This article is about a molecule of one nitrogen atom and one oxygen atom. For other chemical combinations of nitrogen and oxygen, see nitrogen oxide. For the use of nitric oxide as a medication or in biology, see Biological functions of nitric oxide.
Nitric oxide
Skeletal formula showing two lone pairs and one three-electron bond
The ground state electronic configuration of NO is, in united atom notation:[10]
The first two orbitals are actually pure atomic 1sO and 1sN 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=3⁄2orJ=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 dimerizestodinitrogen 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:
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.
The uncatalyzed endothermic reaction of oxygen (O2) and nitrogen (N2), which is effected at high temperature (>2000 °C) by lightning has not been developed into a practical commercial synthesis (see Birkeland–Eyde process):
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.
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]
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]
Nitric oxide, an endothelium-derived relaxing factor (EDRF), is biosynthesized endogenously from L-arginine, oxygen, and NADPH by various nitric oxide synthase (NOS) enzymes.[25] Reduction of inorganic nitrate may also make nitric oxide.[26] One of the main enzymatic targets of nitric oxide is guanylyl cyclase.[27] The binding of nitric oxide to the heme region of the enzyme leads to activation, in the presence of iron.[27] Nitric oxide is highly reactive (having a lifetime of a few seconds), yet diffuses freely across membranes. These attributes make nitric oxide ideal for a transient paracrine (between adjacent cells) and autocrine (within a single cell) signaling molecule.[26] Once nitric oxide is converted to nitrates and nitrites by oxygen and water, cell signaling is deactivated.[27]
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]
^Hou, Y. C.; Janczuk, A.; Wang, P. G. (1999). "Current trends in the development of nitric oxide donors". Current Pharmaceutical Design. 5 (6): 417–441. doi:10.2174/138161280506230110111042. PMID10390607.
^Arulsamy, Navamoney; Bohle, D. Scott (2006). "Synthesis of Diazeniumdiolates from the Reactions of Nitric Oxide with Enolates". J. Org. Chem. 71 (2): 572–581. doi:10.1021/jo051998p. PMID16408967.
^Derosa, Frank; Keefer, Larry K.; Hrabie, Joseph A. (2008). "Nitric Oxide Reacts with Methoxide". The Journal of Organic Chemistry. 73 (3): 1139–1142. doi:10.1021/jo7020423. PMID18184006.
^Fontijn, Arthur; Sabadell, Alberto J.; Ronco, Richard J. (1970). "Homogeneous chemiluminescent measurement of nitric oxide with ozone. Implications for continuous selective monitoring of gaseous air pollutants". Analytical Chemistry. 42 (6): 575–579. doi:10.1021/ac60288a034.
^Nagano, T; Yoshimura, T (2002). "Bioimaging of nitric oxide". Chemical Reviews. 102 (4): 1235–1270. doi:10.1021/cr010152s. PMID11942795.
^Kojima H, Nakatsubo N, Kikuchi K, Kawahara S, Kirino Y, Nagoshi H, Hirata Y, Nagano T (1998). "Detection and imaging of nitric oxide with novel fluorescent indicators: diaminofluoresceins". Anal. Chem. 70 (13): 2446–2453. doi:10.1021/ac9801723. PMID9666719.
^Liu, Hongying; Weng, Lingyan; Yang, Chi (2017-03-28). "A review on nanomaterial-based electrochemical sensors for H2O2, H2S and NO inside cells or released by cells". Microchimica Acta. 184 (5): 1267–1283. doi:10.1007/s00604-017-2179-2. ISSN0026-3672. S2CID21308802.
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