Hyponitrous Acid Research Articles

First real-time isotopic characterisation of N2O from chemodenitrification

Chemodenitrification can be a substantial abiotic source of nitrous oxide (N2O) in soil. The isotopic signature of N2O from this process could support source partitioning, but it is currently unknown in sufficient detail. In this study, we determined the isotopic composition of N2O, produced by the reaction of nitrite (NO2−) with lignin, four lignin derivatives, and three types of soils, online with a quantum cascade laser absorption spectrometer (QCLAS). We present the first dataset of continuous measurements of δ15Nbulk (δ15Nbulk ≡ (δ15Nα + δ15Nβ)/2), δ18O, and site preference (SPN2O, SPN2O ≡ δ15Nα − δ15Nβ) of N2O from chemodenitrification in both chemical assays and soils. Considerable amounts of N2O were produced by chemical reduction of NO2−, indicating that chemodenitrification could dominate N2O emission in NO2−-rich environments. The values of SPN2O varied by more than 20‰ in the reactions of sodium nitrite with organic substances. Contrary to the common assumption that SPN2O values are constant for a distinct N2O source process, our results reveal a considerable shift in SPN2O over time for most experiments. The large SPN2O variability might be explained by the multiple pathways with hyponitrous acid or nitramide as N2O precursors. These findings provide important new information to improve our understanding about the dependency of N2O isotopic signatures on N2O production processes.

Arylglyoxal oximes as putative C-nucleophiles in eliminative nucleophilic substitution process

Reaction of arylglyoxal oximes ArCOCH=NOH (Ar = 4-MeOC6H4, Ph) with 5-arylfurazanopyrazines proceeds as vicarious nucleophilic substitution of hydrogen in pyrazine ring with the elimination of hyponitrous acid, affording 5-(aroylmethylidene)-6-aryl-4H-furazano[3,4-b]pyrazines. Structure of the product was confirmed by X-ray diffraction.

Computational study on the catalytic cycle for reduction of NO to N2 catalyzed by a ruthenium–substituted Keggin-type polyoxometalate

Reaction mechanism corresponding to reduction of NO to N2 by hydroxylammonium chloride (NH2OH⋅HCl) catalyzed by a ruthenium–substituted Keggin-type polyoxometalates (POMs) has been studied by using density functional theory (DFT) method with the M06L functional. A complete catalytic cycle including adsorption of reactant, activation of the hydroxylammonium chloride, formation of hyponitrous acid intermediates ((HNO)2), removal of water molecules to generate N2, has been proposed based on our mechanism study. The key intermediate (HNO)2 toward N2 is identified. The Ru center in this POM complex is proposed to be the active site, responsible not only for the formation of (HNO)2 by the reaction of NO and NH2OH⋅HCl but also for the subsequent two dehydration steps to form N2, which is well in agreement with available experimental literature. The calculated whole energy profiles indicate that the formation of dinitrogen is controlled by thermodynamic factor. Detailed electronic structure analysis indicates that the coordinated NO is a good electrophile and responsible for the catalytic activity because the Ru → NO+ back donating bonding interaction. These results provide a valuable insight to development of the redox catalyst of NO into inorganic POM field.

Nitric oxide coupling to generate N2O promoted by a single-heme system as examined by density functional theory

Bacteria utilize a heme/non-heme enzyme system to detoxify nitric oxide (NO) to N2O. In order to probe the capacity of a single-heme system to mediate this NO-to-N2O transformation, various scenarios for addition of electrons, protons, and a second NO molecule to a heme nitrosyl to generate N2O were explored by density functional theory calculations. We describe, utilizing this single-heme system, several stepwise intermediates along pathways that enable the critical N–N bond formation step yielding the desired Fe–N2O product. We also report a hitherto unreported directional second protonation that results in either productive N2O formation with loss of water, or formation of a non-productive hyponitrous acid adduct Fe{HONNOH}.

