A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols

An experimental and modeling study of irradiated toluene–NO_x–air, toluene–benzaldehyde–NO_x–air, and cresol–NO_x–air mixtures at part-per-million concentrations has been carried out. These mixtures were irradiated at 303 ± 1 K in a 5800-liter Teflon-lined, evacuable environmental chamber, with temperature, humidity, light intensity, spectral distribution, and the concentrations of O₃, NO, NO₂, toluene, PAN, formaldehyde, benzaldehyde, o-cresol, m-nitrotoluene, and methyl nitrate beingmonitored as a function of time. For the toluene and toluene–benzaldehyde–NO_x–air runs a variety of initial reactant concentrations were investigated. Cresol–NO_x–air runs were observed to be much less reactive in terms of O₃ formation and NO to NO₂ conversion rates than toluene–NO_x–air runs, with the relative reactivity of the cresol isomers being in the order meta » ortho > para. The addition of benzaldehyde to toluene–NO_x–air mixtures decreased the reactivity, in agreement with previous studies. Alternative mechanistic pathways for the NO_x photooxidations of aromaticsystems in general are discussed, and the effects of varying these mechanistic alternatives on the model predictions for the toluene and o-cresol–NO_x–air systems are examined. Fits of the calculations to most of the experimental concentration–time profiles could be obtained to within the experimental uncertainty for two of the mechanistic options considered. In both cases it is assumed that (1) O₂ adds to the OH–toluene adduct ～75% of the time forming, after a further addition of O₂, a C₇ bicyclic peroxy radical, and (2) this C₇ bicyclic peroxy radical reacts with NO ～75% of the time to ultimately form α-dicarbonyls and conjugated γ-dicarbonyls (e.g., methylglyoxal + 2-butene-1,4-dial) and ～25% of the time to form organic nitrates. The major uncertainties in the mechanisms concern (1) the structure of the bicyclicperoxy intermediate, and (2) the γ-dicarbonyl photooxidation mechanism. Good fits to the o-cresol concentration–time profiles in the toluene–NO_x runs are obtained if it is assumed that o7-cresol reacts rapidly with NO₃ radicals. However, it is observed that the model underpredicts nitrotoluene yields by a factor of ～10, but this is in any case a minor product. It is concluded that further experimental work will be required toadequately validate the assumptions incorporated in the aromatic photooxidation mechanisms presented here.     

Smog chamber study of H2O2 formation in ethene?NOx and propene?NOx mixtures

Hydrogen peroxide formation in the photooxidation of CO? NO_x, ethene? NO_x, and propene? NO_x mixtures has been determined in the TVA 31 cubic meter smog chamber under the following conditions: [NO_x] ca. 22–46 ppb; ethene = 0.22–1.1 ppm, [propene] = 0.12–0.97 ppm; [H₂O] ca. 8 × 10^?3 ppm. Ethene, propene, NO, NO_x, PAN, HCHO, and CH₃CHO were also monitored. Computer modeling was performed using the gas phase ethene and propene mechanism of the Regional Acid Deposition Model. There is good agreement between the model predicted and observed H₂O₂ concentrations. However, to successfully model all the propene? NO_x experimental results, organic nitrate formation from the reaction of peroxy radicals with NO must be included in the mechanism.     

Updated chemical mechanism for atmospheric photooxidation of toluene

A new reaction mechanism describing the atmospheric photochemical oxidation of toluene is formulated and tested against environmental chamber data from the University of California, Riverside, Statewide Air Pollution Research Center (SAPRC). On simulations of toluene—NO_x and toluene—benzaldehyde—NO_x irradiations, the average predicted O₃ and PAN maxima are within 3% of the experimental values. Simulations performed with the new mechanism are used to investigate various mechanistic paths, and to gain insight into areas where our understanding is not complete. Specific areas that are investigated include benzaldehyde photolysis, organic nitrate formation, alternate ring fragmentation pathways, and conjugated γ-dicarbonyl condensation to the aerosol phase.     

