemistry
Std molar entropy (S⦵298)	210.76 J/(K·mol)
Std enthalpy of formation (ΔfH⦵298)	90.29 kJ/mol
Pharmacology
ATC code	R07AX01 (WHO)
License data	EU EMA: by INN
Routes of administration	Inhalation
Pharmacokinetics:
Bioavailability	good
Metabolism	via pulmonary capillary bed
Biological half-life	2–6 seconds
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Fatal if inhaled Causes severe burns Causes eye damage Corrosive to the respiratory tract [4]
GHS labelling:
Pictograms	[3][4]
Signal word	Danger
Hazard statements	H270, H280, H314, H330[3][4]
Precautionary statements	P220, P244, P260, P280, P303+P361+P353+P315, P304+P340+P315, P305+P351+P338+P315, P370+P376, P403, P405[3][4]
NFPA 704 (fire diamond)	3 0 3 OX
Lethal dose or concentration (LD, LC):
LC50 (median concentration)	315 ppm (rabbit, 15 min) 854 ppm (rat, 4 h) 2500 ppm (mouse, 12 min)[5]
LCLo (lowest published)	320 ppm (mouse)[5]
Safety data sheet (SDS)	External SDS
Related compounds
Related nitrogen oxides	Dinitrogen pentoxide Dinitrogen tetroxide Dinitrogen trioxide Nitrogen dioxide Nitrous oxide Nitroxyl (reduced form) Hydroxylamine (hydrogenated form)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Nitric oxide (nitrogen oxide or nitrogen monoxide

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.

Dipole

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]

Reactions

With di- and triatomic molecules

Upon condensing to a liquid, nitric oxide

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

Organic chemistry

The addition of a nitric oxide

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]

Coordination complexes

Nitric oxide reacts with

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.

Detection and assay

Nitric oxide concentration can be determined using a chemiluminescent reaction involving ozone.^[17] A sample containing nitric oxide is mixed with a large quantity of ozone. The nitric oxide reacts with the ozone to produce oxygen and nitrogen dioxide, accompanied with emission of light (chemiluminescence):

NO + O₃ → NO₂ + O₂ + hν

which can be measured with a photodetector. The amount of light produced is proportional to the amount of nitric oxide in the sample.

Other methods of testing include electroanalysis (amperometric approach), where ·NO reacts with an electrode to induce a current or voltage change. The detection of NO radicals in biological tissues is particularly difficult due to the short lifetime and concentration of these radicals in tissues. One of the few practical methods is spin trapping of nitric oxide with iron-dithiocarbamate complexes and subsequent detection of the mono-nitrosyl-iron complex with electron paramagnetic resonance (EPR).^[18][19]

A group of

Acid rain deposition

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.

Ozone depletion

^•NO participates in

NO is a

The endothelium (inner lining) of blood vessels uses nitric oxide to signal the surrounding smooth muscle to relax, resulting in vasodilation and increasing blood flow.^[26] Sildenafil (Viagra) is a drug that uses the nitric oxide pathway. Sildenafil does not produce nitric oxide, but enhances the signals that are downstream of the nitric oxide pathway by protecting cyclic guanosine monophosphate (cGMP) from degradation by cGMP-specific phosphodiesterase type 5 (PDE5) in the corpus cavernosum, allowing for the signal to be enhanced, and thus vasodilation.^[25] Another endogenous gaseous transmitter, hydrogen sulfide (H₂S) works with NO to induce vasodilation and angiogenesis in a cooperative manner.^[28][29]

Nasal breathing produces nitric oxide within the body, while oral breathing does not.^[30][31]

Occupational safety and health

In the U.S., the

Liquid nitrogen oxide is very sensitive to detonation even in the absence of fuel, and can be initiated as readily as nitroglycerin. Detonation of the endothermic liquid oxide close to its b.p. (-152°C) generated a 100 kbar pulse and fragmented the test equipment. It is the simplest molecule that is capable of detonation in all three phases. The liquid oxide is sensitive and may explode during distillation, and this has been the cause of industrial accidents.[33] Gaseous nitric oxide detonates at about 2300 m/s, but as a solid it can reach a detonation velocity of 6100 m/s.^[34]

References

Notes