Reduktive Dimerisierung von NO an Dirutheniumkomplexen und Bildung eines tridentaten 2, 2‐Bis(diphenylphosphanyl)ethanolato‐Liganden

AbstractThe reaction of the trans‐hyponitrito complex [Ru2(CO)4(μ‐η2‐ONNO)(μ‐H)(μ‐PtBu2)(μ‐dppen)] (1, dppen = Ph2PC(=CH2)PPh2) with tetrafluorido boric acid afforded the new complex salt [Ru2(CO)4(μ‐η2‐ONNOH)(μ‐H)(μ‐PtBu2)(μ‐dppen)]BF4 (2) containing the monoprotonate hyponitrous acid as the ligand in the cationic complex. Complex 1 showed a nucleophilic reactivity towards the trimethyloxonium cation resulting in the monoester derivative of the hyponitrous acid [Ru2(CO)4(μ‐η2‐ONNOMe)(μ‐H)(μ‐PtBu2)(μ‐dppen)]BF4 (3). During heating of compound 2 in ethanol under reflux for a short time nitrous oxide was liberated affording unexpectedly a new tridentate 2, 2‐bis(diphenylphosphanyl)ethanolato ligand formed by an intramolecular attack of an intermediate hydroxido ligand towards the unsaturated carbon carbon double bond in the bridging dppen ligand. Thus the complex salt [Ru2(CO)4{μ‐η3‐OCH2CH(PPh2)2}(μ‐H)(μ‐PtBu2)]BF4 (4) was formed in good yields. The new compounds 2, 3, and 4 were characterized by spectroscopic means as well as their molecular structures were determined in the crystal.

ChemInform Abstract: Kinetics and Mechanism of the Oxidation of Hyponitrous Acid with Chromium(VI) in Acid Perchlorate Solutions.

ChemInform Abstract: Kinetics and Mechanism of the Oxidation of Hyponitrous Acid with Chromium(VI) in Acid Perchlorate Solutions.

Hyponitrite complexes: Crystal and molecular structure of [Ru2(CO)4(μ-PtBu2)(μ-Ph2PCH2PPh2)(μ-η2-ONNOMe)

Abstract The new trans-hyponitrite derivative complex [Ru2(CO)4(μ-PtBu2)(μ-dppm)(μ-η2-ONNOMe)] (2, dppm = Ph2PCH2PPh2) was prepared by deprotonation of [Ru2(CO)4(μ-H)(μ-PtBu2)(μ-dppm)(μ-η2-ONNOMe)][BF4] (1) with the base DBU (1.8-diazabicyclo[5.4.0]undec-7-ene). The latter complex salt has been obtained in an improved synthesis starting from the trans-hyponitrite complex [Ru2(CO)4(μ-H)(μ-PtBu2)(μ-dppm)(μ-η2-ONNO)]. Compound 2 has been characterized by spectroscopic methods as well as by X-ray diffraction and represents the first neutral complex bearing a deprotonated monoester of the hyponitrous acid as the bridging ligand.

The shy Angeli and his elusive creature: the HNO route to vasodilation

what we now call Angeli's salt (AS) was discovered more than a century ago by the Italian chemist Angelo Angeli (Fig. 1) . He found that “salts of nitroxylaminic acid are readily resolved into the corresponding nitrites and the unsaturated residue nitroxyl” (HNO), proposing their use as suppliers of HNO (3). However, by analogy with the genial but very reclusive personality of the discoverer (17), the physiological properties of HNO didn't draw too much attention until early 1990s. In their seminal work, Fukuto and colleagues (19) reported that HNO elicits vasorelaxation in both the rabbit aorta and bovine intrapulmonary artery by a soluble guanylate cyclase (sGC)- dependent pathway. Later, it was evident that the thiol-donating agent l-cysteine can discriminate the vasodilatative profile of HNO from that of nitric oxide (NO·) or nitrosothiols: AS/HNO action can be blocked by this agent, whereas the NO· effect is potentiated (36). In combination with the suggestion that NO· is the only species able to directly activate sGC (9), these data led to the conclusion that HNO-induced vasorelaxation is due to its intracellular conversion to NO· with subsequent activation of sGC. For several years, this idea has relegated HNO to a mere precursor of the presence and biological activity of NO·, de facto negating to HNO its own right to exist.

A Stable Hyponitrite-Bridged Iron Porphyrin Complex

The coupling of two nitric oxide (NO) molecules in heme active sites is an important contributor to the conversion of NO to nitrous oxide (N(2)O) by heme-containing enzymes. Several formulations for the presumed heme-Fe{N(2)O(2)}(n-) intermediates have been proposed previously, however, no crystal structures of heme-Fe{N(2)O(2)}(n-) systems have been reported to date. We report the first isolation and characterization of a stable bimetallic hyponitrite iron porphyrin, [(OEP)Fe](2)(mu-N(2)O(2)), prepared from the reaction of [(OEP)Fe](2)(mu-O) with hyponitrous acid. Density functional theoretical calculations were performed on the model compound [(porphine)Fe](2)(mu-N(2)O(2)) to characterize its electronic structure and properties.

