Isomers of N2O3: Observation of trans-cis N2O3 in solid Ar

Nitric Oxide Dioxygenation by O2 Adducts of Manganese Porphyrins

We describe here nitric oxide dioxygenation (NOD) by the dioxygen manganese porphyrin adducts Mn(Por)(η²-O₂) (Por^2- = the meso-tetra-phenyl or meso-tetra-p-tolylporphyrinato dianions, TPP^2- and TTP^2-). The Mn(Por)(η²-O₂) was assembled by adding O₂ to sublimed layers of Mn^II(Por). When NO was introduced and the temperature was slowly raised from 80 to 120 K, new IR bands with correlated intensities grew concomitant with depletion of the υ(O₂) band. Isotope labeling experiments with ¹⁸O₂, ¹⁵NO, and N¹⁸O combined with DFT calculations provide the basis for identifying the initial intermediates as the six-coordinate peroxynitrito complexes (ON)Mn(Por)(η¹-OONO). Further warming to room temperature led to formation of the nitrato complexes Mn(Por)(η¹-ONO₂), thereby demonstrating the ability of these metal centers to promote NOD. However, comparable quantities of the nitrito complexes Mn(Por)(η¹-ONO) are also formed. In contrast, when the analogous reactions were initiated with the weak σ-donor ligand tetrahydrofuran or dimethyl sulfide present in the layers, formation of Mn(Por)(η¹-ONO₂) is strongly favored (∼90%). The latter are formed via a 6-coordinate intermediate (L)Mn(Por)(η¹-ONO₂) (L = THF or DMS) that loses L upon warming. These reaction patterns are compared to those observed previously with analogous iron and cobalt porphyrin complexes.

355 nm Photodissociation of N2O3 Revealed by Velocity-Mapped Ion Imaging

Dinitrogen trioxide is proposed as a precursor to forming nitrous acid, an important source of hydroxyl radicals in the atmosphere. The spectroscopy and properties of N₂O₃ have been studied at high pressures or low temperatures, but there are no reports of its gas-phase photodissociation. This study investigates the 355 nm photodissociation of N₂O₃ in a DC-sliced velocity-mapped ion imaging apparatus using linearly polarized nanosecond pump and probe lasers. The N₂O₃ sample was generated by expanding NO and NO₂ seeded in a He carrier gas. After photodissociation, a high fraction of the available energy ends up in translation of the products. Time-dependent density functional theory calculations confirm the parallel transition dipole assignment if the dissociation occurs from a nonplanar N₂O₃ conformation. The vector correlations are nearly at the physical limits for a system where μ||v⊥J. The DC-sliced velocity-mapped ion imaging technique is well-suited to investigate N₂O₃ photodissociation since it resolves product speeds and differentiates among the sources of NO⁺ in an expansion containing NO, NO₂, HONO, and N₂O₃.

Heterogeneous Reactions between Toluene and NO2 on Mineral Particles under Simulated Atmospheric Conditions

Heterogeneous reactions between organic and inorganic gases with aerosols are important for the study of smog occurrence and development. In this study, heterogeneous reactions between toluene and NO₂ with three atmospheric mineral particles in the presence or absence of UV light were investigated. The three mineral particles were SiO₂, α-Fe₂O₃, and BS (butlerite and szmolnokite). In a dark environment, benzaldehyde was produced on α-Fe₂O₃. For BS, nitrotoluene and benzaldehyde were obtained. No aromatic products were produced in the absence of NO₂ in the system. In the presence of UV irradiation, benzaldehyde was detected on the SiO₂ surface. Identical products were produced in the presence and absence of UV light over α-Fe₂O₃ and BS. UV light promoted nitrite to nitrate on mineral particles surface. On the basisi of the X-ray photoelectron spectroscopy (XPS) results, a portion of BS was reduced from Fe³⁺ to Fe²⁺ with the adsorption of toluene or the reaction with toluene and NO₂. Sulfate may play a key role in the generation of nitrotoluene on BS particles. From this research, the heterogeneous reactions between organic and inorganic gases with aerosols that occur during smog events will be better understood.

