Study of the HNO + HNO and HNO + NO reactions by intracavity laser spectroscopy

Flash photolysis of CH₃CHO and H₂CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N₂O + H₂O. The rate constant of this reaction was determined to be k₁ = (1.5 ± 0.8) × 10^?15 cm³/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k₂ = (5 ± 1.5) × 10^?19 cm³/s. NO₂ was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed.     

Theoretical interpretation of the kinetics and mechanisms of the HNO + HNO and HNO + 2NO reactions with a unified model

Previously measured decay rates of HNO in the presence of NO have been kinetically modeled on the basis of thermochemical data calculated with the BAC-MP4 technique. The results of this modeling, aided by TST-RRKM calculations for the association of HNO and the isomerization, decomposition, and stabilization of the many dimers of HNO, reveal that the decay of HNO under NO-lean conditions occurs primarily by association forming cis- and trans-(HNO)₂ at temperatures below 420 K. N₂O, which is a relatively minor product, is believed to be formed by H₂O elimination from cis-HON ? NOH, a product of succesive isomerization reactions: trans-(HNO)₂^? → HN(OH)NO^? → HN(O)NOH^? → cis-HON NOH^?. The calculated rate constants, which fit experimental data quantitatively, can be represented by k = 10^16.2 × T^?2.40e^?590/T cm³/mol sec for the HNO recombination reaction and k = 10^?2.44T^3.98e^?600/T cm³/mol sec for N₂O formation in the temperature range 80–420 K, at a total pressure of 710 torr H₂ or He. Under NO-rich conditions, HNO reacts predominantly by the exothermic termolecular reaction, HNO + 2NO → HN(NO)ONO → HN NO + NO₂, with a rate contant of (6 ± 1) × 10⁹ cm⁶/mol² sec at room temperature, based on both HNO decay and NO₂ production. All existing thermal kinetic data on HNO + HNO and HNO + 2NO processes can be satisfactorily rationalized with a unified model based on the thermochemical data obtained by BAC-MP4 calculations.     

Preparation and reactivity of metal-containing monomers

The kinetics and mechanism of thermal decay of an unsaturated copper carboxylate, Cu(OCOCH=CH₂)₂ (CuAcr₂), have been studied. At 190–240°C the rate of thermal decay can be adequately described by a set of zero and first order rate equations. The initial rate of decay,W ₀, is equal to 1.7·10¹⁷exp[-48500/(RT)] s^–1. The decay products of CuAcr₂ were analyzed by IR and mass spectroscopy as well as by optical microscopy. It has been established that the thermal decay of the monomer under study is accompanied by polymerization, fragmentation, and recombination processes in the solid phase which produce polymeric agglomerates.For part 25, seeIzv.Akad.Nauk, Ser.Khim., 1993, 76 [Russ. Chem. Bull., 1993,42, 66].Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 303–307, February, 1993.     

Isotopic effects on the time-dependences of 420 nm ice luminescence excited by UV light

The kinetics of the 420 nm luminescence emitted from H₂O and D₂O polycrystalline Ih ices have been studied over the 77 to 162 K temperature range. In the case of both H₂O and D₂O ices, it was found that the luminescence rise and decay curves consisted of two luminescence components, and superimposing two first-order curves with different rate constants gave the best fit to the decay and rise curves. The mean lifetimes of the two luminescence components were 1.08 ± 0.03 s and 2.47 ± 0.03 s. The rate constants were found to have negligible temperature dependences, which led to activation energies well below those obtained for either activation-limited processes or even diffusion-limited processes. Furthermore, it was found that the luminescence kinetics were not affected by isotopic substitution of D for H in the ice lattice. These observations suggest that the rate-determining step in the mechanism for the production of the luminescence is a slow (probably spinforbidden) electronic transition that can occur at two different rates due to the presence of two different types of trapping sites in the ice lattice. A possible candidate for the electronic transition is the ⁴Σ → X ²Π transition of excited OH^. radicals and not the previously suggested and ubiquitous A ²Σ⁺ → X ²Π transition of this species. Published in Russian in Kinetika i Kataliz 2006, Vol. 47, No. 5, pp. 709–721. This text was submitted by the authors in English.     