Formation of ring-retaining products from the OH radical-initiated reactions of benzene and toluene

The aromatic ring-retaining products formed from the gas–phase reactions of the OH radical with benzene and toluene, in the presence of NO_x, have been identified and their formation yields determined. These products, and their formation yields, are as follows: from benzene – phenol, 0.236 ± 0.044; nitrobenzene, {(0.0336 ± 0.0078) + (3.07 ± 0.92) × 10^?16[NO₂]}; from toluene – benzaldehyde, 0.0645 ± 0.0080; benzyl nitrate, 0.0084 ± 0.0017; o?cresol, 0.204 ± 0.027; m? + p?cresol, 0.048 ± 0.009; m-nitrotoluene, {(0.0135 ± 0.0029) + (1.90 ± 0.25) × 10^?16[NO₂]}, where the NO₂ concentration is in molecule cm^?3 units. The formation yields of o- and p-nitrotoluene from toluene were ca. 0.07 and 0.35 that of m-nitrotoluene, respectively. The observations that the nitro-aromatic yields do not extrapolate to zero as the NO₂ concentration approaches zero are not consistent with current chemical mechanisms for these OH radical-initiated reactions, and suggest that under the experimental conditions employed in this study the hydroxycyclohexadienyl radicals formed from OH radical addition to the aromatic ring react with NO₂ rather than with O₂. However, these data concerning the nitroaromatic yields are consistent with our previous conclusions that many of the nitrated polycyclic aromatic hydrocarbons present in ambient air are formed, at least in part, in the atmosphere from OH radical reactions.     

Products of the gas-phase reactions of aromatic hydrocarbons: Effect of NO2 concentration

The formation yields of selected products of the OH radical-initiated reactions of toluene, o-xylene, and 1,2,3,-trimethylbenzene have been measured in the absence of NO_x and in the presence of varying concentrations of NO and NO₂. The formation yield of o-cresol from toluene increased from 0.123 ± 0.022 in the absence of NO_x to 0.160 ± 0.008 for an average NO₂ concentration of 1.7 × 10¹⁴ molecule cm³. The formation yield of 2,3-butanedione from o-xylene was 0.092 ± 0.013 in the absence of NO_x, and in the presence of NO_x decreased from 0.16 at an average NO₂ concentration of (7–8) × 10¹² molecule cm^?3 to 0.09 at an average NO₂ concentration of ca. 7 × 10¹³ molecule cm^?3. The formation yield of 2,3-butanedione from 1,2,3-trimethylbenzene increased from 0.18 in the absence of NO_x to 0.444 ± 0.053 in the presence of ca. (0.16–3.6) × 10¹³ molecule cm^?3 of NO₂. These product data are consistent with literature kinetic data showing that the hydroxycyclohexadienyl radicals formed by OH radical addition to the aromatic ring react with both O₂ and NO₂ and with the NO₂ reaction rate constants being ca. 10⁵ higher than the O₂ reaction rate constants at room temperature. Under typical tropospheric conditions the reactions of the hydroxycyclohexadienyl radicals with O₂ will dominate over their reactions with NO₂. © 1994 John Wiley & Sons, Inc.     

Computer modeling of smog chamber data: Progress in validation of a detailed mechanism for the photooxidation of propene and n-butane in photochemical smog

A detailed mechanism is presented for reactions occurring during irradiation of part-per-million concentrations of propene and/or n-butane and oxides of nitrogen in air. Data from an extensive series of well-characterized smog chamber experiments carried out in our 5800-liter evacuable chamber–solar simulator facility designed for providing data suitable for quantitative model validation were used to elucidate several unknown or uncertain kinetic parameters and details of the reaction mechanism. The mechanism was then tested against the data base from the smog chamber runs. In general, most calculated concentration–time profiles agreed with experiments to within the experimental uncertainties. Fits were usually attained to within ～±20% or better for ozone, NO, propene, and n-butane, to within ～±30% or better for NO₂, PAN, methyl ethyl ketone, 2-butyl nitrate, butyraldehyde, and (in runs not containing propene) methyl nitrate, to within ?±50% or better for the minor products 1-butyl nitrate and propene oxide, and to within a factor of 2 for methyl nitrate in propene-containing runs. Propionaldehyde was consistently underpredicted in all runs; it is probably a chamber contaminant. For formaldehyde and acetaldehyde, the major products in both systems, fits to within ?±20% were often obtained, yet for a number of experiments, significantly greater discrepancies were observed, probably as a result of experimental and/or analytical problems. The good fits to experimental data were attained only after adjusting several rate constants or rate constant ratios related to uncertainties concerning chamber effects or the chemical mechanism. The largest uncertainty concerns the necessity to include in the mechanism a significant rate of radical input from unknown sources in the smog chamber. Other areas where fundamental kinetic and mechanistic data are most needed before a predictive, detailed propene + n-butane-NO_x-air smog model can be completely validated concern other chamber effects, the O₃ + propene mechanism, decomposition rates of substituted alkoxy radicals, primary quantum yields for radical production as a function of wavelength for aldehyde and ketone photolyses, and the mechanisms and rates of reactions of peroxy radicals with NO and NO².     