Six-coordinate nitrito and nitrato complexes of manganese porphyrin

Reaction of small increments of NO2 gas with sublimed amorphous layers of Mn(II)(TPP) (TPP = meso-tetra-phenylporphyrinato dianion) in a vacuum cryostat leads to formation of the 5-coordinate monodentate nitrato complex Mn(III)(TPP)(η(1)-ONO2) (II). This transformation proceeds through the two distinct steps with initial formation of the five coordinate O-nitrito complex Mn(III)(TPP)(η(1)-ONO) (I) as demonstrated by the electronic absorption spectra and by FTIR spectra using differently labeled nitrogen dioxide. A plausible mechanism for the second stage of reaction is offered based on the spectral changes observed upon subsequent interaction of (15)NO2 and NO2 with the layered Mn(TPP). Low-temperature interaction of I and II with the vapors of various ligands L (L = O-, S-, and N-donors) leads to formation of the 6-coordinate O-nitrito Mn(III)(TPP)(L)(η(1)-ONO) and monodentate nitrato Mn(III)(TPP)(L)(η(1)-ONO2) complexes, respectively. Formation of the 6-coordinate O-nitrito complex is accompanied by the shifts of the ν(N═O) band to lower frequency and of the ν(N-O) band to higher frequency. The frequency difference between these bands Δν = ν(N═O) - ν(N-O) is a function of L and is smaller for the stronger bases. Reaction of excess NH3 with I leads to formation of Mn(TPP)(NH3)(η(1)-ONO) and of the cation [Mn(TPP)(NH3)2](+) plus ionic nitrite. The nitrito complexes are relatively unstable, but several of the nitrato species can be observed in the solid state at room temperature. For example, the tetrahydrofuran complex Mn(TPP)(THF)(η(1)-ONO2) is stable in the presence of THF vapors (∼5 mm), but it loses this ligand upon high vacuum pumping at RT. When L = dimethylsulfide (DMS), the nitrato complex is stable only to ∼-30 °C. Reactions of II with the N-donor ligands NH3, pyridine, or 1-methylimidazole are more complex. With these ligands, the nitrato complexes Mn(III)(TPP)(L)(η(1)-ONO2) and the cationic complexes [Mn(TPP)(L)2](+) coexist in the layer at room temperature, the latter formed as a result of NO3(-) displacement when L is in excess.

Nitric oxide interaction with oxy-coboglobin models containing trans-pyridine ligand: two reaction pathways

The oxy-cobolglobin models of the general formula (Py)Co(Por)(O2) (Por = meso-tetraphenyl- and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low-temperature interaction of Py and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Similar to the recently studied (NH3)Co(Por)(O2) system (Kurtikyan et al. J. Am. Chem. Soc., 2012, 134, 13671-13680), this interaction leads to the nitric oxide dioxygenation reaction with the formation of thermally unstable nitrato complexes (Py)Co(Por)(η(1)-ONO2). The reaction proceeds through the formation of the six-coordinate peroxynitrite adducts (Py)Co(Por)(OONO), as was demonstrated by FTIR measurements with the use of isotopically labeled (18)O2, (15)NO, N(18)O, and (15)N(18)O species and DFT calculations. In contrast to the ammonia system, however, the binding of dioxygen in (Py)Co(Por)(O2) is weaker and the second reaction pathway takes place due to autoxidation of NO by rebound O2 that in NO excess gives N2O3 and N2O4 species adsorbed in the layer. This leads eventually to partial formation of (Py)Co(Por)(NO) and (Py)Co(Por)(NO2) as a result of NO and NO2 reactions with five-coordinate Co(Por)(Py) complexes that are present in the layer after the O2 has been released. The former is thermally unstable and at room temperature passes to the five-coordinate nitrosyl complex, while the latter is a stable compound. In these experiments at 210 K, the layer consists mostly of six-coordinate nitrato complexes and some minor quantities of six-coordinate nitro and nitrosyl species. Their relative quantities depend on the experimental conditions, and the yield of nitrato species is proportional to the relative quantity of peroxynitrite intermediate. Using differently labeled nitrogen oxide isotopomers in different stages of the process the formation of the caged radical pair after homolytic disruption of the O-O bond in peroxynitrite moiety is clearly shown. The composition of the layers upon farther warming to room temperature depends on the experimental conditions. In vacuo the six-coordinate nitrato complexes decompose to give nitrate anion and oxidized cationic complex Co(III)(Por)(Py)2. In the presence of NO excess, however, the nitro-pyridine complexes (Py)Co(Por)(NO2) are predominantly formed formally indicating the oxo-transfer reactivity of (Py)Co(Por)(η(1)-ONO2) with regard to NO. Using differently labeled nitrogen in nitric oxide and coordinated nitrate a plausible mechanism of this reaction is suggested based on the isotope distribution in the nitro complexes formed.

Nitric oxide dioxygenation reaction by oxy-coboglobin models: in-situ low-temperature FTIR characterization of coordinated peroxynitrite

The oxy-cobolglobin models of the general formula (NH(3))Co(Por)(O(2)) (Por = meso-tetra-phenyl and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low temperature interaction of NH(3) and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Upon warming the layers from 80 to 120 K a set of new IR bands grows with correlating intensities along with the consumption of the ν(O(2)) band. Isotope labeling experiments with (18)O(2), (15)NO and N(18)O along with DFT calculations provides a basis for assigning them to the six-coordinate peroxynitrite complexes (NH(3))Co(Por)(OONO). Over the course of warming the layers from 140 to 170 K these complexes decompose and there are spectral features suggesting the formation of nitrogen dioxide NO(2). Upon keeping the layers at 180-210 K the bands of NO(2) gradually decrease in intensity and the set of new bands grows in the range of 1480, 1270, and 980 cm(-1). These bands have their isotopic counterparts when (15)NO, (18)O(2) and N(18)O are used in the experiments and certainly belong to the 6-coordinate nitrato complexes (NH(3))Co(Por)(η(1)-ONO(2)) demonstrating the ability of oxy coboglobin models to promote the nitric oxide dioxygenation (NOD) reaction similar to oxy-hemes. As in the case of Hb, Mb and model iron-porphyrins, the six-coordinate nitrato complexes are not stable at room temperature and dissociate to give nitrate anion and oxidized cationic complex Co(III)(Por)(NH(3))(1,2).