Atmospheric chemistry of FCOx radicals: Kinetic and mechanistic study of the FC(O)O2 + NO2 reaction

A pulse radiolysis system was used to study the kinetics of the reaction of FC(O)O₂ radicals with NO₂. By monitoring the rate of the decay of NO₂ using its absorption at 400 nm the reaction rate constant was determined to be (5.5 ± 0.6) × 10^?12 cm³ molecule^?1 s^?1 at 296 K and 500–1000 mbar pressure of SF₆ diluent. A long path length Fourier transform infrared spectrometer was used to investigate the thermal stability of the product FC(O)O₂NO₂. The rate of thermal decomposition of FC(O)O₂NO₂ was independent of the total pressure of N₂ diluent over the range 100–700 torr and was fit by the expression k_?3 = 6.0 × 10¹⁶ exp(?14150/T) s^?1. The results are discussed in the context of the atmospheric chemistry of FCO_x radicals. © 1995 John Wiley & Sons, Inc.     

15N Chemically Induced Dynamic Nuclear Polarization (15N‐CIDNP) Investigations of the Peroxynitrite Decay and Nitration of N‐Acetyl‐L‐tyrosine

During the decay of (¹⁵N)peroxynitrite (O?¹⁵NOO ^? ) in the presence of N‐acetyl‐L ‐tyrosine (Tyrac) in neutral solution and at 268 K, the ¹⁵N‐NMR signals of ¹⁵NO and ¹⁵NO show emission (E) and enhanced absorption (A) as it has already been observed by Butler and co‐workers in the presence of L ‐tyrosine (Tyr). The effects are built up in radical pairs [CO , ¹⁵NO ]^S formed by O? O bond scission of the (¹⁵N)peroxynitrite? CO₂ adduct (O?¹⁵NO? OCO ). In the absence of Tyrac and Tyr, the peroxynitrite decay rate is enhanced, and ¹⁵N‐CIDNP does not occur. This is explained by a chain reaction during the peroxynitrite decay involving N₂O₃ and radicals NO ^. and NO . The interpretation is supported by ¹⁵N‐CIDNP observed with (¹⁵N)peroxynitrite generated in situ during reaction of H₂O₂ with N‐acetyl‐N‐(¹⁵N)nitroso‐dl ‐tryptophan ((¹⁵N)NANT) at 298 K and pH 7.5. In the presence of Na¹⁵NO₂ at pH 7.5 and in acidic solution, ¹⁵N‐CIDNP appears in the nitration products of Tyrac, 1‐(¹⁵N)nitro‐N‐acetyl‐L ‐tyrosine (1‐¹⁵NO₂‐Tyrac) and 3‐(¹⁵N)nitro‐N‐acetyl‐L ‐tyrosine (3‐¹⁵NO₂‐Tyrac). The effects are built up in radical pairs [Tyrac ^. , ¹⁵NO ]^F formed by encounters of independently generated radicals Tyrac ^. and ¹⁵NO . Quantitative ¹⁵N‐CIDNP studies show that nitrogen dioxide dependent reactions are the main if not the only pathways for yielding both nitrate and nitrated products.     

The possible resurrection of the Chapman mechanism for atmospheric sodium chemiluminescence and ruminations on NaO reaction dynamics

