Rate constants for quenching of O2(1Δg) with sulfur compounds

The rate constants for the quenching of O₂(¹Δ_g) with carbon disulfide, dimethyl sulfide, dimethyl disulfide, diallyl disulfide, ethyl mercaptan, and thiophene have been determined in a discharge flow system in the absence of oxygen atoms. The rate constants are found to be (6.5 ± 0.6) × 10⁴, (1.8 ± 0.2) × 10⁴, and (3.5 ± 0.6) × 10³ L/mol · s for dimethyl sulfide, ethyl mercaptan, and thiophene, respectively. The other compounds have rate constants <9.9 × 10² L/mol · s. In the case of dimethyl sulfide, even when NO₂ concentration is more than what is required to remove oxygen atoms completely, the rate constants are found to vary with different amounts of NO₂. No correlation is found to exist between the logarithm of the rate constants and the ionization potentials of the compounds.     

Reaction of hydrogen atoms and O2(1Δg)

The reaction of atomic hydrogen with O₂(¹Δ_g) has been investigated as a function of temperature, using a fast discharge-flow apparatus equipped for EPR detection of free radical species. The rate constant for the overall reaction was measured as (1.46 ± 0.49) × 10^?11 exp(-4000 ± 200 cal/mol/RT) cm³/s. Evidence is presented which suggests that the reaction occurs principally via abstraction, H + O₂(¹Δ_g) → OH + O, rather than via physical quenching, H + O₂(¹Δ_g) → H + O₂(X³Σ_g^?).     

Quenching of O2(1Δg) by aliphatic amines

The quenching rate constants of O₂(¹Δ_g) with n-butylamine, diethylamine, dipropylamine, dibutylamine, and tripropylamine have been determined in a discharge flow system. The rate constants are found to be (1.6 ± 0.2) × 10³, (8.5 ± 0.6) × 10⁴, (9.8 ± 0.5) × 10⁴, (2.1 ± 0.1) × 10⁵, and (8.6 ± 0.5) × 10⁵ 1 mol^?1 s^?1, respectively. The rate constants are found to increase in the order, tertiary amine → secondary amine → primary amine. The “inductive effect” of alkyl substitution is also found to increase the rate constant in a given series of amines.     

Photooxidation of Tryptophan: O2(1Δg) versus Electron-Transfer Pathway

Abstract— Tris (2,2'-bipyridyl)ruthenium(II)chloride hexahydrate (Ru[bpy]₃²+) free in solution and adsorbed onto antimony-doped SnO₂ colloidal particles was used as a photosensitizer for a comparison of the O₂(¹Δ_g) and electron-transfer-mediated photooxidation of tryptophan (TRP), respectively. Quenching of excited Ru(bpy)₃²+ by O₂(³σ_g^?) in an aerated aqueous solution leads only to the formation of O₂(¹Δ_g) (φ₄= 0.18) and this compound was used as a type II photosensitizer. Excitation of Ru(bpy)₃²+ adsorbed onto Sb/SnO₂ results in a fast injection of an electron into the conduction band of the semiconductor and accordingly to the formation of Ru(bpy)₃²+ and was used for the sensitization of the electron-transfer-mediated photooxidation. The Ru(bpy)₃³+ is reduced by TRP with a bimolecular rate constant k_Q= 5.9 × 10⁸M^?1 s^?1, while O₂(¹Δ_g) is quenched by TRP with k_t= 7.1 × 10⁷M^?1 s^?1 (chemical + physical quenching). Relative rate constants for the photooxidation of TRP (k_c) via both pathways were determined using fluorescence emission spectroscopy. With N_p, the rate of photons absorbed, being constant for both pathways we obtained k_c= (372/N_p) M^?1 s^?1 for the O₂(¹Δ_g) pathway and k_c≥ (25013/N_p) M^?1 s^?1 for the electron-transfer pathway, respectively. Thus the photooxidation of Trp is more than two orders of magnitude more efficient when it is initiated by electron transfer than when initiated by O₂(¹Δ_g).     