Reaction of the NO3 radical with CO: Determination of an upper limit for the rate constant using FTIR spectroscopy

An upper limit for the reaction rate of CO with the nitrate radical NO₃ has been determined equal to 4 × 10^?19 cm⁺³ molec^?1 s^?1 at 295 ± 2 K. In the experiment the isotopic species C¹³O¹⁶ and C¹³O¹⁸ mixed at 1–2 ppmv level in synthetic air have been reacted with NO₃ and the reaction followed using long path infrared absorption FT spectroscopy. The result is of interest in the studies on the role played by NO₃ in nighttime tropospheric chemistry.     

Formation of ring-retaining products from the OH radical-initated reactions of o-, m-, and p-xylene

The ring-retaining products formed from the OH radical-initiated reactions of o-, m-, and p-xylene in the presence of NO_x have been identified and their formation yields determined. Experiments were carried out at 298 ± 2 K and in the presence of 740 torr total pressure of air. The products observed, and their yields, were: from o-xylene, o-tolualdehyde, 0.0453; 2-methylbenzyl nitrate, (0.0135 + 5.5 × 10^?17 [NO₂]); 2,3-dimethylphenol, 0.097; 3,4-dimethyl-phenol, 0.064; 3-nitro-o-xylene, 0.0059; 4-nitro-o-xylene, (0.0111 + 9.9 × 10^?17 [NO₂]); from m-xylene, m-tolualdehyde, 0.0331; 3-methylbenzyl nitrate, 0.0061; 2,4-dimethylphenol, 0.099; 2,6-dimethylphenol, 0.111; 4-nitro-m-xylene, 0.0018; 5-nitro-m-xylene, (0.0032 + 1.6 × 10^?17 [NO₂]); from p-xylene, p-tolualdehyde, 0.0701; 4-methylbenzyl nitrate, 0.0082; 2,5-dimethylphenol, 0.188, 2-nitro-p-xylene, (0.0120 + 2.8 × 10^?17 [NO₂]), where the NO₂ concentration is in molecule cm^?3 units. The nitro-xylene data are consistent with our recent product study of the corresponding reactions of benzene and toluene and indicate that under the experimental conditions employed the dimethylhydroxycyclohexadienyl radicals reacted with NO₂ and not with O₂. When combined with literature ring-cleavage product yields, these data show that ca. 55–80% of the reaction pathways are accounted for.     

An investigation of the dark formation of nitrous acid in environmental chambers

The formation of nitrous acid (HONO) in the dark from initial concentrations of NO₂ of 0.1–20 ppm in air, and the concurrent disappearance of NO₂, were monitored quantitatively by UV differential optical absorption spectroscopy in two different environmental chambers of ca.4300- and 5800-L volume (both with surface/volume ratios of 3.4 m^?1). In these environmental chambers the initial HONO formation rate was first order in the NO₂ concentration and increased with the water vapor concentration. However, the HONO formation rate was independent of the NO concentration and relatively insensitive to temperature. The initial pseudo-first-order consumption rate of NO₂ was (2.8 ± 1.2) × 10^?4 min^?1 in the 5800-L Teflon-coated evacuable chamber and (1.6 ± 0.5) × 10^?4 min^?1 in a 4300-L all-Teflon reaction chamber at ca.300 K and ca.50% RH. The initial HONO yields were ca.40–50% of the NO₂ reacted in the evacuable chamber and ca.10–30% in the all-Teflon chamber. Nitric oxide formation was observed during the later stages of the reaction in the evacuable chamber, but ca.50% of the nitrogen could not be accounted for, and gas phase HNO₃ was not detected. The implications of these data concerning radical sources in environmental chamber irradiations of NO_x? organic-air mixtures, and of HONO formation in polluted atmospheres, are discussed.     