A critical reappraisal and kinetic modeling is presented of a sodium/N₂O system. The analysis provides a compelling argument that previously reported kinetic decay rates may refer not to the reaction of O with NaO but rather to that of O with Na₂O. The system shows pronounced propensity for a significant portion of the NaO to be quantitatively converted to Na₂O on a time scale commensurate with that of the Na/N₂O titration reaction. The O atom in the system does appear to originate from the photolysis of a residual level of NaO. The observed Na(²P) chemiluminescence, used to track the O atom decay rates, can be consistent with the O + NaO reaction as previously surmised. It is unlikely that the alternate O + Na₂ and O + Na₂O chemiluminescent channels can generate the observed intensity levels. This reanalysis, which provides for the observed first order dependences on N₂O(Na₂O) and O atom concentrations has significant implications for the Chapman atmospheric mechanism of the sodium airglow. Its conclusions resurrect the viability of the original scheme which requires efficient branching of the O + NaO reaction to Na(²P). Recent suggestions invoking the participation of NaO(A²Σ⁺) require the latter to have a metastable nature with respect to its radiative and collisional quenching (N₂, O₂) channels, for which there is no current evidence. An additional evaluation of the rate constant measurements for the fast reactions of NaO with NO or CO indicates that these most probably are kinetically complex and involve longlived transition states. Their rate constants are predicted to have small negative activation energies and pressure dependences. In the case of NaO with CO this may explain its low Na(²P) chemiluminescent efficiency. For the NaO + O reaction, a rate constant of about 4 ×10^?11 cm³ molecule^?1s^?1 is predicted at room temperature. This is similar to that used in earlier atmospheric models. Its magnitude circumvents the consequences of the reaction's large entropy decrease which otherwise implies too large a cross-section for the reverse reaction. A smaller value also is more likely to be consistent with a normal short-lived collisional transition state, which will allow for a more significant Na(²P) quantum efficiency. © 1993 John Wiley & Sons, Inc.     

Hydrophobic N‐Diazeniumdiolates and the Aqueous Interface of Sodium Dodecyl Sulfate (SDS) Micelles

Zwitterionic diazeniumdiolates of the form RN[N(O)NO^?](CH₂)₂NH₂⁺R, where R=CH₃ ( 1 ), (CH₂)₃CH₃ ( 2 ), (CH₂)₅CH₃ ( 3 ), and (CH₂)₇CH₃ ( 4 ) were synthesized by reaction of the corresponding diamines with nitric oxide. Spectrophotometrically determined pK_a(O) values, attributed to protonation at the terminal oxygen of the diazeniumdiolate group, show shifts to higher values in dependence of the chain lengths of R. The pH dependence of the decomposition of NO donors 1 – 3 was studied in buffered solution between pH 5 and 8 at 22 °C, from which pK_a(N) values for protonation at the amino nitrogen, leading to release of NO, were estimated. It is shown that the decomposition of these diazeniumdiolates is markedly catalyzed by anionic SDS micelles. First‐order rate constants for the decay of 1 – 4 were determined in phosphate buffer pH 7.4 at 22 °C as a function of SDS concentration. Micellar binding constants, K_SM, for the association of diazeniumdiolates 1 – 3 with the SDS micelles were also determined, again showing a significant increase with increasing length of the alkyl side chains. The decomposition of 1 – 3 in micellar solution is quantitatively described by using the pseudo‐phase ion‐exchange (PIE) model, in which the degree of micellar catalysis is taken into account through the ratio of the second‐order rate constants (k_2m/k_2w) for decay in the micelles and in the bulk aqueous phase. The decay kinetics of 1 – 3 were further studied in the presence of cosolvents and nonionic surfactants, but no effect on the rate of NO release was observed. The kinetic data are discussed in terms of association to the micelle–aqueous phase interface of the negatively charged micelles. The apparent interfacial pH value of SDS micelles was evaluated from comparison of the pH dependence of the first‐order decay rate constants of 2 and 3 in neat buffer and the rate data obtained for the surfactant‐mediated decay. For a bulk phase of pH 7.4, an interfacial pH of 5.7–5.8 was determined, consistent with the distribution of H⁺ in the vicinity of the negatively charged micelles. The data demonstrate the utility of 2 and 3 as probes for the determination of the apparent pH value in the Stern region of anionic micelles.     

Preparation,Crystal Structure,Vibrational Spectra,and Thermal Behavior of Rb2H2P2O6·2H2O

Single crystals of Rb₂H₂P₂O₆ · 2H₂O could be obtained from aqueous solutions of hypodiphosphoric acid and rubidium carbonate. Its crystal structure was determined by X‐ray diffraction and it crystallizes in the monoclinic space group P2₁/c with Z = 4. The salt‐like title compound consists of [H₂P₂O₆]^2– units in staggered P₂O₆‐skeleton conformation, Rb⁺ cations, and H₂O molecules, held together by intermolecular hydrogen bonds of the type O ··· O. The vibrational spectra (IR/FIR and Raman) of the rubidium salt were recorded and an assignment of the vibrational modes is proposed based on the point group C_2h for the P₂O₆‐skeleton of the anion. The thermal behavior of Rb₂H₂P₂O₆ · 2H₂O is dominated by a complex TG decay indicating a simultaneous H₂O delivery coupled with a disproportionation of [H₂P₂O₆]^2–, what is also supported by Raman spectra of heated samples.     