The rate of destruction of O2 (1Δg) by atomic hydrogen

An analysis of the atmospheric observations of O₂(¹δg) in the dayglow and twilight confirms the crude experimental assessments of Brown [1] and the conclusions that can be made from recent work of Westenberg, Roscoe, and DeHaas [2] that the reaction is relatively slow, much slower than was expected or can be explained easily in theoretical terms. Using a value for the concentration of atomic hydrogen at 85 km, an upper limit rate of 3×10^?13 cm³ molecule^?1 sec^?1 would be compatible with current atmospheric models. An evaluation of the available data for the rates of several reactions involving ground state or electronically excited species, for which the values are reasonably well established, is included to better analyze the general effects of an electronically excited reactant. This further illustrates the unusual slowness of the H + O₂(¹Δg) reaction.     

Generation of high O2 (a1Δg) concentrations in O2/H2 mixtures

The effect of hydrogen on the concentration of singlet oxygen in the a¹Δ_g and b¹Σ states, generated from a microwave discharge in O₂ and in an O₂/Ar mixture, was studied in flow reactors. The addition of hydrogen, in a range of 0.01–1 of concentration of the O₂, increased the yields of singlet oxygen by factor of 5–20. In addition to the higher O₂ (a and b) concentrations, the addition of hydrogen removed the usual NO₂ fluorescence, making observation of the O₂(b → X) transition at 762 nm much easier in the flow reactor. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 12–17, 2006     

Yields of O2(1Σg+) and O2(1Δg) in the H + O2 reaction system,and the quenching of O2(1Σg+) by atomic hydrogen

The generation of metastable O₂(¹Σg⁺) and O₂(¹Δg) in the H + O₂ system of reactions was studied by the flow discharge chemiluminescence detection method. In addition to the O₂(¹Σg⁺) and O₂(¹Δg) emissions, strong OH(v = 2) → OH(v = 0), OH(v = 3) → OH(v = 1), HO₂(²A′₀₀₀) → HO₂(²A″₀₀₀), HO₂(²A′₀₀₁) → HO₂(²A″₀₀₀), and H O₂(²A″₂₀₀) → HO₂(²A″₀₀₀) emissions were detected in the H + O₂ system. The rate constants for the quenching of O₂(¹Σg⁺) by H and H₂ were determined to be (5.1 ± 1.4) × 10^?13 and (7.1 ± 0.1) × 10^?13 cm³ s^?1, respectively. An upper limit for the branching ratio to produce O₂(¹Σg⁺) by the H + HO₂ reaction was calculated to be 2.1%. The contributions from other reactions producing singlet oxygen were investigated.     

Collision-induced emission of O2(b1Σ → a1Δg) in the gas phase

In flow tube studies of the quenching of O₂(b¹Σ), broad band emission of O₂(b):M collision complexes was found to appear under the discrete rotational lines of the 0–0 band of the b¹Σ → a¹Δ_g electric quadrupole transition at higher oxygen pressures and on addition of foreign gases. Bimolecular rate constants for the collision-induced emission processes have been derived from the ratio of the intensities of the discrete lines and the continuum as well as from low-resolution measurements of the relative intensities of the b → a and b → X bands as a function of O₂ and added gas pressure. They range from ≈10^?21 cm³ s^?1 for He to ≈4 × 10^?19 cm³ s^?1 for PCl₃ vapor.     

The reaction of 2,5-dimethylpyrrole with O2(a1Δg) in the gas phase

The chemical reaction of 2,5-dimethylpyrrole (C₆H₉N) with O₂(¹Δ_g) was studied in the gas phase in an isothermal flow reactor at room temperature and low pressures. The C₆H₉N concentration profiles were studied under pseudo-first order conditions [C₆H₉N]_° ? [O₂(¹Δ_g)] with mass-spectrometric detection of C₆H₉N. O₂(¹Δ_g) was produced either in a microwave discharge or in a chemical reaction. The value for the rate constant: was measured. The rate constant is compared to the value obtained for the quenching process. The primary product C₆H₉NO₂ was detected by mass spectrometry and the reaction mechanism is proposed. The possibility of using this reaction as a gas phase titration reaction for O₂(¹Δ_g) is discussed.     