Variation in stoichiometry and mechanism of some redox systems

Synopsis 1. Introduction 2. Reduction of (S 2 O 2- 8 + Ag 1 ) system in acid solutions. Competence of intermediate SO - 4 to react with H 2 O/substrate/Ag I leading to a variation in the stoichiometry and mechanism. (a) Oxidation of (b) Oxidation of Ce' (c) Oxidation of hyponitrous acid (H 2 N 2 O 2 ) 3. Reactions of hydrazine in acid solutions. Competence of hydrazyl radical (N 2 H 3 ) to get dimerized and decomposed/oxidised leading to a variation in the stoichiometry and mechanism. (a) Oxidation by Ce (b) Oxidation by TI III 4. Reactions of hydroxylamine. Ratio of the initial reactant-concentrations deciding the stoichiometry and mechanism. Intermediates sensitive to light. (a) Oxidation by peroxomonophosphoric acid (H 3 PO 5 ) in acid solutions (b) Oxidation by TI III in acid solutions (c) Oxidation by Fe III in buffered solutions 5. Reactions of hyponitrous acid. Competence of intermediatee oxyhyponitrous acid (H2N 2 O 3 ) and HNO 2 to decompose/get oxidised leading to a variation in the stoichiometry and mechanism. (a) Oxidation by TI III in acid solutions (b) Oxidation by Ce in acid solutions (c) Oxidation by iodine in buffered solutions (d) Oxidation by Fe(CN) 3- 6 in alkaline solutions 6. Complicated analysis in a simple system of TI III -Sb III in acid perchlorate solutions for 1 : 1 stoichiometry.

A theoretical study on nitric oxide reductase activity in a ba3-type heme-copper oxidase

The mechanism of nitric oxide reduction in a ba 3-type heme-copper oxidase has been investigated using density functional theory (B3LYP). Four possible mechanisms have been studied and free energy surfaces for the whole catalytic cycle including proton and electron transfers have been constructed by comparison to experimental data. The first nitric oxide coordinates to heme a 3 and is partly reduced having some nitroxyl anion character ( 3NO −), and it is thus activated toward the attack by the second NO. In this reaction step a cyclic hyponitrous acid anhydride intermediate with the two oxygens coordinating to Cu B is formed. The cyclic hyponitrous acid anhydride is quite stable in a local minimum with high barriers for both the backward and forward reactions and should thus be observable experimentally. To break the N O bond and form nitrous oxide, the hyponitrous acid anhydride must be protonated, the latter appearing to be an endergonic process. The endergonicity of the proton transfer makes the barrier of breaking the N O bond directly after the protonation too high. It is suggested that an electron should enter the catalytic cycle at this stage in order to break the N O bond and form N 2O at a feasible rate. The cleavage of the N O bond is the rate limiting step in the reaction mechanism and it has a barrier of 17.3 kcal/mol, close to the experimental value of 19.5 kcal/mol. The overall exergonicity is fitted to experimental data and is 45.6 kcal/mol.

The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives

High level ab initio and density functional theory calculations have been carried out to investigate the relative stability of the different conformers of hyponitrous acid and its mono- and dithio-derivatives. Geometries and vibrational frequencies were obtained at the B3LYP/6-311+G(d,p) level and final energies through B3LYP/6-311++G(3df,2pd) single point calculations. The reliability of this theoretical scheme has been assessed by comparing these DFT results with those obtained at the G3 level of theory, for some suitable cases. The cis conformers of hyponitrous acid and its mono- and dithio-derivatives are systematically more stable than the trans ones because in the cis conformation a dative interaction between the nitrogen-lone pairs and the σNX^* antibonding orbital is significantly favored. Quite interestingly, in general, the conformers presenting an intramolecular hydrogen bond (IHB) are not the global minima of the corresponding potential energy surfaces and only for hyponitrous acid the conformer with a OH ⋅s O IHB is slightly more stable than the cis conformer without IHB. The low stability of the tautomers with IHB is closely related with another weak intramolecular interaction which involves the lone-pairs of the chalcogen atoms and the πNN* antibondig orbital, and which is significantly perturbed when the IHB is formed.