The flash photolysis of di-μ-oxo-bis(oxo-molybdate(V)) acid aqueous solutions in the presence of excess thiocyanate

The flash photolysis of the mono and di-thiocyanate complexes of μ-oxo bis(oxo-molybdenum(V)) (Mo₂O₄(NCS)_x(H₂O)_6?x^(2?x)+ where x = 1, 2) has been studied in the system composed of perchloric acid solutions of Mo₂O₄(H₂O)₆²⁺ in the presence of excess thiocyanate. Irradiation with light in the wavelength range 250–340 nm induced the formation of three reaction intermediates identified as μ-oxo-bis (oxo-molybdenum(V)) species: Mo₂O₃(H₂O)₈⁴⁺ (A), Mo₂O₃(NCS) (H₂O)₇³⁺ (B), and Mo₂O₃(NCS)₂(H₂O)₆²⁺ (C). A rapid equilibria between the transient species is established before they decay following a first order kinetics. Intermediate A is the only species effective in promoting the observed decay leading to the end products Mo(VI) and H₂. The relative absorption spectrum of species A, B, and C are reported, as well as the measured stability constants for the formation of B and C in the temperature range 20–45°C. An overall mechanism is proposed.     

On line radiochemical separation of the168mLu isomer (T1/2=6.7 min) and study of its gamma-ray spectrum

The^168mLu (T _1/2=6.7 min) (EC + β⁺) decay was reinvestigated by direct γ and γ-γ coincidence measurements using a continuous radiochemical separation from its¹⁶⁸Hf parent produced by¹⁵⁶Gd(¹⁶O,4n) reaction. Energies and intensities of 171 γ-transitions assigned to¹⁶⁸Lu decay were accurately measured, among the 97 were found for the first time.     

Conversion of Nitric Oxide into Nitrous Oxide as Triggered by the Polarization of Coordinated NO by Hydrogen Bonding

Reduction of the {Co(NO)}⁸ cobalt–nitrosyl N‐confused porphyrin (NCP) [Co(CTPPMe)(NO)] ( 1 ) produced electron‐rich {Co(NO)}⁹ [Co(CTPPMe)(NO)][Co(Cp*)₂] ( 2 ), which was necessary for NO‐to‐N₂O conversion. Complex 2 was NO‐reduction‐silent in neat THF, but was partially activated to a hydrogen‐bonded species 2 ??? MeOH in THF/MeOH (1:1, v/v). This species coupling with 2 transformed NO into N₂O, which was fragmented from an [N₂O₂]‐bridging intermediate. An intense IR peak at 1622 cm^?1 was ascribed to ν(NO) in an [N₂O₂]‐containing intermediate. Time–course ESI(?) mass spectra supported the presence of the dimeric [Co(NCP)]₂(N₂O₂) intermediate. Five complete NO‐to‐N₂O conversion cycles were possible without significant decay in the amount of N₂O produced.     

Deactivation of O2(A3Σ) by O2, O,and Ar

O₂ in the A³Σ_u⁺ state has been prepared in a discharge flow system by recombining oxygen atoms on a nickel surface. The decay of this excited state was followed by observing the emission between 280 and 400 nm. The wall deactivation was observed to approach unit efficiency. Rate constants were determined to be 0.9 × 10^?11, 2.9 × 10^?13, and 8.6 × 10^?16 cm³/molecule sec for the quenching of O₂(A³Σ_u⁺) by O, O₂, and Ar, respectively.     