Rate constants for the quenching of metastable O2(1Σ) molecules

Singlet molecular oxygen, O₂(¹Σ), is one of the important intermediate species in the atmospheres of Earth, Mars, and Venus. To elucidate the chemistry of this excited molecular oxygen, a series of kinetic measurements have been undertaken using the flow-discharge/optical-emission technique. By monitoring the characteristic emission (762 nm for ¹Σ), the quenching rates for several important molecules have been obtained at room temperature. The following table summarizes measurements.

Kinetic studies of the deactivation of O2(1Σg+) and O(1D)

The kinetics of the deactivation of O₂(¹Σ_g⁺) is studied in real time. O₂(¹Σ_g⁺) is generated in this system by the O(¹D) + O₂ reaction following O₃laser flash photolysis in the presence of excess O₂, and it is monitored by its characteristic emission band at 762 nm. Quenching rate constants were obtained for O₂, O₃, N₂, CO₂, H₂O, CF₄and the rare gases. Since O(¹D) is the precursor for the formation of O₂(¹Σ_g⁺), the addition of an O(¹D) quencher effectively lowers the initial concentration of O₂(¹Σ_g⁺). By measuring the initial intensity of the 762 nm fluorescence signal, the relative quenching efficiencies were determined for O(¹D) quenching by N₂, CO₂, Xe, and Kr with respect to O₂; the results are in good agreement with literature values.     

Collisional deactivation of O2(1Σg+)

Relaxation rates for O₂(¹Σ_g⁺) by nonradiative pathways have been determined using the fast-flow technique. O₂(¹Σ_g⁺) is formed from O₂(¹Δ_g) by an energy pooling process. O₂(¹Δ_g) is generated by passing purified oxygen through a microwave discharge. Oxygen atoms are removed by distilling mercury vapor through the discharge zone. It has been observed that the wall loss rate for O₂(¹Σ_g⁺) decreases with increasing pressure of oxygen and thus appears to be diffusion controlled. Quenching rate constants for O₂, N₂, and He have been determined and found to be (1.5 ± 0.1) × 10⁴, (1.0 ± 0.05) × 10⁶ and (1.2 ± 0.1) × 10⁵ l./mol·sec, respectively.     

Rate constants in the photopolymerization of H2O2–isoprene systems

The polymerization reactions of isoprene with the use of H₂O₂ as the photoinitiator have been studied in acetone solutions. The lifetime of chain radicals, τ_s, was evaluated to be 0.29 by the rotating sector technique. The rate constants k_p and k_t were found to be 19.1 and 3.40 × 10⁷ L/mol- s, respectively.     

Farbstoffsensibilisierte Photo-Oxygenierung von(−)-cis-Pulegol und (+)-Pulegon. Zur Addition von O2(1Δg) an Doppelbindungen mit allylständiger. Hydroxyl- oder Carbonyl-Gruppe

(?)-cis-Pulegol ( 1 ) and (+)-pulegone ( 2 ) readily add one mole of oxygen on dyesensitized photo-oxygenation in solution, forming almost exclusively allyl hydroperoxides. The product distributions and structures of the peroxides ( B–E ) have been derived from analysis of their reduction products. Compound 1 yielded only the diols 4, 5 and 6 , besides the epoxyketones 7 and 8 formed directly from the hydroperoxide C (proportions of 4–8 , ～21:56:13:4:6), while 2 gave the keto-alcohols 9,10 and 11 (～80:6:14) as the sole products. The course of addition of O₂(¹Δ_g) on 1 and 2 was found in all cases to be independent of solvent and photosensitizer.     