The atmospheric chemistry of the HC(O)CO radical

The chemistry of the HC(O)CO radical, produced in the oxidation of glyoxal, has been studied under conditions relevant to the lower atmosphere using an environmental chamber/Fourier Transform infrared spectrometric system. The chemistry of HC(O)CO was studied over the range 224–317 K at 700 Torr total pressure and was found to be governed by competition between unimolecular decomposition [to HCO and CO, reaction (5)] and reaction with O₂ [to form HO₂ and 2CO, reaction (6a), or HC(O)C(O)O₂, reaction (6b)]. The rate coefficient for decomposition relative to that of reaction with O₂ increases with increasing temperature. Assuming a value for k₆ of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, the following expression for the unimolecular decomposition is obtained at 700 Torr, k₅ = 1.4⁺⁹/_−1.1 × 10¹² exp(−3160 ± 500/T). The rate coefficients for reactions (6a) and (6b) are about equal, with no strong dependence on temperature. The reaction of HC(O)C(O)O₂ with NO₂ was also studied. Final product analysis was consistent with the formation of HCO, CO₂, and NO₃ as the major products in this reaction; no evidence for the PAN‐type species, HC(O)C(O)O₂NO₂, was found even at the lowest temperature studied (224 K). The UV‐visible absorption spectrum of glyoxal is also reported; results are in substantive agreement with previous studies. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 149–156, 2001     

Electron-impact fragmentation of aliphatic polynitro compounds

The mass spectra of two series of aliphatic polynitro compounds are reported and discussed. The fragmentation patterns of aliphatic nitro and polynitro compounds are similar in that no appreciable molecular ion current is observed; however, there are several other features in the fragmentation of aliphatic polynitro compounds which differ from that of nitroalkane spectra. Both series of compounds studied-C(NO₂)_x(CH₃)_4?x, where x = 4 to 0 and C₂(NO₂)_x(CH₃)_6?x, where x = 6,4,2-show a decrease in the number and intensity of alkylions with an increase in the NO⁺ and NO₂⁺ ion current as x increases. The main ions resulting from the more nitrated compounds are [NO]⁺, [NO₂]⁺, [CO₂]^+. and [CH₃CO]⁺, whose noncharged counterparts are the principal species produced in the detonation of these compounds. This similarity of the products of the two processes suggests the use of mass spectroscopy for the investigation of the initial explosive processes. The principal fragmentation pathways of the polynitroalkanes have been elucidated by exact mass measurements and the observation of metastable ion transitions.     

Experimental investigation of chamber-dependent radical sources

While environmental chamber data have been widely used to generate and validate computer models of the chemistry occurring in polluted atmospheres, the effects of the chambers on the gas-phase chemistry being studied have been poorly characterized. In order to investigate such chamber effects, a series of NO_x—air irradiations, with trace levels of organics present to monitor OH radical concentrations, have been carried out in four different environmental chambers (ranging in volume from ～100 to 40,000 L) at varying temperatures, humidities, pressures, and reaction conditions. In addition, a number of control experiments have been carried out to validate the technique for measuring OH radical levels in these irradiations. The data show that unknown sources of OH radicals are present in all of the chambers studied. The data are consistent with the presence of two distinct radical sources: (1) the photolysis of initially present HONO, whose importance increases with increasing NO₂/NO concentration ratios, but which is a minor contributor to the overall radical flux after 30–60 min of irradiation, and (2) a constant (for these NO_x—air irradiations) radical source which dominates beyond approximately the first 60 min of irradiation. The radical input rates, after the first ∽30–60 min of irradiation, are independent of the NO concentration, increase with increasing temperature, humidity, and NO₂ concentration, are proportional to light intensity, and are dependent on the chamber employed. Although the exact nature of this radical source is still undetermined, results of experiments reported here allow a number of possible mechanisms to be ruled, out, and these are discussed.     