ChemInform Abstract: Transition States and Mechanisms of the Hetero‐Diels—Alder Reactions of Hyponitrous Acid, Nitrosoalkanes, Nitrosoarenes, and Nitrosocarbonyl Compounds.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Transition states and mechanisms of the hetero-Diels-Alder reactions of hyponitrous acid, nitrosoalkanes, nitrosoarenes, and nitrosocarbonyl compounds.

The transition states and energetics of Diels-Alder reactions for a variety of nitroso compounds with dienes were explored with density functional theory using the B3LYP functional and 6-31G basis set. The reactions involve HNO, various nitrosoalkanes and arenes (RNO and ArNO), and nitrosoformaldehyde (CHONO) as dienophiles with butadiene and a series of 1- and 2-substituted dienes. The mechanisms, activation energies, energies of reaction, stereoselectivities, and regioselectivities are predicted for these reactions. These predictions are compared to available experimental data. The mechanism is found to be concerted but involves highly asynchronous transition states. Although it is not evident in the products, we find that the endo path is very strongly favored over the exo alternative due to repulsion between the diene and nitrogen's lone pair. A range of experimental regioselectivities are reproduced by calculations and are found to hinge on a very sensitive balance between FMO interactions, electrostatics, and steric effects. A series of generalizations for predicting regioselectivity for untried diene-dienophile pairs are made.

Correlation of the Product E/Z Framework Geometry and O/O vs O/N Regioselectivity in the Dialkylation of Hyponitrite

The products from the alkylation of silver hyponitrite with tert-butyl bromide, tert-amyl bromide, p-tert-butyl benzylbromide, and chlorotriethylsilane have been determined. In the reaction of tert-butyl bromide the formation of three new products, namely, (Z)-N-tert-butyl-N‘-tert-butoxydiazene-N-oxide {(CH3)3CN(O)NOC(CH3)3}, 1D, trans-mono-O-tert-butylhyponitrous acid {(CH3)3CONNOH}, 2A, and (Z)-N-tert-butyl-N‘-hydroxydiazene-N-oxide {(CH3)3CN(O)NNOH}, 2D, is observed together with the formation of the known trans-di-O-tert-butyl hyponitrite {(CH3)3CONNOC(CH3)3}, 1A. The reaction with tert-amyl bromide also yielded the corresponding tert-amyl derivatives. However, from the reactions of p-tert-butylbenzyl bromide and chlorotriethylsilane with silver hyponitrite, the corresponding trans-di-O-tert-alkylhyponitrites were obtained as the only alkylated products. The differential reactivity of the two sets of halides with silver hyponitrite is explained in terms of the higher stability of the cations generated from the former halides in comparison to those generated from the latter halides. The thermal decomposition data and UV−visible spectroscopic data are reported for all of the products. Crystallographic structural data are reported for two of the products, namely, (Z)-N-tert-butyl-N‘-tert-butoxydiazene-N-oxide, 1D, as its hyponitrous acid adduct, (CH3)3CN(O)NOC(CH3)3·1/2HONNOH, and trans-di-O-p-tert-butylbenzyl hyponitrite, {p(CH3)3C}C6H4CH2ONNOCH2C6H4-p-{C(CH3)3}. Theoretical calculations using ab initio density functional theory (B3LYP) are reported for the hyponitrite dianion, trans-mono-O-methylhyponitrite anion, (Z)-N-methyl-N‘-hydroxylatodiazene-N-oxide anion, and for the methylation transition states. On the basis of the products isolated from the reactions of tert-butyl bromide and tert-amyl bromide and the theoretical calculations, a pathway is proposed to account for the observed product distributions where O/O-dialkylation is found in the trans(E) products and O/N-dialkylation is found in the cis(Z) products.