Engineering Platinum–Oxygen Dual Catalytic Sites via Charge Transfer towards Highly Efficient Hydrogen Evolution

A dual‐site catalyst allows for a synergetic reaction in the close proximity to enhance catalysis. It is highly desirable to create dual‐site interfaces in single‐atom system to maximize the effect. Herein, we report a cation‐deficient electrostatic anchorage route to fabricate an atomically dispersed platinum–titania catalyst (Pt₁^O1/Ti_1?xO₂), which shows greatly enhanced hydrogen evolution activity, surpassing that of the commercial Pt/C catalyst in mass by a factor of 53.2. Operando techniques and density functional calculations reveal that Pt₁^O1/Ti_1?xO₂ experiences a Pt?O dual‐site catalytic pathway, where the inherent charge transfer within the dual sites encourages the jointly coupling protons and plays the key role during the Volmer–Tafel process. There is almost no decay in the activity of Pt₁^O1/Ti_1?xO₂ over 300 000 cycles, meaning 30 times of enhancement in stability compared to the commercial Pt/C catalysts (10 000 cycles).     

Pyrolytic production of Se (3P) from carbon diselenide. II. Rate of CSe2 decay

Pyrolytic decay of carbon diselenide was monitored by ultraviolet absorption spectroscopy in reflected shock waves in the temperature range of 1600–2600°K. The temperature dependence of the absorption coefficient of CSe₂ at 2308 Å was determined and was used to provide kinetic information along with a deconvolution procedure which accounted for and removed systematic distortions of the fast time-resolved absorbance profile. For temperatures of 1600–2600°K and argon densities of 1.5–7.0 × 10^?5 mol/cm³ dilute (1.0–9.0 × 10^?9 mol/cm³) CSe₂ pyrolyzed with measured first-order decay rates in the range of log₁₀ k₁ (sec^?1) = 3.0?5.7; at midrange (2100°K and 4.3 × 10^?5 mol/cm³ in Ar) k₁ ≈ 3 × 10⁴ sec^?1. The decay probably occurs via a unimolecular low-pressure process, first order in both CSe₂ and Ar, for which k₂ ± 10⁹ cm³/mol·sec at 2100°K. The deconvoluted data yield Arrhenius activation energies of 53.2 kcal/mol under second-order treatment, but the activation energy is less reliable than the general magnitude of the rate constant. A comparison of CSe₂ with other molecules which are isoelectronic in their valence shells (CO₂, CS₂, OCS, and N₂O) is made.     

Engineering Platinum–Oxygen Dual Catalytic Sites via Charge Transfer towards Highly Efficient Hydrogen Evolution

A dual-site catalyst allows for a synergetic reaction in the close proximity to enhance catalysis. It is highly desirable to create dual-site interfaces in single-atom system to maximize the effect. Herein, we report a cation-deficient electrostatic anchorage route to fabricate an atomically dispersed platinum–titania catalyst (Pt₁^O1/Ti_1−xO₂), which shows greatly enhanced hydrogen evolution activity, surpassing that of the commercial Pt/C catalyst in mass by a factor of 53.2. Operando techniques and density functional calculations reveal that Pt₁^O1/Ti_1−xO₂ experiences a Pt−O dual-site catalytic pathway, where the inherent charge transfer within the dual sites encourages the jointly coupling protons and plays the key role during the Volmer–Tafel process. There is almost no decay in the activity of Pt₁^O1/Ti_1−xO₂ over 300 000 cycles, meaning 30 times of enhancement in stability compared to the commercial Pt/C catalysts (10 000 cycles).     

Instrumental analysis of phosphorus in organic material using high energy beta-counting after photoactivation (IPAA)

C, N, O, F and P can be analyzed by instrumental photon activation analysis (IPAA) including decay curve analysis. The interference of ³⁰P (T _1/2 = 149.9 s) by ¹⁵O (T _1/2 = 122.2 s) can be ruled out by direct positron measurement making use of the largely different maximum β⁺-energies of both nuclides (3.24 MeV and 1.73 MeV, respectively). Interference by carbon (¹¹C) can be avoided by sub-threshold activation with 17 MeV bremsstrahlung. The short half-life of ³⁰P allows a high productivity of the method. Reliability was demonstrated in the range of 0.2%–2% P (detection limit = 40 μg/g). Analysis of a certified reference material (BCR-CRM 063) yielded excellent agreement with the certified data.     