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO₃RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo₂CT_x : Fe (T_x are oxo, hydroxy and fluoro surface termination groups). Mo₂CT_x : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo₂CT_x) and features a Faradaic efficiency (FE) and an NH₃ yield rate of 41 % and 3.2 μmol h⁻¹ mg⁻¹, respectively, for the reduction of NO₃⁻ to NH₄⁺ in acidic media and 70 % and 12.9 μmol h⁻¹ mg⁻¹ in neutral media. Regardless of the media, Mo₂CT_x : Fe outperforms monometallic Mo₂CT_x owing to a more facile reductive defunctionalization of T_x groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a T_x vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO₃RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.     

Nitration of Simple Aromatics with NO2 under Air Atmosphere in the Presence of Novel Brønsted Acidic Ionic Liquids

Abstract

Aromatics nitrate with NO₂/air catalyzed by novel Brønsted acidic ionic liquids (ILs) without any volatile chlorinated organic solvent under mild conditions. The ILs employed were caprolactam based, [Caprolactam]X (X^?=pTSO^?, BSO^?, BF₄ ^?, NO₃ ^?), which are of relatively lower cost and lower toxicity than traditional imidazolium‐based ILs. The nitration reactions were carried out at ?15 to ?0°C first, then at room temperature for a longer time with a little excessive NO₂ (ca. 1.4 eqv.) for moderate yield (for toluene). The IL could be reused four times.     

Rate coefficients for reactions of OH radicals with CH3D,CH2D2, CHD3, and CD4

Fourier transform infrared (FTIR) smog chamber techniques were used to investigate the atmospheric chemistry of the isotopologues of methane. Relative rate measurements were performed to determine the kinetics of the reaction of the isotopologues of methane with OH radicals in cm³ molecule⁻¹ s⁻¹ units: k(CH₃D + OH) = (5.19 ± 0.90) × 10⁻¹⁵, k(CH₂D₂ + OH) = (4.11 ± 0.74) × 10⁻¹⁵, k(CHD₃ + OH) = (2.14 ± 0.43) × 10⁻¹⁵, and k(CD₄ + OH) = (1.17 ± 0.19) × 10⁻¹⁵ in 700 Torr of air diluent at 296 ± 2 K. Using the determined OH rate coefficients, the atmospheric lifetimes for CH_4–xD_x (x = 1–4) were estimated to be 6.1, 7.7, 14.8, and 27.0 years, respectively. The results are discussed in relation to previous measurements of these rate coefficients.     

The kinetics and mechanisms of the HO2-NO2 reactions; the significance of peroxynitric acid formation in photochemical smog

Peroxynitric acid has been identified by Fourier transform, infrared, long-path spectroscopy as a product of the UV-irradiated dilute mixtures of HONO, CO, No_x in synthetic air at 24 ± 2°C. The characteristic peaks of this compound are identical to those observed by Niki, in irradiated Cl₂, H₂, NO₂, air mixtures and Hanst, in irradiated Cl₂, CH₂O, NO₂, air mixtures. The concentration of the reactants and products of the HONO, CO, NO_x, air system have been determined as a function of irradiation time. The changes observed were computer simulated using a combination of thirty elementary reactions. The data suggests that both reactions (1) and (2) occur as a result of HO₂-NO₂ interactions in the gas phase: HO₂ + NO₂ å HONO + O₂ (1); HO₂ + NO₂(+M) å HO₂NO₂(+M)(2). The data give the preliminary estimates: k₁/k₂ ≈0.7 ±0.4; k₁/k₃ ≈ 0.043 ± 0.02; k₂/k₃ ≈ 0.058 ± 0.02, where reaction (3) is: HO₂ + NO å HO + NO₂. These data and computer simulations of sunlight-irradiated, NO_x, hydrocarbon, aldehyde-polluted atmospheres suggest that peroxynitric acid may be formed in urban atmospheres at rates which are comparable to those observed for peroxyacetyl nitrate (PAN), and it is concluded that HO₂NO₂ may contribute to the “oxidant” found in these atmopheres.     