Isomers of GeNO and Ge(NO)2: Production and infrared absorption of GeNO and ONGeNO in solid Ar

Crystalline germanium was ablated with light at 532 nm from a frequency-doubled neodymium: yttrium aluminum garnet laser, and the resultant plume reacted with NO before deposition onto a substrate at 13 K. Lines in group A at 1543.8 and 3059.7 cm(-1) that become enhanced at the initial stage of irradiation at 308 or 193 nm and also after annealing are attributed to nu1 and 2nu1 of GeNO. Lines in group B at 1645.5 and 1482.8 cm(-1) that become diminished after further irradiation of the matrix at 308 or 193 nm but become enhanced after annealing are attributed to symmetric NO stretch (nu1) and antisymmetric NO stretch (nu7) of ONGeNO. The assignments were derived based on wave numbers and isotopic ratios observed in the experiments with 15N- and 18O-isotopic substitutions and predicted with quantum-chemical calculations. Quantum-chemical calculations with density-functional theories (B3LYP and BLYP/aug-cc-pVTZ) predict four stable isomers of GeNO, six isomers of Ge2NO, and four isomers of Ge(NO)2, with linear GeNO, cyc-GeNGeO, and cyc-GeONNO having the least energies, respectively. The formation mechanisms of GeNO and ONGeNO are discussed. In addition, a weak line at 1417.0 cm(-1) and two additional lines associated with minor matrix sites at 1423.0 and 1420.3 cm(-1) are assigned to GeNO-.

Isomers of HSCO: IR absorption spectra of t-HSCO in solid Ar

Irradiation of an Ar matrix sample containing H2S and CO (or OCS) with an ArF excimer laser at 193 nm yields trans-HSCO (denoted t-HSCO). New lines at 1823.3, 931.6, and 553.3 cm(-1) appear after photolysis and their intensity enhances after annealing; secondary photolysis at 248 nm diminishes these lines and produces OCS and CO. These lines are assigned to C-O stretching, HSC-bending, and C-S stretching modes of t-HSCO, respectively, based on results of 13C-isotopic experiments and theoretical calculations. Theoretical calculations using density-functional theories (B3LYP and PW91PW91) predict four stable isomers of HSCO: t-HSCO, c-HSCO, HC(O)S, and c-HOCS, listed in increasing order of energy. According to calculations with B3LYP/aug-cc-pVTZ, t-HSCO is planar, with bond lengths of 1.34 A (H-S), 1.81 A (S-C), and 1.17 A (C-O), and angles angle HSC congruent with 93.4 degrees and angle SCO congruent with 128.3 degrees; it is more stable than c-HSCO and HC(O)S by approximately 9 kJ mol(-1) and more stable than c-HOCS by approximately 65 kJ mol(-1). Calculated vibrational wave numbers, IR intensities, and 13C-isotopic shifts for t-HSCO fit satisfactorily with experimental results. This new spectral identification of t-HSCO provides information for future investigations of its roles in atmospheric chemistry.

Is N2O3 the main nitrosating intermediate in aerated nitric oxide (NO) solutions in vivo? If so, where, when, and which one?

The widespread opinion that N(2)O(3) as a product of NO oxidation is the only nitros(yl)ating agent under aerobic conditions is based on experiments in homogeneous buffered water solutions. In vivo NO is oxidized in heterogeneous media and this opinion is not correct. The equilibrium in the system being dependent on temperature and DeltaG((sol)) for NO, NO(2), isomers of both N(2)O(3), and N(2)O(4). For polar solvents including water, DeltaG((sol)) for N(2)O(3) is high enough, and a stationary concentration of N(2)O(3) in the mixture with other oxides is sufficient to guarantee the hydrolysis of N(2)O(3) to nitrite. In heterogeneous media, the mixture contains solvates NO(2(sol)), N(2)O(3(sol)), and N(2)O(4(sol)) at stationary nonequilibrium concentrations. As far as DeltaG((sol)) is decreased in heterogeneous mixtures with low polar solvents and/or at increased temperatures, the equilibrium in such a system shifts to NO(2). Although NO(2) is a reactive free radical, it almost does not react with water. In contrast, the reaction with most functional protein groups efficiently proceeds by a radical type with the formation of nitrite and new radicals (X) further stabilized in various forms. Therefore, the ratio of the nitrosylated and nitrated products yields depends on actual concentrations of all NO(x).