Reductions by hyponitrite †

Hyponitrites, salts of the weak acid HO–NN–OH, hyponitrous acid, react, in aqueous solution (pH 4–8), with an array of inorganic oxidants. With the high potential reagents Br2 and [Fe(bipy)3]3+ in excess, conversion is mainly to nitrate, whereas with I2 and [IrCl6]2−, the product is nitrite. Kinetic acidity patterns indicate that nearly all oxidations proceed largely through the conjugate base, HN2O2−. With I2, I3−, and Br2 there is also evidence also for a doubly deprotonated route, possibly involving a halogen–N2O22− complex. The 1e− metal oxidants [Fe(bipy)3]3+ and [IrCl6]2− probably operate through a series of outer-sphere 1e− steps in which the initial transfer is rate-determining, whereas with I2 and Br2, reaction via a halogen bridge, resulting in net transfer of X+ (a 2e− change) is more likely. None of the reaction profiles exhibits irregularities attributable to the intervention of an intermediate formed or destroyed on a time scale commensurate with the initial reaction.

Synthesis and Characterization of Alkylammonium Hyponitrites and Base-Stabilized Hyponitrous Acid Salts

A number of alkylammonium and bipyridinium salts of hyponitrous acid, namely, N,N,N‘,N‘-tetraethylethylenediammonium (3 and 4), N,N,N‘,N‘-tetramethylethylenediammonium (5), triethylenediammonium (6), diquinuclidinium (7), 2,2‘-bipyridinium (8), 4,4‘-bipyridinium (9), 4,4‘-trimethylenebis(1-methylpiperidinium) (10), 4,4‘-trimethylenepiperidinium (11), and bis(triethylammonium) (12) hyponitrite salts, have been synthesized by the reaction of the corresponding amine in either anhydrous diethyl ether or absolute ethanol with hyponitrous acid solution in anhydrous diethyl ether. Single-crystal X-ray crystallographic data were obtained for sodium hyponitrite (1), and the thermal decomposition behavior of the salt was examined. The new salts were characterized by IR and Raman spectroscopic data and elemental analyses. Compounds 4 and 7−9 were also characterized by single-crystal X-ray crystallography. The hyponitrite anions in 1 and 7−9 exhibit similar structural features. The anions are planar with average NN and N−O bond distances of 1.237 and 1.380 Å, respectively. The monoprotonated hyponitrite anion and solvated hyponitrous acid molecules present in 4 also exhibit close structural similarity to the dianion in the crystals of 1 and 7−9. Most of these salts are soluble in organic solvents. The UV−vis spectra of the sodium and alkylammonium salts (1, 3−7, and 10−12) in aqueous 0.1 M NaOH exhibit an absorption peak at 248 nm (λmax) with a molar extinction coefficient of 7033 ± 153 M-1 cm-1. Differential scanning calorimetric data for the salts reveal exothermic decomposition of the hyponitrite species. Significantly, the pentahydrate and anhydrous forms of Na2N2O2 exhibit distinct thermal decomposition behavior. The thermogram of the pentahydrate exhibits two exotherms at ca. 96 and 382 °C, whereas that of the anhydrous Na2N2O2 exhibits a single exotherm at ca. 382 °C. The exotherm at 96 °C observed for the pentahydrate is explained in terms of a coupled phase transition−dehydration process. The alkylammonium and bipyridinium salts undergo exothermic decomposition at a considerably lower temperature in the range 67−170 °C. Electrochemistry of the salts in acetonitrile solvent reveal an irreversible oxidation at ∼1.80 V vs an Ag/AgCl reference electrode.

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

We investigated the heterogeneous processes that contribute towards the for- mation of N2O in an environment that comes as closely as possible to exhaust conditions containing NO and SO2 among other constituents. The simultaneous presence of NO, SO2, O2, and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N2O. The maximum rate of N2O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO2 or HONO significantly in- creases the rate constant for N2O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N2O formation for the three different nitrogen reactants reveal the following series of increasing reactivity: indicating the probable se- NO , NO , HONO, 2 quential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N2O are largest at of a H2SO4/H2O solution. The pH 5 0 results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N2O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O2 to NO2 mainly in the gas phase, with the presence of both H2O and active surfaces significantly accelerating NO2 production. Subsequently, NO2 reacts with excess NO to obtain HONO which reacts with S(IV) to result in N2O and H2SO4 through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid. q 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29: 869- 891, 1997.

ChemInform Abstract: Stoichiometry, Kinetics and Mechanism of the Silver(I) Catalyzed Oxidation of Hyponitrous Acid with Peroxodisulfate Ion in Acid Perchlorate Solutions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Stoichiometry, Kinetics, and Mechanism of the Oxidation of Hyponitrous Acid by Iodine in Acetate Buffers.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.