Experimental measurements and kinetic modeling of CO/H2/O2/NOx conversion at high pressure

This paper presents results from lean CO/H₂/O₂/NO_x oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high‐pressure laminar flow reactor designed to conduct well‐defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H₂/O₂, CO/CO₂, and NO_x chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS‐QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO₂ ? H₂O + O₂, CO + OH ? [HOCO] ? CO₂ + H, HOCO + OH ? CO + H₂O₂, NO₂ + H₂ ? HNO₂ + H, NO₂ + HO₂ ? HONO/HNO₂ + O₂, and HNO₂(+M) ? HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H₂/O₂, H₂/O₂/NO₂, and CO/H₂O/O₂ at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H₂O₂ → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008     

NOx-Sorbing Metal Oxides, MnOx–CeO2. Oxidative NO Adsorption and NOx–H2 Reaction

(n)MnO_x–(1?n)CeO₂ binary oxides have been studied for the sorptive NO removal and subsequent reduction of NO_x sorbed to N₂ at low temperatures (≤150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO^? ₃) and/or nitrite (NO^? ₂) species on the surface depending on temperature, O₂ concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NO_x toward H₂ was examined for MnO_x–CeO₂ impregnated with Pd, which is known as a nonselective catalyst toward NO–H₂ reaction in the presence of excess oxygen. The Pd/MnO_x–CeO₂ catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H₂ at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnO_x–CeO₂ surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H₂–O₂ combustion, H₂–NO reaction was suppressed to a certain extent in the presence of O₂. Nevertheless, Pd/MnO_x–CeO₂ attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H₂, and 6% O₂ in He at as low as 150 °C, compared to ca. 30% conversion for Pd/γ–Al₂O₃ at the same temperature. The combination of NO_x-sorbing materials and H₂-activation catalysts is expected to pave the way to development of novel NO_x-sorbing catalysts for selective deNO_x at very low temperatures.     

Syntheses and structures of cobalt(II), nickel(II), and copper(II) complexes with N,N,N′,N′-tetraalkylpyridine-2,6-dicarboxamides (O-daap) containing nitrate as the counter ion

Reactions of M(NO₃)₂?·?xH₂O [M?=?Co(II), Ni(II), and Cu(II)] with N,N,N′,N′-tetraalkylpyridine-2,6-dicarboxamides(O-daap) in CH₃CN yield [Co(O-dmap)(NO₃)₂] (1), [Co(O-deap)(NO₃)₂] (2), [Co(O-dpap)(NO₃)₂] (3), [Ni(O-dmap)(H₂O)₃](NO₃)₂] (4), [Ni(O-deap)(H₂O)₂(NO₃)](NO₃)] (5), [Cu(O-deap)(NO₃)₂] (6), and [Cu(O-dpap)(NO₃)₂] (7). X-ray crystal structures of 1, 2, 4, 5, and 7 reveal that O-daap ligands coordinate tridentate to each metal, O–N–O, with nitrate playing a vital role in molecular and crystal structures of all the complexes. The coordination geometry in the two Co(II) complexes, 1 and 2, is approximately pentagonal bipyramidal with nitrate bonded in a slightly unsymmetrical bidentate chelating mode. [Ni(dmap)(H₂O)₃](NO₃)₂ (4) and [Ni(deap)(H₂O)₂(NO₃)](NO₃) (5) exhibit octahedral geometry, the former containing uncoordinated nitrate while the latter has one nitrate coordinated unidentate and the other nitrate outside the coordination sphere. The Cu(II) in [Cu(dpap)(NO₃)₂] (7) occupies a distorted square pyramidal geometry and is linked to two unidentate nitrates, although one nitrate is also involved in a weak interaction with the metal through its other oxygen. IR spectra and other physical studies are consistent with their crystal structural data. O-dmap?=?N,N,N′,N′-tetramethylpyridine-2,6-dicarboxamides; O-deap?=?N,N,N′,N′-tetraethylpyridine-2,6-dicarboxamides; and O-dpap?=?N,N,N′,N′-tetraisopropylpyridine-2,6-dicarboxamides.     

Mechanisms of Photochemical Air Pollution

Selected aspects of the chemistry of photochemical air pollution is discussed and some important, unresolved problems dilineated. The reactive species considered include NO₂, O₃, O(³P), O(¹D), O₂(¹Δ_g), OH and HO₂. Both the kinetics and mechanicsms of the reactions constituting the major tropospheric sources and sinks of these species are treated where available. The application of this information in both computer and smog chamber simulations of photochemical smog is discussed.