 | |
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| Names | |
|---|---|
| IUPAC name Azanone | |
| Systematic IUPAC name Oxidanimine[1] | |
| Other names Hydrogen nitroxide Hydrogen oxonitrate(I) | |
| Identifiers | |
3D model (JSmol) | |
| ChEMBL | |
| ChemSpider |
|
| MeSH | Nitroxyl |
| UNII | |
| |
| |
| Properties | |
| HNO | |
| Molar mass | 31.014 g·mol−1 |
| logP | 0.74 |
| Structure | |
| Digonal | |
| Bent | |
| Thermochemistry | |
| 33.88 J K−1 mol−1 | |
Std molar entropy(S⦵298) | 220.91 J K−1 mol−1 |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Nitroxyl (common name) orazanone (IUPAC name)[2] is the chemical compound HNO. It is well known in the gas phase.[3][4] Nitroxyl can be formed as a short-lived intermediate in the solution phase. The conjugate base, NO−, nitroxide anion, is thereduced form ofnitric oxide (NO) and isisoelectronic withdioxygen. The bond dissociation energy of H−NO is 49.5 kcal/mol (207 kJ/mol), which is unusually weak for a bond to the hydrogen atom.
Nitroxyl is produced from the reagentsAngeli's salt (Na2N2O3) andPiloty's acid (PhSO2NHOH).[5] Other notable studies on the production of HNO exploit cycloadducts of acyl nitroso species, which are known to decompose via hydrolysis to HNO and acyl acid. Uponphotolysis these compounds release the acyl nitroso species which then further decompose.[6]HNO is generated viaorganic oxidation ofcyclohexanone oxime withlead tetraacetate to form 1-nitrosocyclohexyl acetate:[7]
This compound can behydrolyzed underbasic conditions in aphosphate buffer to HNO,acetic acid, andcyclohexanone.
Dichloramine reacts with thehydroxide ion, which is always present in water, to yield nitroxyl and thechloride ion.[8]
Alkali metals react withnitric oxide to give salts of the formMNO (M = metal).[9] However, generation of the (unstable) free acid from these salts is not entirely straightforward (see below).
Nitroxyl is aweak acid, withpKa of about 11, the conjugate base being the triplet state of NO−, sometimes callednitroxide. Nitroxyl itself, however, is a singlet ground state. Thus, deprotonation of nitroxyl uniquely involves the forbidden spin crossing from the singlet state starting material to triplet state product:
Due to thespin-forbidden nature of deprotonation, proton abstraction is many orders of magnitude slower (k =4.9×104 M−1 s−1 for deprotonation by OH−) than what one would expect for a heteroatom proton-transfer process (processes that are so fast that they are sometimesdiffusion-controlled).
TheKa of starting from or ending with the electronic excited states has also been determined. When process of deprotonating singlet state HNO to obtain singlet state NO− has a pKa is about 23. On the other hand, when deprotonating triplet HNO to obtain triplet NO−, the pKa is about −1.8.[10][11]
Nitroxyl rapidly decomposes by a bimolecular pathway tonitrous oxide (k at 298 K =8×106 M s):[10]
The reaction proceeds via dimerization tohyponitrous acid, H2N2O2, which subsequently undergoes dehydration. Therefore, HNO is generally preparedin situ as described above.
Nitroxyl is very reactive towards nucleophiles, includingthiols. The initial adduct rearranges to asulfinamide:[11]
In biological samples, nitroxyl can be detected usingfluorescent sensors, many of which are based on the reduction of copper(II) to copper(I) with concomitant increase in fluorescence.[12]
Nitroxyl donors, known asnitroso compounds, show potential in the treatment of heart failure and ongoing research is focused on finding new molecules for this task.[citation needed]