Synthesis and Crystal Structures of Alkali and Alkaline Earth Metal Complexes of 3,5-Dinitropyrazolyl-4-carboxylate

The synthesis and crystal structures of 3,5-dinitro-1H-pyrazolyl-4-carboxylic acid (H₂dnpzc) and its four complexes with Ca²⁺, Ba²⁺, Na⁺ and K⁺ are reported in this paper. Ca(dnpzc) · 5H₂O exhibits a 1D polymeric structure, whereas Ba(dnpzc) · 4H₂O possesses a 2D structure. The structure of Na₂(dnpzc) · 4H₂O consists of 2D layers of [Na(dnpzc)]^–_n and 1D chains of [Na(H₂O)₃]⁺_n. K₂(dnpzc) · H₂O has a true 3D structure. It was observed that the doubly deprotonated ligand (dnpzc^2–) can act as a versatile bridge to form polymeric structures by varying combinations of its 8 potential donor atoms (two carboxy O atoms, two pyrazolyl N atoms and four nitro O atoms). Particularly in the structure of K₂(dnpzc) · H₂O, all the 8 donor atoms of dnpzc^2– take part in the coordination and as many as 10 potassium atoms are connected by one ligand.     

Effect of Ethylene Glycol on the Molecular Organization of H2O in Comparison with Methanol and Glycerol: A Calorimetric Study

Excess partial molar enthalpies of ethylene glycol, H ^E _EG, in binary ethylene glycol–H₂O, and those of 1-propanol, H ^E _IP, in ternary 1-propanol–ethylene glycol (or methanol)–H₂O were determined at 25°C. From these data, the solute–solute interaction functions, H ^E _EG–EG = N(H ^E _EG/n _EG) and H ^E _1P–1P = N(H ^E _1P/n _1P), were calculated by graphical differentiation without resorting to curve fitting. Using these, together with the partial molar volume data, the effect of ethylene glycol on the molecular organization of H₂O was investigated in comparison with methanol and glycerol. We found that there are three concentration regions, in each of which the mixing scheme is qualitatively different from the other regions. Mixing scheme III operative in the solute-rich region is such that the solute molecules are in a similar situation as in the pure state, most likely in clusters of its own kind. Mixing scheme II, in the intermediate region, consists of two kinds of clusters each rich in solute and in H₂O, respectively. Thus, the bond percolation nature of the hydrogen bond network of liquid H₂O is lost. Mixing scheme I is a progressive modification of liquid H₂O by the solute, but the basic characteristics of liquid H₂O are still retained. In particular, the bond percolation of the hydrogen bond network is still intact. Similar to glycerol, ethylene glycol participates in the hydrogen bond network of H₂O via-OH groups, and reduces the global average of the hydrogen bond probability and the fluctuations inherent in liquid H₂O. In contrast to glycerol, there is also a sign of a weak hydrophobic effect caused by ethylene glycol. However, how these hydrophobic and hydrophilic effects of ethylene glycol work together in modifying the molecular organization of H₂O in mixing scheme I is yet to be elucidated.     

Shock tube studies of the N2O/Ar and N2O/H2/Ar systems

N₂O decay has been monitored via infrared emission for a series of mixtures containing N₂O/Ar and N₂O/H₂/Ar. These mixtures were studied behind reflected shock waves in the temperature interval of 1950–3075°K with total concentrations ranging from 1.2 to 2.5 × 10¹⁸ molec/cm³. In all cases the N₂O decayed exponentially, and a rate constant k_obs was obtained. Runs without added H₂ could be described by the following Arrhenius parameters: log A = ?9.72 ± 0.08 (in units of cm³/molec · sec) and E_A = 203.5 ± 3.6 kJ/mole. Addition of 0.01% and 0.1% H₂ was observed to increase the decay rate; the largest increase occurred between 2250 and 2500°K with 0.1% H₂, where k_obs doubled. Mixtures with no added H₂ were analyzed by numerical integration of the following reactions: Quantitative agreement between calculations and observations were obtained with both high and low choices for k₂ and k₃. The additional reactions were included in the analysis of the mixtures containing H₂. Here agreement was obtained only when low values were assigned to k₂ and k₃. The combinations of k₁ ← k₃ which agreed with all the data were k₁ = 3.25 × 10^?10 exp (?215 kJ/RT) and k₂ = k₃ = 1.91 × 10^?11 exp (-105 kJ/RT).