Singlet-Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water

In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (k_q) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL₃] complexes (where L stands for 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrazine (bpz), 4,7-diphenyl-1,10-phenanthroline (dip), diphenyl-1,10-phenanthroline-4,7-disulfonate (dpds), and 1, 10-phenanthroline-5-octadecanamide (poda)) in H₂O and in MeOH. These compounds are singlet-oxygen (O₂(¹Δ_g)) sensitizers, and quantum yields of singlet-oxygen production (Ψ_Δ) in both solvents are also reported. Values of k_q and Ψ_Δ depend on the nature of the ligand L and on the solvent, Ψ_Δ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL₃] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (?) is unity for all compounds in the series investigated. Changing from MeOH to H₂O has several remarkable effects: higher k_q and lower Ψ_Δ values are observed; ? drops to ca. 0.5 except for [Ru(bpz)₃]²⁺. In fact, [Ru(bpz)₃]²⁺ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest k_q and Ψ_Δ values and a ? equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge-transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. Ru^II Complexes based on the 4,7-diphenyl-1, 10-phenanthroline (dip) ligand are very efficient and stable singlet-oxygen sensitizers with Ψ_Δ values close to unity in air-saturated MeOH.     

Kinetics of the reactions of singlet molecular oxygen (O21Δg) with organic compounds in the gas phase

The reactions of singlet molecular oxygen (O₂¹Δ_g) with a series of organic compounds have been studied in the gas phase at 298°K. The concentration of singlet molecular oxygen was determined by titration with 2,5-dimethylfuran. The titration technique was checked using a photoionization technique. Absolute rate constants were measured on the basis of the loss of organic reactant and, in some cases, of singlet molecular oxygen. It was found that the usual method of producing singlet molecular oxygen in the gas phase can also, under some conditions, allow reactive species other than singlet molecular oxygen to enter the reactor, leading to serious errors in the determination of rate constants. This problem was eliminated by carrying out the rate measurements in the presence of a small amount of nitrogen dioxide a radical scavenger.     

Triaquatris(2,6‐dinitrophenolato‐κ2O1,O2)yttrium(III)

In the title compound, [Y(C₆H₃N₂O₅)₃(H₂O)₃], the Y atom is nine‐coordinate with a slightly distorted tricapped trigonal prismatic coordination geometry. The Y^III ion is coordinated to three bidentate 2,6‐di­nitro­phenolate ligands and three water mol­ecules. The Y—O bond distances are in the range 2.217 (3)–2.754 (4) Å, with the Y—O distances from the nitro groups being longer than those from the water mol­ecules and the phenol groups. The coordinated NO₂ groups are almost coplanar with the benzene rings.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Microheterogeneous Systems

The quantum yield (Φ_Δ) of singlet oxygen (O₂(¹Δ_g) production by 9H‐fluoren‐9‐one (FLU) is very sensitive to the nature of the solvent (0.02 in a highly polar and protic solvent, such as MeOH, to 1.0 in apolar solvents). This high sensitivity has been used for probing the interaction of FLU with micellar media and microemulsions based on anionic (sodium dodecyl sulfate, SDS; bis‐(2‐ethylhexyl)sodium sulfosuccinate, AOT), cationic (cetyltrimethylammonium chloride, CTAC) and nonionic (Triton X‐100, TX) surfactants. Values of Φ_Δ of FLU vary in a wide range (0.05–1.0) in both microheterogeneous media and neat solvent, and provide information on the microenvironment of FLU, i.e., on its localization within organized media. In ionic and nonionic micellar media, as well as in four‐component microemulsions, FLU is, to various extents, exposed to solvation by the polar and protic components of the microheterogeneous systems (water and/or butan‐1‐ol) in the micellar interfacial region (Φ_Δ=0.05–0.30). In contrast, in AOT reverse micelles (consisting of AOT as surfactant, cyclohexane as hydrophobic component, and water), FLU is located in the hydrophobic continuous pseudophase, and is totally separated from the micellar water pools (Φ_Δ≈1.0).